Handbook: The Cranium, Cranial Nerves, and Meninges PDF

Unit 1 The Cranium, Cranial Nerves and Meninges Dural Venous Sinuses, Blood Supply of Head & Neck Intended Learning Outcomes On completing this Unit and after attending the prosections session in the Dissection Room...

Unit 1 The Cranium, Cranial Nerves and Meninges Dural Venous Sinuses, Blood Supply of Head & Neck Intended Learning Outcomes On completing this Unit and after attending the prosections session in the Dissection Room along with the applied anatomy seminar, you should be able to: 1. Identify the bones of the cranium. 2. Identify the cranial fossae, their contents and foramina. 3. Identify the cranial nerves and know their functions. 4. Identify the meningeal layers of the brain and spinal cord. 5. Understand the folds of dura mater which subdivide the cranium. 6. Identify the dural venous sinuses and their drainage pattern. 7. Know the main arteries that supply the head and neck. 8. Understand the common clinical applications associated with the skull and meninges. 1 Summary 1. The skull, excluding the ossicles, is composed of 22 bones. Some are paired whereas others are single. All but one of these bones are firmly attached to each other via sutures and are immovable. The only movable bone is the mandible. The cranial vault ossifies in membrane to form the frontal, parietal, occipital and squamous temporal bones. The skull base ossifies in cartilage and becomes ossified around the cranial nerves to form foramina. 2. The cranial cavity is divided into the anterior, middle and posterior cranial fossae. These house different parts of the brain and contain foramina through which the 12 pairs of cranial nerves exit from the skull. Their branches are distributed in the head and neck, with the exception of the vagus nerve, which continues into the thorax and abdomen. 3. The three meningeal layers are the outer dura mater, the arachnoid mater, and the innermost pia mater, which adheres to the surface of the brain. Three dural folds, the falx cerebri, falx cerebelli, and tentorium cerebelli, extend into the major brain fissures. 4. The dura mater consists of a double layer of connective tissue. Where the layers separate, primarily at the bases of the dural folds, they form the dural venous sinuses. These venous sinuses collect most of the venous return from the brain and also cerebrospinal fluid. They ultimately drain into the internal jugular vein. 5. The spinal cord is protected by the vertebral column within which it is surrounded by three layers of meninges. The emerging nerve roots exit through intervertebral foramina. The spinal cord terminates in the conus medullaris and becomes the cauda equina at the lower border of L1. The dural sac terminates at S2. 6. The brain is supplied by the internal carotid artery and the vertebral artery, a branch of the subclavian artery. At the base of the skull is an anastomotic circle, the circle of Willis, which is formed from these arteries and their communicating branches. 7. The external carotid artery gives branches to the face, scalp and viscera of the neck. One of its terminal branches, the maxillary artery, gives important meningeal branches. 2 Self-Directed Learning THE SKULL 1. Read Bones of the Skull 2. Read Development of the Skull 3. Read Regions of the head The skull vault houses the brain, and comprises 4 unpaired bones (ethmoid, frontal, sphenoid and occipital) and 2 paired bones (temporal and parietal). The bones of the facial skeleton will be studied in Unit 2. Bones of the Skull Vault Ethmoid cribriform plate with the crista galli, perpendicular plate superior and middle conchae, ethmoidal air cells Frontal contains air sinuses; in two parts at birth but usually fuses early in life superior ciliary ridges and supra-orbital notches Sphenoid greater and lesser wings, medial and lateral pterygoid plates sella turcica for the pituitary gland optic canals, superior orbital fissures foramen rotundum, foramen ovale, foramen spinosum Occipital squamous and basal parts, hypoglossal canals, foramen magnum occipital condyles, occipital protuberance, superior nuchal line Temporal squamous part: zygomatic process, mandibular fossa petromastoid part: middle and inner ear, internal auditory meatus, facial canal, air cells in the mastoid process tympanic part: external auditory meatus, attachment of tympanic membrane styloid process: attachment for ligaments and muscles, adjacent to stylomastoid foramen, where CN VII emerges Parietal contains foramina for emissary veins that connect scalp veins with venous dural sinuses inside the skull THE CRANIAL CAVITY 4. Read Bones of the Cranium & Face 5. Read Cranial Cavity – Anterior & Middle fossae 6. Read Cranial Cavity Posterior Fossa & Foramen Magnum 7. Read Cranial nerves & special senses The floor of the cranial cavity is a three-tiered structure, with the lowest tier posteriorly and the highest anteriorly. Each tier is said to form the floor of a cranial fossa. Anterior Cranial Fossa The cribriform plate of the ethmoid lies in the middle of the floor, with olfactory filaments passing through its foramina. There is a midline projection, the crista galli, to which the falx cerebri attaches. Elsewhere, the anterior fossa is occupied by the frontal lobes of the brain. 3 Middle Cranial Fossa The central portion is formed by the body of the sphenoid, which has a deep depression for the pituitary gland, the pituitary fossa. The pituitary fossa and clinoid processes together form the sella turcica. Laterally, the middle cranial fossa houses the temporal lobes of the brain. A crescent of foramina is described in the floor. Most lateral is the foramen spinosum for the middle meningeal vessels. Anteromedially lies the foramen ovale. Between the tip of the petrous temporal bone and sella turcica is the cartilage-filled foramen lacerum. On each side where the anteromedial walls meet the floor is the foramen rotundum and superior to this, lying between the greater and lesser wings of the sphenoid, is the superior orbital fissure. Posterior Cranial Fossa The posterior cranial fossa houses the cerebellum and much of the brainstem. In its floor is the foramen magnum, and anterior to the occipital condyles lie the hypoglossal canals. Between each petrous temporal and occipital bone is the jugular foramen, and on the posterior wall of the petrous temporal bone is the internal acoustic meatus. FIG.1.1 The Cranial Fossae, Foramina and Cranial Nerves Key: ACF, MCF, PCF = anterior, middle and posterior cranial fossae; G = trigeminal ganglion 1 = frontal crest; 2 = cribriform plate; 3 = optic canal; 4 = superior orbital fissure 5 = foramen rotundum; 6 = foramen ovale; 7 = foramen spinosum; 8 = foramen lacerum 9 = internal auditory meatus; 10 = jugular foramen; 11 = hypoglossal canal; 12 = foramen magnum 4 THE MENINGES 8. Read Dura Mater and its Vessels 9. Read Cranial Vault These are three layers of connective tissue membranes that invest the spinal cord and brain. Dura Mater and Dural Folds This is the outermost layer and consists of dense connective tissue. The cranial dura mater is divided into an outer endosteal layer, which is continuous with the pericranium through the cranial sutures and foramina, and an inner meningeal layer. These two layers are united except where they separate to enclose the venous sinuses. Only one layer surrounds the spinal cord. The meningeal layer is reflected to form four septa or dural folds. The two main folds are the falx cerebri between the two cerebral hemispheres, and the tentorium cerebelli, separating the cerebellum from the posterior parts of the cerebral hemispheres. The falx cerebelli lies between the cerebellar hemispheres. In addition, a small horizontal shelf of dura mater, the diaphragma sellae, covers the pituitary fossa in the sella turcica of the sphenoid bone. Pia and Arachnoid Mater The pia mater is a delicate, highly vascular layer which closely covers the brain and spinal cord. The arachnoid mater is a thin non-vascular layer between the dura and pia. The cranial pia is thinner than that of the spinal cord, and also differs in being only loosely attached to the surface of the brain. The spinal cord has a ventral median fissure into which the pia follows; similarly, the brain has numerous grooves and sulci which is invested by pia. The cranial arachnoid also differs from that of the spinal cord. Cranial arachnoid is thicker than spinal arachnoid, and is connected to cranial pia by numerous fibrous strands that cross the fluid-filled subarachnoid space. As the arachnoid does not line the grooves on the surface of the brain or spinal cord, the sulci of the cerebral hemispheres are filled with cerebrospinal fluid (CSF). There are larger accumulations of CSF at sites where cranial arachnoid bridges across larger grooves in the brain surface, the subarachnoid cisterns. Meningeal Spaces The spinal epidural space lies between the dura and vertebral periosteum, containing loose connective tissue, venous plexuses and lymphatics. The cranial epidural (extradural) space is a potential space between the endosteal layer and skull, and is only realised pathologically. The subdural space is a potential space between dura and arachnoid. The subarachnoid space contains cerebrospinal fluid, bathing the brain and spinal cord. In the adult, it extends to the termination of the lumbar cistern, at the level of the second sacral vertebra. FIG.1.2 Disposition of the Meninges (Coronal section through the superior sagittal sinus) Key: 1 = outer endosteal layer of dura mater 2 = inner meningeal layer of dura mater 3 = arachnoid mater 4 = pia mater 5 = falx cerebri (dura mater) V = dural venous sinus (superior sagittal sinus) S = subarachnoid 5 Dural Venous Sinuses 10. Read Dural Venous Sinuses At certain sites, the two layers of dura mater separate to enclose the dural venous sinuses, which are lined by vascular endothelium, with no valves or muscular tissue in their walls. They form a complex network of venous channels which drain blood from the brain and cranium. The superior sagittal sinus begins at the crista galli and runs in the superior margin of the falx cerebri, forming a median groove in the cranial vault. It usually drains into the right transverse sinus. The inferior sagittal sinus runs in the posteroinferior border of the falx cerebri. It joins the great cerebral vein and the right and left basal veins to form the straight sinus. The straight sinus runs in the junction of the falx cerebri and tentorium cerebelli, continuing as the left transverse sinus. The transverse sinus runs on each side in the lateral margin of the tentorium cerebelli. It receives the superior petrosal sinus and continues into the sigmoid sinus. The sigmoid sinus is an S-shaped sinus that deeply grooves the petrous temporal bone. It curves forward onto the occipital bone and passes through the jugular foramen to join the inferior petrosal sinus and form the internal jugular vein. The mastoid air cells lie lateral to it. The superior petrosal sinus runs in the edge of the tentorium cerebelli, where it attaches to the petrous temporal bone and thus connects the cavernous sinus with the transverse sinus. The inferior petrosal sinus lies in the groove between the occipital bone and petrous temporal bone. It connects and drains the cavernous sinus into the internal jugular vein. The cavernous sinus lies on either side of the body of the sphenoid. Numerous delicate, interlacing strands of connective tissue create a cavern-like meshwork in one blood-filled space. It receives the superior and inferior ophthalmic veins, thus forming extracranial to intracranial venous connections via the facial vein and the pterygoid plexus. The internal carotid artery runs through the sinus with the abducent nerve (VI). The oculomotor (III), trochlear (IV), ophthalmic (Va) and maxillary (Vb) nerves lie in its lateral wall. FIG.1.3 The Dural Venous Sinuses Key: 1 = superior sagittal sinus; 2 = inferior sagittal sinus; 3 = transverse sinus 4 = straight sinus; 5 = inferior petrosal sinus; 6 = sigmoid sinus 7 = superior petrosal sinus 8 = cavernous sinus 6 THE SPINAL CORD 11. Read Structure of the Spinal Cord 12. Read Spinal Cord and Its Blood Supply 13. Read Vertebral Canal 14. Read Position of nerve roots The spinal cord begins at the foramen magnum and ends at the lower border of L1, tapering to end as the conus medullaris, which is attached by a stalk of pia mater, the filum terminale, to the end of the dural sac at S2. The cauda equina consists of the roots of the spinal nerves from L2 to Co which pass caudal to the conus to exit at their respective intervertebral foramina. The spinal nerve roots emerge from the ventral and dorsal aspects of the cord as the anterior motor and posterior sensory roots. Each spinal root is covered by three meningeal layers. At each level the two roots join to form a mixed spinal nerve in the intervertebral foramen. Where the spinal roots unite, the meninges fuse with the epineurium of the spinal nerve. FIG.1.4 The Distal End of the Spinal Cord (Posterior view with dura mater opened) Key: 1 = dorsal root ganglion 2 = denticulate ligament 3 = spinal nerve 4 = conus medullaris 5 = cauda equina 6 = filum terminale BLOOD SUPPLY OF THE HEAD AND NECK The Vertebral Artery (Intracranial course) 15. Read Arteries of the brain 16. Read Internal Carotid Artery 17. Read External Carotid Artery 18. View Angiogram of arteries of the head (1) & (2) The vertebral artery travels up the neck to enter the cranial cavity through the foramen magnum. It gives a small posterior meningeal branch, the anterior and posterior spinal arteries, branches to the medulla, and the posterior inferior cerebellar artery. On the ventral surface of the brainstem, the two vertebral arteries merge to form the single basilar artery in the midline groove on the ventral surface of the pons. The basilar artery gives branches to the brainstem, the anterior inferior cerebellar and superior cerebellar arteries. More anteriorly, it divides into the posterior cerebral arteries that supply the occipital lobes of the cerebrum. 7 The Internal Carotid Artery (Intracranial course) The internal carotid artery enters the carotid foramen at the base of the skull. It then makes a right- angled turn to travel anteromedially towards the apex of the petrous temporal bone, where it emerges superior to the cartilage filling the foramen lacerum. It then makes another right-angled turn upwards into the cavernous sinus, and runs anteriorly within it, grooving the body of the sphenoid. It then turns upwards, pierces the dural roof of the cavernous sinus and gives off the ophthalmic artery. Now lying within the subarachnoid space, it turns backwards lateral to the optic chiasma. On reaching the lateral edge of the posterior clinoid process, it turns superiorly towards the brain, giving the anterior and middle cerebral arteries. FIG.1.5 Course of the Internal Carotid Artery Key: T = petrous part of temporal bone; S = body of sphenoid ICA = internal carotid artery 1 = carotid canal; 2 = foramen lacerum 3 = cavernous sinus (grey); 4 = anterior clinoid process 5 = abducent nerve (lateral to ICA) 6 = ophthalmic artery The Circle of Willis This is an anastomotic connection between the vertebral and internal carotid arteries at the base of the brain in the region of the optic chiasma and pituitary stalk. The anterior cerebral arteries are connected by the anterior communicating arteries, and each posterior cerebral artery is joined to the corresponding internal carotid artery by a posterior communicating artery. This allows blood from the vessels on one side of the body to reach those on the other side, or for blood from the vertebral distribution to reach the carotid distribution when necessary. No veins run alongside the intracranial parts of either the vertebral or internal carotid arteries. Instead, veins from the brain follow independent courses to the dural sinuses. FIG.1.6 The Circle of Willis Key: The arteries: 1 = anterior cerebral; 2 = anterior communicating 3 = internal carotid; 4 = middle cerebral 5 = posterior communicating; 6 = posterior cerebral 7 = superior cerebellar; 8 = pontine 9 = labyrinthine; 10 = anterior inferior cerebellar 11 = posterior inferior cerebellar; 12 = vertebral 13 = anterior spinal; 14 = basilar 8 The External Carotid Artery This artery extends from the bifurcation of the common carotid artery at the level of the upper border of the thyroid cartilage to the neck of the mandible. Within the parotid gland, it divides into the superficial temporal and maxillary arteries. The other main branches of the external carotid artery are the superior thyroid, lingual, facial and occipital arteries. The middle meningeal artery arises from the maxillary artery and enters the skull through the foramen spinosum. It runs between dura mater and bone, dividing into two main branches. The anterior branch passes deep to the pterion, where it is liable to be torn in skull fractures. The pterion refers to the area on the lateral aspect of the skull where the sutures of the parietal, frontal, temporal bones and the greater wing of the sphenoid meet. FIG.1.7 The External Carotid Artery (Lateral view) The arteries: ICA & ECA = internal and external carotid 1 = superior thyroid; 2 = lingual; 3 = facial 4 = occipital; 5 = maxillary; 6 = superficial temporal 7 = middle meningeal, passing through FS = foramen spinosum 9 Clinical Applications 1. Skull fractures Fractures of the vault of the skull heal by fibrous tissue, which may not ossify. The fracture line may then be permanently visible on radiographs. 2. The sutures of the skull The sutures of the skull grow in response to tension within them generated by intracranial pressure. Thus, the size of the cranial vault is not controlled genetically but is a function of what is occurring inside the cranium. If the newborn is microcephalic and the brain does not grow adequately, the cranial vault stays small. If the cranial contents become excessively voluminous, as in hydrocephalus, the cranial vault responds by excessive enlargement. 3. Lumbar puncture Lumbar puncture is performed to withdraw a sample of cerebrospinal fluid for examination. As the spinal cord terminates at the lower border of L1, the needle is often inserted at the L3/4 space. The procedure may be diagnostic, e.g. in suspected cases of meningitis, or therapeutic, e.g. for spinal anaesthesia or chemotherapy. 4. Intracranial haemorrhage Intracranial haemorrhage can be life-threatening and is divided into different types. An extradural haemorrhage occurs when meningeal arteries are ruptured and bleed, lifting the periosteum away from the bone. In a subdural haemorrhage, cerebral veins draining into the dural venous sinuses may tear, sometimes after a minor head injury, especially in the elderly. In a subarachnoid haemorrhage, the main arteries to the brain which lie in the sub- arachnoid space may tear following a head injury or from an aneurysm. An intracerebral haemorrhage may be spontaneous, as in haemorrhagic strokes, or follow head injury. 5. Cavernous sinus thrombosis Infections of the face pose the very serious threat of spread to the cavernous sinus. One route is from the communication established between the facial vein and the cavernous sinus by the superior ophthalmic vein. 6. Vertebrobasilar insufficiency Also known as vertebral basilar ischaemia, this refers to a set of symptoms which occurs as a result of decreased blood flow to the posterior cerebral circulation. This will affect the medulla, cerebellum, pons, midbrain, thalamus and occipital cortex. Symptoms include vertigo, which may be brought on by head turning, resulting in occlusion of the ipsilateral vertebral artery. In addition, patients may also experience diplopia and blurred vision. 10  Unit 2 The Facial Skeleton, Scalp and Face The Orbit and Associated Cranial Nerves Intended Learning Outcomes On completing this Unit and after attending the prosections session in the Dissection Room along with the applied anatomy seminar, you should be able to: 1. Identify the bones of the facial skeleton. 2. Identify the layers of the scalp. 3. Identify the facial muscles and the branches of the facial nerve supplying them. 4. Describe the components of the bony orbit, the different parts of the eye and the flow of lacrimal fluid. 5. Identify the extraocular muscles and their nerve supply. 6. Understand the autonomic nerve supply to structures in the orbit, and the concept of hitchhiking of autonomic nerves on cranial nerves to reach their effector organs. 7. Describe the vessels of the face and the sites for potential intracranial spread of infection. 8. Understand the common clinical applications associated with injuries to the facial nerve and to structures in the orbit. 11 Summary 1. The facial skeleton comprises 6 paired bones, which are the maxillae, inferior conchae, nasal, lacrimal, palatine and zygomatic bones; and 2 unpaired bones: the mandible and vomer. 2. The scalp consists of 5 layers: Skin, Connective tissue, Aponeurosis, Loose areolar tissue, and Periosteum. 3. The muscles of facial expression are all derived from the second pharyngeal arch. They are sphincters and dilators, with the main muscles circumscribing the orbit and oral cavity. 4. The bones of the orbit form a pyramid, with the orbital rim forming the base and the optic canal at the apex. Its walls are formed by the ethmoid, frontal, lacrimal, sphenoid and zygomatic bones, and the maxilla. 5. The superior division of CN III innervates levator palpebrae superioris and superior rectus. The inferior division supplies the medial rectus, inferior rectus and inferior oblique. Superior oblique and lateral rectus are supplied by CN IV and CN VI respectively. 6. The ciliary ganglion is a parasympathetic ganglion located between the optic nerve and the lateral rectus muscle. Postganglionic axons in the short ciliary nerves innervate the sphincter pupillae and ciliary muscles for pupillary constriction and accommodation respectively. 7. The lacrimal gland consists of orbital and palpebral parts. Lacrimal fluid flows from the lacrimal gland, through lacrimal puncta and canaliculi to the lacrimal sac, which discharges into the inferior meatus of the nose via the nasolacrimal duct. 12 Self-Directed Learning 1. Read The Scalp. 2. Read Skin of the Head. 3. Read The Face. THE SCALP The SCALP consists of five layers: Skin, Connective tissue, Aponeurosis, Loose connective tissue, and Periosteum. The Aponeurosis The superficial connective tissue layer of the scalp is unique in being fibrous and tightly bound both to skin and the underlying aponeurosis, so that there is no movement between the two. Sliding of the scalp over the cranium is only possible because a very loose areolar connective tissue, the subaponeurotic fascia, is interposed between the aponeurosis and the pericranium. Specialisation of the subcutaneous tissue of the scalp has important clinical consequences. The dense fibrous subcutaneous tissue tends to hold the walls of superficial blood vessels open even when they are cut, so scalp wounds tend to bleed profusely and require suturing. If a wound penetrates the aponeurosis, a transverse tear will gape due to the pull of the occipitalis and frontalis muscles across the defect, whereas a sagittal tear is more easily sutured. Any wound that penetrates the aponeurosis is serious as infection can enter the subaponeurotic space and spread over the entire surface of the cranial vault with little obstruction. FIG.2.1 Layers of the Scalp Key: (B = bone of skull) S = skin, C = connective tissue, A = aponeurosis L = loose areolar tissue, P = pericranium The Vessels and Nerves These run between the aponeurosis and skin. The scalp is supplied by branches of the external and internal carotid arteries: superficial temporal and posterior auricular arteries (ECA); supraorbital and supratrochlear arteries (ICA). All these vessels anastomose freely with each other. Cutaneous nerves follow the blood vessels. Posteriorly, the scalp is supplied by the greater occipital and third occipital nerves, branches of the posterior rami of C2 and C3 respectively and laterally by the lesser occipital (C2) and auriculotemporal (branch of Vc) nerves. Anteriorly and up to the vertex, it is supplied by the zygomaticotemporal (branch of Vb) nerve, and the supratrochlear and supraorbital nerves (branches of Va). 13  FIG.2.2 Nerves and Arteries of the Scalp Key: Branches of internal carotid artery: 1 = supratrochlear; 2 = supraorbital Branches of external carotid artery: 3 = superficial temporal; 4 = posterior auricular; 5 = occipital Nerves: 6 = supratrochlear; 7 = supraorbital; 8 = zygomaticotemporal (Vb) 9 = auriculotemporal (Vc); 10, 11 = lesser and greater occipital (C2) THE FACIAL NERVE (CN VII) 4. Read The Facial Nerve. 5. Read Front of Scalp and Face. 6. Read Side of Scalp and Face. 7. Read Styloid and Digastric Regions. 8. Read Muscles of Facial Expression. The facial nerve is the source of branchiomotor innervation to all the muscles that are derived from the second pharyngeal arch. In addition, it carries preganglionic parasympathetic fibres destined for the lacrimal, submandibular, sublingual and nasal mucous glands, and taste fibres from the anterior two-thirds of the tongue via its chorda tympani branch. It is also partially responsible for general sensation of the skin lining the external auditory meatus. Course The facial nerve leaves the brainstem as two roots: a larger motor root and a smaller nervus intermedius containing parasympathetic and sensory fibres. The two roots enter the internal acoustic meatus with the vestibulocochlear nerve (CN VIII), and join to form the facial nerve, which enters the narrow facial canal which lies within the petrous temporal bone. On the medial wall of the middle ear, the facial nerve bends posteriorly. At the end, there is a swelling, the facial (geniculate) ganglion. On reaching the posterior wall of the tympanic cavity, the nerve runs inferiorly to emerge from the skull at the stylomastoid foramen. After exiting the skull, the facial nerve enters the parotid gland and divides into upper and lower divisions. The upper division gives the temporal, zygomatic and buccal branches, while the lower division gives the mandibular and cervical branches. 14 Branches Before emerging from the skull, the facial nerve gives the following branches: greater petrosal nerve, arising at the geniculate ganglion and carrying parasympathetic fibres to the lacrimal gland via the pterygopalatine ganglion; nerve to stapedius, a muscle which dampens over-vibration of the tympanic membrane and ossicles; and the chorda tympani, which joins the lingual nerve. It mediates taste from the anterior two thirds of the tongue, and also carries preganglionic parasympathetic secretomotor fibres to the submandibular and sublingual glands. After exiting from the stylomastoid foramen, the following branches are given off: muscular branches to the posterior belly of digastric, stylohyoid and occipitalis; branches to the muscles of facial expression, namely temporal, to orbicularis oculi and frontalis; zygomatic, to the muscles of the eyelid; buccal, to buccinator, orbicularis oris and muscles of the external nose; mandibular, to the muscles of the lower lip and chin; and cervical, to platysma. FIG.2.3 The Facial Nerve (Schematic diagram showing branches) Key: IAM = internal auditory meatus, ME = middle ear (dark grey) SMF = stylomastoid foramen, P = parotid gland (light grey) Branches in middle ear: 1 = greater petrosal nerve;2 = nerve to stapedius; 3 = chorda tympani Branches before entering parotid gland: 4 = to occipitalis; 5 = to stylohyoid; 6 = to posterior belly of digastric Branches to face: T = temporal; Z = zygomatic; B = buccal; M = mandibular; C = cervical 15 THE ORBIT AND EYE 9. Read Walls and Apertures of Orbit. 10. Read Orbital Cavity and Conjunctival Sac. 11. Read Eyelids and Orbital Septum. THE BONY ORBIT The orbit is a socket formed by the bones of the face around the optic nerve and eyeball. It is cone- shaped, with the apex pointing posteromedially. Note that the two medial walls are almost parallel to each other, while the two lateral walls are virtually at right angles. Its roof is the orbital plate of the frontal bone, which separates the frontal lobes of the brain from the orbital contents. The floor is formed almost entirely by the maxilla with its air sinus. The lateral wall is formed by the zygomatic bone and greater wing of the sphenoid, and separates the temporalis muscle from the eye. The medial wall is formed mainly by the labyrinth of the ethmoid. Anteriorly, both the lacrimal bone and the frontal process of the maxilla contribute to the orbit. Posterior to the inferomedial angle of the orbital rim is a depression, the lacrimal fossa, which houses the lacrimal sac. Beneath the fossa, the orbital floor has a large foramen for the nasolacrimal duct which opens into the nasal cavity. FIG.2.4 Bony Walls of the Right Orbit Key: 1 = optic canal; 2 = orbital plate of ethmoid bone; 3 = lacrimal bone; 4 = lacrimal fossa 5 = orbital process of palatine bone; 6 = infraorbital foramen; 7 = orbital plate of maxilla 8 = inferior orbital fissure; 9 = zygomatic bone; 10 = greater wing of sphenoid 11 = superior orbital fissure; 12 = lesser wing of sphenoid; 13 = supraorbital foramen THE MUSCLES AND NERVES OF THE EYE 12. Read Muscles of Orbit. 13. Read Cranial Nerve II and the Visual Pathway. 14. Read Cranial Nerves III, IV and VI. 15. Read The Lacrimal Gland and Apparatus. 16 Muscles The muscles of the eye can be divided into 3 groups: the intraocular ciliary muscle and pupillary dilators and constrictors, the 6 extraocular muscles which move the eyeball, and the levator palpebrae superioris, also extraocular, which elevates the upper eyelid. The muscles that move the eyeball are small skeletal muscles, with four straight muscles (the recti) and two oblique muscles. The recti arise from a tendinous ring at the back of the bony orbit, and pass forwards to attach to the eyeball just behind the junction of the cornea and sclera, i.e. just anterior to the equator of the eyeball. Three recti (superior, inferior and medial) and inferior oblique are supplied by the oculomotor nerve (CN III), superior oblique is supplied by the trochlear nerve (CN IV) and lateral rectus by the abducent nerve (CN VI). The medial rectus adducts the eyeball, and lateral rectus abducts it. Due to the different long axes of the orbit and eyeball, superior rectus elevates and adducts the eyeball, while inferior rectus depresses and adducts it. Due to their attachments posterior to the equator of the eyeball, superior oblique depresses and abducts the eyeball, and inferior oblique elevates and abducts it. Any coordinated movement of the eyeballs involves at least three muscles. Nerves The Optic Nerve (CN II) This is a purely sensory nerve, whose axons originate in the retinal cells. The two nerves partially decussate to form the optic chiasma, where the optic tracts continue posteriorly to the lateral geniculate body inferior to the thalamus. As an outgrowth of the brain, the optic nerve is surrounded by pia, arachnoid, and dura up to the point where it pierces the sclera. There is a thin but definite subarachnoid space. Thus, any increase in intracranial pressure is transmitted around the nerve via the cerebrospinal fluid, leading to the appearance of papilloedema, i.e. swelling of the optic nerve disc. The Oculomotor Nerve (CN III) After exiting the midbrain, the oculomotor nerve pierces the roof of the cavernous sinus and runs in its lateral wall. Emerging from the sinus, it divides into superior and inferior divisions, which pass into the orbit through the superior orbital fissure and the tendinous ring. The superior division supplies levator palpebrae superioris and superior rectus. The inferior division supplies the inferior rectus, medial rectus and inferior oblique muscles, and also carries parasympathetic axons for the ciliary muscle and sphincter pupillae. Preganglionic parasympathetic fibres leave the inferior division to reach the ciliary ganglion, which lies between the lateral surface of the optic nerve and the lateral rectus muscle. Postsynaptic postganglionic axons leave the ganglion via 10-12 short ciliary nerves. These run forwards in the eyeball to reach the ciliary and sphincter pupillae muscles. The Trochlear Nerve (CN IV) After exiting the brain and entering the cavernous sinus, the trochlear nerve runs forwards to enter the orbit through the superior orbital fissure, turning medially above levator palpebrae superioris to supply superior oblique. The Abducent Nerve (CN VI) The abducent nerve pierces the dura on the posterior aspect of the clivus and travels superolaterally to enter the cavernous sinus, lying just lateral to the internal carotid artery. It leaves the sinus to pass through the superior orbital fissure, supplying lateral rectus. 17  FIG.2.5 Nerves and Muscles of the Eye Key: SOF = superior orbital fissure; OC = optic canal; SR, IR, MR, LR = superior, inferior, medial and lateral recti. Note the origin of the four recti from the common tendinous ring. SO, IO = superior and inferior obliques; LPS = levator palpebrae superioris 1 = CN II; 2 = central artery of the retina; 3 = ophthalmic artery; 4 = superior ophthalmic vein 5 = lacrimal n.; 6 = frontal n.; 7 = CN IV; 8 = superior division CN III; 9 = nasociliary nerve 10 = CN VI; 11 = inferior division CN III; 12 = inferior ophthalmic vein. The Ophthalmic Nerve (CN Va) The ophthalmic nerve passes into the cavernous sinus, where it picks up postganglionic sympathetic fibres from the internal carotid plexus. The sympathetic axons are distributed with its branches to supply the vessels of the orbit, vessels and sweat glands of the forehead, and dilator pupillae. Its three main branches are the frontal, lacrimal, and nasociliary nerves. BLOOD SUPPLY OF THE EYE The ophthalmic artery is the first major branch of the internal carotid artery. After entering the orbit, it gives the central artery of the retina and branches to the extraocular muscles. It ascends on the lateral surface of the optic nerve and gives the lacrimal artery. This artery has a recurrent meningeal branch that anastomoses with the anterior branch of the middle meningeal artery, creating an anastomosis between the internal and external carotid arteries. The superior ophthalmic vein leaves the orbit through the superior orbital fissure to enter the cavernous sinus. The inferior ophthalmic vein may join the superior vein, pass separately through the superior orbital fissure to join the cavernous sinus, or pass through the inferior orbital fissure to join with the pterygoid venous plexus in the infratemporal fossa. 18 THE LACRIMAL APPARATUS The Eyelids The eyelids are two movable folds which protect the eye. When they are open, the angles of junction of the upper and lower lids are known as the medial and lateral canthi. The opening between the eyelids is the palpebral fissure. The tarsal plates are laminae of condensed connective tissue in each lid. The superior tarsal plate receives the main insertion of levator palpebrae superioris. Modified sebaceous glands, (tarsal or Meibomian glands) open onto the skin posterior to the eyelashes, producing an oily secretion to make the lid margins waterproof and reduce evaporation of tears on the cornea. The Lacrimal Gland At the root of each eyelid, the palpebral conjunctiva turns into the bulbar conjunctiva by reflecting onto the anterior aspect of the eyeball. Closing the lids creates a conjunctival sac, with the lines of reflection known as the superior and inferior conjunctival fornices. The lacrimal gland occupies a depression in the superolateral angle of the orbit. The ducts (12-14) open into the lateral part of the superior conjunctival fornix. Two structures bulge into the conjunctival sac medially: the lacrimal caruncle, a small elevation and the plica semilunaris, a crescenteric fold. They protrude into the lacrimal lake, where tears are swept on blinking. From the lake, tears enter the lacrimal canaliculi, which open into the lacrimal sac in a groove formed by the lacrimal bone and frontal process of the maxilla. The nasolacrimal duct emerges from the lower end to open into the inferior meatus of the nose. This opening is guarded by a fold of mucous membrane which prevents air from being forced up the duct into the sac when blowing one’s nose. FIG.2.6 The Lacrimal Apparatus Key: 1 = lacrimal gland 2 = upper and lower lacrimal canaliculi 3 = lacrimal sac 4 = nasolacrimal duct 19 Clinical Applications 1. Facial skin Facial skin varies in thickness. It is especially delicate around the eyes, where there is little subcutaneous fat and many muscles inserting into the skin. There is no deep fascia. As the face is involved in almost continuous movements, there are many skin creases. With age, these become pronounced as the skin loses its elasticity, with crow’s feet around the eyes. 2. Bell’s palsy Bell’s palsy is an inflammatory disease of unknown aetiology, which affects the facial nerve in the facial canal, just above the stylomastoid foramen. In this condition, all the facial muscles on one side are paralysed. 3. Facial nerve injury The facial nerve is at risk in parotid surgery, as its branches pass through the gland. It can also be injured in operations in the submandibular region, as its mandibular branch dips below the mandible before passing up to supply the lower lip muscles. As babies do not have a mastoid process at birth, the facial nerve is at risk of injury in a forceps delivery. 4. Paralysis of levator palpebrae superioris (LPS) This causes ptosis, i.e. drooping of the upper eyelid. Complete ptosis indicates a lesion of the oculomotor nerve, whereas partial ptosis may indicate a disruption to the sympathetic supply only and thus part of LPS, as in Horner’s syndrome. To keep the eye open normally, both the sympathetic supply to the smooth muscle fibres and the CN III supply to the striated muscle of LPS must be fully intact. 5. Damage to the oculomotor nerve In addition to complete ptosis, oculomotor nerve damage will also result in paralysis of most of the extraocular muscles, with only the lateral rectus and superior oblique intact. The eyeball is immobile, with a downwards and outwards gaze at rest. As the two eyes do not point in the same direction, the patient will experience double vision (diplopia). When parasympathetic supply to the constrictor pupillae is disrupted, the pupil is dilated and does not constrict when light is shone into the eye or when the eye focuses on a close object (i.e. lack of accommodation), as the ciliary muscle is also paralysed. 6. Trochlear nerve lesions In isolated lesions of the trochlear nerve, which are uncommon, disability is limited to paralysis of superior oblique. When the eyeball is maximally adducted, the line of pull of the superior oblique will coincide with the visual axis of the eyeball. The superior oblique is therefore tested in this position as it will depress the eye due to its insertion posterior to the equator of the eyeball. Hence the patient is asked to look inwards and then downwards. The patient will also complain of diplopia during activities that involve looking down, e.g. reading and walking downstairs. 7. Abducent nerve lesions This is the most frequently damaged among the nerves that innervate extraocular muscles. It is the first nerve to be affected by raised intracranial pressure. An aneurysm of the internal carotid artery within the cavernous sinus or tumours at the base of the brain may also compress it. Abducent nerve injury results in lateral rectus palsy. The eyeball assumes an adducted position at rest, due to the unopposed pull of medial rectus. 20  Unit 3 The Nose and Paranasal Sinuses The External and Middle Ear Intended Learning Outcomes On completing this Unit and after attending the prosections session in the Dissection Room along with the applied anatomy seminar, you should be able to: 1. Identify the bony margins of the nasal cavity and the cartilages which contribute to it. 2. Identify the features on the lateral wall of the nasal cavity. 3. Know the location of the paranasal sinuses, their drainage and relations to surrounding structures. 4. Identify the features of the temporal bone and the structures housed within it. 5. Identify the features of the external ear, and describe the path of sound transmission. 6. Know the main features of the middle ear, its contents and relations to surrounding structures. 7. Be familiar with the appearance of the tympanic membrane on otoscopy under normal circumstances and understand how this may change in pathological conditions. 8. Know the common clinical applications associated with the nose, paranasal sinuses, external and middle. 21 Summary 1. The skeleton of the external nose is partly bony and partly cartilaginous, with the nasal bones meeting in the midline. 2. The nasal cavity extends from the external nares (the nostrils) to the internal nares (the posterior nasal apertures or choanae) which open into the nasopharynx. 3. The nasal cavity is separated from the anterior cranial fossa by the cribriform plate of the ethmoid, through which olfactory nerves pass to reach the olfactory bulb. 4. The nasal septum divides the nasal cavity into two halves. Each half comprises olfactory, vestibular and respiratory parts with different types of mucosa present. 5. Projecting into the nasal cavity from the lateral wall are the superior, middle and inferior conchae. A meatus lies below the corresponding concha. 6. The paranasal sinuses open into the lateral wall of the nasal cavity. They all drain into the middle meatus, except the sphenoidal sinus, which drains into the sphenoethmoidal recess, and the posterior ethmoidal air cells which drain into the superior meatus. 7. The external ear consists of the auricle (pinna) and the external auditory meatus. The pinna consists of elastic cartilage covered with skin, with numerous ridges and depressions. The external auditory canal is formed partly of cartilage and partly of bone. 8. The tympanic membrane separates the external ear and middle ear, which lies within the temporal bone. It communicates with the nasopharynx via the auditory (Eustachian) tube and contains the auditory ossicles: the malleus, incus and stapes. 22 Self-Directed Learning 1. Read Bones of the Nose. 2. Read Cranial Nerve I and Olfactory Pathway. 3. Read Nose and Nasal Cavity. 4. Read Nose and Paranasal Sinuses. 5. Watch Rhinoscopy. THE EXTERNAL NOSE The external nose is composed partly of bone and partly of hyaline cartilage, with the nasal bones meeting in the midline superiorly. There is an opening below the nasal bones between the two maxillae, which meet to form its lower boundary. The nasal cartilages are attached to the edge of the opening. The lateral boundary of the external nasal aperture (naris) is formed by fibrofatty tissue posteriorly and the major alar cartilage anteriorly. In the midline, the alar cartilages turn inwards to form the medial boundary of the aperture below the nasal septum. The hairy skin of the external nose extends into the nasal cavity for a short distance and is known as the vestibule of the nose. THE NASAL CAVITY The nasal cavity is divided into left and right halves by the nasal septum, which forms its medial walls. The septum is formed by the perpendicular plate of the ethmoid superiorly, by the vomer posteriorly and by cartilage anteriorly. The roof is arched, and is formed anteriorly by the nasal bones, in the middle by the cribriform plate of the ethmoid, and posteriorly by the inferior surface of the body of the sphenoid. Olfactory nerves pass through the cribriform plate to reach the olfactory bulb. The floor consists of the palatine processes of the maxillae anteriorly, and the horizontal plates of the palatine bones posteriorly. The lateral wall is formed mainly by the maxilla and three conchae. The superior and middle conchae are part of the ethmoid, while the inferior concha is a separate bone. The conchae are large, medially-directed bony elements covered by highly vascular mucous membranes. Air is heated and humidified as it is drawn past the conchae during breathing. Beneath each shelf-like concha is a meatus into which the paranasal sinuses open. 23  FIG.3.1 Lateral Wall of the Nasal Cavity Key: Parts of the conchae have been removed to show the openings. F = frontal sinus; S = sphenoidal sinus E = Eustachian (auditory) tube Openings of sinuses: 1 = sphenoidal (sphenoethmoidal recess) 2 = posterior ethmoidal (superior meatus) 3 = maxillary; 4 = frontal and anterior ethmoidal air cells 5 = middle ethmoidal air cells on bulla ethmoidalis The nasolacrimal duct opens into the inferior meatus (6). Blood Supply The nasal cavity receives blood from three sources. The predominant supply is from a branch of the maxillary artery, the sphenopalatine artery. The other arteries that contribute are the anterior ethmoidal branch of the ophthalmic artery and the labial branch of the facial artery. Venous blood leaves the region in veins that course alongside the arteries, and drain into the facial, ophthalmic, and sphenopalatine veins. The latter form part of the pterygoid plexus of veins in the infratemporal fossa. Mucous Membrane The olfactory part, which is responsible for smell, is in the roof and adjacent parts of the medial and lateral walls, and is covered by columnar epithelium. The rest is respiratory epithelium, except for the vestibule, which is lined with stratified squamous epithelium. Nerve Supply Olfactory nerves come from cell bodies in the olfactory mucous membrane, and pass through openings in the cribriform plate of the ethmoid bone. General sensation is mediated by branches of the ophthalmic and maxillary divisions of the trigeminal nerve which accompany the vascular supply. Parasympathetic secretomotor supply to the mucous glands is mediated by postganglionic fibres from the pterygopalatine ganglion. The preganglionic fibres are carried by the greater petrosal nerve, a branch of the facial nerve. 24 THE PARANASAL SINUSES These are a series of membrane-lined cavities within the facial bones. Each sinus is lined with ciliated mucous columnar epithelium. The ciliary currents are directed spirally towards the openings of the sinuses. Each sinus is named by the bone in which it lies. The Frontal Sinus This lies above the orbit and can be quite extensive. It communicates with the nasal cavity by an opening in the anterior end of the middle meatus. The Maxillary Sinus This is large and occupies most of the maxilla. It lies lateral to the nasal cavity, above the upper molar teeth. Its floor extends to a lower level than the floor of the nasal cavity, so that its opening lies above the floor. It drains into the middle meatus. The Ethmoidal Sinuses These are small, interconnected air cells which lie between the orbit and the nose, and are divided into three cavities: anterior, middle, and posterior. The lateral wall of the nose has a prominent bulge, the bulla of the ethmoid, which contains the middle ethmoidal air cells. Beneath the bulla is a curved opening, the hiatus semilunaris, which contains the openings of the frontal sinus, anterior ethmoidal air cells and maxillary sinus. The posterior ethmoidal air cells open into the superior meatus. The Sphenoidal Sinus This opens into the nasal cavity in a small triangular space just above the superior concha known as the sphenoethmoidal recess. Important relations include the pituitary gland superiorly and the cavernous sinus laterally. The paranasal sinuses are thought to act as resonance chambers during speech, but may also function to lighten the bones of the facial skeleton. FIG.3.2 The Paranasal Sinuses (Coronal section) Key: O = orbit; B = olfactory bulb Sinuses: F = frontal; A, P = anterior and posterior ethmoidal; M = maxillary Conchae: 1 = superior, 2 = middle, and 3 = inferior conchae Note the openings of the sinuses, using FIG.3.1 for reference. 25 THE EAR 6. Read External Ear and Tympanic Membrane. 7. Read Walls and Contents of Middle Ear. 8. Read Site of Tympanic Cavity and Inner Ear. 9. Read Temporal Bone. 10. Read Temporal Bone: Interior and Ossicles. The ear can be subdivided into the external ear, middle ear, and internal ear. The External Ear This consists of the auricle, also known as the pinna, and the external auditory meatus, which extends to the tympanic membrane, dividing the external ear from the middle ear. The auricle This consists of elastic cartilage covered with skin, with ridges and depressions. Anterior to the meatal opening is the tragus, with the concha posteriorly. The outer edge is the helix and there is an inferior projection, the lobule, which is formed by fibrofatty tissue. FIG.3.3 Features of the Auricle Key: 1 = helix; 2 = antihelix; 3 = crura of antihelix; 4 = crus of helix 5 = tragus; 6 = external auditory meatus; 7 = antitragus; 8 = lobule The external auditory meatus This extends from the concha of the auricle to the tympanic membrane and is about 3cm long. The canal is directed medially and slightly anteriorly. The lateral third is formed of cartilage and is continuous with the auricle. It is deficient posterosuperiorly, where it is completed by fibrous tissue. The medial two-thirds is bony, with a groove around the sides and floor for the attachment of the tympanic membrane. The external auditory meatus is lined by skin with hairs and ceruminous glands which secrete wax. 26 The Middle Ear The middle ear, also known as the tympanic cavity, is essentially a hollowed-out cave in the petrous part of the temporal bone. Boundaries The roof of the tympanic cavity is formed by a plate of bone, which separates the middle ear from the middle cranial fossa, the meninges, and the temporal lobe of the brain. Its floor is a thin plate of bone separating the cavity from the jugular fossa, which houses the commencement of the internal jugular vein, and the carotid canal. The lateral wall is formed by the tympanic membrane, which is inserted into the groove in the bony portion of the external auditory meatus. It is placed obliquely downwards and medially, and bulges into the middle ear so that its lateral surface is concave. The handle of the malleus is attached to its medial surface. Its upper part is thin and loose. FIG.3.4 The Right Tympanic Membrane (Otoscopic appearance) Key: 1 = annulus; 2 = pars flaccida; 3 = lateral process of malleus; 4 = handle of malleus 5 = umbo (most depressed part); 6 = cone of light The pars tensa of the tympanic membrane is shaded in grey. The medial wall consists of the oval window, which leads to the internal ear. There is also a bulge, the promontory, which is produced by the first turn of the cochlea. The facial nerve runs posteriorly and then inferiorly in the bony facial canal. The ossicles The tympanic cavity contains the three auditory ossicles, the malleus, incus, and stapes, which articulate with each other to transmit vibrations of the membrane to the internal ear. The malleus has a lateral process and handle which are attached to the tympanic membrane, and a rounded superior head which articulates with the body of the incus. The incus articulates via its long process with the stapes. The base (footplate) of the stapes sits in the oval window (fenestra vestibuli). Movement of the base will cause vibration of the fluid in the internal ear, stimulating its auditory receptors. Two small muscles, the tensor tympani, supplied by the mandibular nerve, and the stapedius, supplied by the facial nerve, are attached to the malleus and stapes respectively and act to modify the transmission of sound waves. 27 Connections The auditory (Eustachian) tube opens off the anterior wall of the tympanic cavity and connects it with the nasopharynx. It has a medial cartilaginous part and a lateral bony part. Its role is to allow air to enter or escape from the middle ear in order to equalise pressure between the tympanic cavity and the external environment. Posteriorly, the tympanic cavity communicates via the aditus with a hollow air-filled cavity, the mastoid antrum. It is small at birth but enlarges as the mastoid process enlarges. It is separated by a thin plate of bone from the posterior cranial fossa with the cerebellum and sigmoid sinus. Superiorly, it is related to the temporal lobe of the brain. Blood and nerve supply The middle ear is supplied by branches of the internal carotid and maxillary arteries. It is innervated by the glossopharyngeal nerve through the tympanic plexus. FIG.3.5 Contents of the Middle Ear (Lateral wall displaced) Key: T = tympanic membrane; E = Eustachian tube; M = malleus; I = incus 1= aditus to mastoid antrum; 2 = chorda tympani; 3 = stylomastoid foramen; 4 = stapedius muscle (VII) 5 = round window; 6 = tympanic branch of IX; 7 = promontory (with tympanic plexus) 8 = lesser petrosal nerve; 9 = tendon of tensor tympani; 10 = footplate of stapes on oval window 11 = facial nerve in bony canal; 12 = prominence of lateral semicircular canal Note that VII has already given off the greater petrosal nerve at the geniculate ganglion. You may find this 3D model of the contents of the middle ear and inner ear, developed by the University of Dundee from MRI data, useful in appreciating the 3D relations of the contents within this confined space in the temporal bone. 28 The Internal Ear This contains the sense organs for hearing and balance. It consists of the osseous labyrinth (with the vestibule, semicircular canals, and cochlea), which contains the membranous labyrinth. The utricle and saccule are located in the vestibule and are responsible for static equilibrium, while the semicircular ducts in the canals are concerned with dynamic equilibrium. The cochlear duct is the organ of hearing. They are all supplied by the vestibulocochlear nerve (CN VIII). Hearing, balance and associated pathways are covered in detail in the NHB Course. 29 Clinical Applications 1. Cerebrospinal fluid (CSF) rhinorrhoea This refers to drainage of CSF through the nose, which may be the result of a fracture of the skull base, most likely the anterior cranial fossa with involvement of the cribriform plate. The most serious complication is a purulent meningitis, which is potentially fatal. The patient may also have anosmia (loss of the sense of smell) due to tearing of the fine olfactory nerve filaments which pass through the cribriform plate. 2. Little’s area (also known as Kiesselbach Plexus) This is an area on the anterior aspect of the nasal septum that is richly vascularised. Inflammation of this area is a common cause of epistaxis (nosebleeds). 3. Sinusitis Sinusitis refers to inflammation of the paranasal sinuses. Sinus infection can cause headache or a feeling of pressure in the eyes, nose, cheeks, or on one side of the head. Patients may also present with a cough, fever, halitosis, and nasal congestion with thick secretions. Maxillary sinusitis can cause pain or pressure in the maxillary area. Patients may present with toothache. The opening of this sinus is high on its medial wall, making drainage difficult and infection more likely. 4. Transphenoidal surgery In transphenoidal hypophysectomy, an endoscope is inserted through the nose and the sphenoidal air sinus. This enables removal of pituitary gland tumours that are either confined to the sella turcica or extend only to the suprasellar area. It is now the procedure of choice as it has significant advantages over open craniotomy. 5. Middle ear infection The auditory tube provides a pathway for infection to pass from the nasopharynx to the middle ear. Middle ear infection, or otitis media, is a common childhood condition. In severe cases, the tympanic membrane may rupture. Before the advent of antibiotics, untreated middle ear infection could lead to a mastoid abscess. This can have serious consequences, as the mastoid air cells are only separated from the cerebellum and temporal lobe of the brain by thin plates of bone, leading to meningitis and cerebral abscesses. 6. Hearing loss As hearing requires a long chain of energy conduction before neuronal stimulation, hearing loss can be a result of either defects or disturbances in conduction (conduction deafness), or defects or disturbances in the sensory organ or neuronal pathways (sensorineural deafness). 7. Acoustic neuroma An acoustic neuroma is the name commonly used for a vestibular schwannoma, which is a benign intracranial tumour of the myelin-forming cells of the vestibulocochlear nerve. Patients present with sensorineural hearing loss (deafness) and disturbances of balance and gait. Surgery may be performed through a translabyrinthine approach which is safe but destroys hearing in the affected ear. The facial nerve is also at risk. 30  Unit 4 The Infratemporal Fossa and Oral Cavity The Mandible and Temporomandibular Joint Intended Learning Outcomes On completing this Unit and after attending the prosections session in the Dissection Room along with the applied anatomy seminar, you should be able to: 1. Identify the boundaries of the infratemporal fossa. 2. Define the oral cavity, its boundaries and contents. 3. Identify the features of the mandible. the temporomandibular joint and its movements. 4. Describe the muscles of mastication, their attachments and functions. 5. Know the parts of the tongue, its extrinsic and intrinsic muscles, nerve and blood supply. 6. Describe the parotid gland, the pathway of its duct, nerve and blood supply. 7. Describe the submandibular and sublingual glands, how their secretions drain into the oral cavity, their nerve and blood supply. 8. Know the common clinical applications associated with the structures of the oral cavity, salivary glands, mandible and temporomandibular joint. 31 Summary 1. The infratemporal fossa lies between the pharynx and the ramus of the mandible. It is bounded anteriorly by the maxilla, and posteriorly by the styloid process. It contains the pterygoid muscles, branches of the mandibular nerve and maxillary artery. 2. The oral cavity consists of the vestibule and the oral cavity proper. The vestibule is the cleft between the lips and cheeks externally and the gums and teeth internally. The oral cavity proper is bounded by the alveolar arches, teeth and gums, palate and tongue. It communicates posteriorly with the oropharynx. 3. The temporomandibular joint is a synovial joint between the head of the mandible and the inferior surface of the temporal bone. Its intra-articular disc creates a double joint cavity. 4. The muscles of mastication are temporalis, masseter, and the medial and lateral pterygoids. They are all supplied by the mandibular branch (Vc) of the trigeminal nerve. 5. The extrinsic muscles of the tongue are attached to bone, and alter its position. The intrinsic muscles lie within the substance of the tongue to change its shape. They are all supplied by the hypoglossal nerve (CN XII), except palatoglossus, which is supplied by the pharyngeal plexus. 6. The parotid gland is the largest salivary gland, lying inferior to the external acoustic meatus, between the mastoid process posteriorly and the mandible anteriorly. Its duct opens in the mouth on a papilla opposite the second upper molar tooth. 7. The submandibular gland lies under cover of the body of the mandible and has superficial and deep lobes. Its duct opens on a papilla beside the lingual frenulum. The sublingual glands open via numerous ducts in the floor of the mouth. 32 Self-Directed Learning THE TRIGEMINAL NERVE 1. Read the Trigeminal nerve. Intracranial Course The trigeminal nerve contains both somatic sensory and motor fibres. It leaves the brainstem in two separate bundles: the large sensory root containing all the sensory axons, and the small, medial motor root containing all the motor axons. After leaving the brain, the two roots pass forwards below the tentorium cerebelli through an oval opening in the dura mater, on the apex of the petrous temporal bone, at the trigeminal impression. Divisions The sensory cell bodies are located as a mass along the sensory root, known as the trigeminal ganglion. From its anterior edge, its three divisions are given off: the ophthalmic (Va), maxillary (Vb), and mandibular (Vc) nerves. The motor root joins the mandibular division. The ophthalmic division enters the cavernous sinus to travel to the superior orbital fissure. The maxillary division passes forwards to the foramen rotundum. The mandibular division, which contains both sensory and motor fibres, exits the cranial cavity through the foramen ovale. FIG.4.1 Sensory Distribution of Trigeminal Nerve Key: Va = ophthalmic division of V Vb = maxillary division of V Vc = mandibular division of V 1 = supraorbital nerve 2 = infraorbital nerve 3 = mental nerve 4 = auriculotemporal nerve Branches From the ophthalmic nerve: frontal gives supraorbital and supratrochlear nerves supplies the skin of the forehead and anterior half of the scalp lacrimal supplies the skin of the upper eyelid carries parasympathetic fibres from pterygopalatine ganglion to lacrimal gland nasociliary supplies ethmoidal and sphenoidal sinuses, and the root of the nose gives long ciliary nerves carrying sensory fibres from the cornea and sympathetic fibres to dilator pupillae From the maxillary nerve: zygomatic gives zygomaticotemporal and zygomaticofacial nerves supplies the skin of the lateral part of the forehead and over the cheek ganglionic gives branches to the pterygopalatine ganglion infraorbital supplies the maxillary teeth 33 From the mandibular nerve: The following are its motor branches and the muscles they supply: nerve to medial pterygoid medial pterygoid, tensor tympani, tensor palate deep temporal temporalis lateral pterygoid lateral pterygoid nerve to masseter masseter The following are sensory branches and their areas of distribution: (long) buccal to mucuous membrane and skin of the cheek auriculotemporal to temporomandibular joint, skin of the upper half of the ear and external auditory meatus, scalp above the meatus inferior alveolar* to mandibular teeth as mental nerve to skin of lower lip lingual joined by chorda tympani from CN VII To anterior two-thirds of tongue, floor of mouth sensory branches to submandibular ganglion *the inferior alveolar nerve also contains motor fibres which are given off in the nerve to mylohyoid. This supplies both mylohoid and the anterior belly of digastric. It is also important to note that branches of the trigeminal nerve play very important roles in conveying postganglionic parasympathetic fibres from autonomic ganglia to their respective effector organs. The Temporal and Infratemporal Fossae 2. Read Infratemporal Region. 3. Read Pterygopalatine Fossa. The space occupied by the temporalis muscle is the temporal fossa. The space deep to the superior half of the mandibular ramus is the infratemporal fossa. Its boundaries are as follows: Roof greater wing of the sphenoid Medial wall lateral pterygoid plate and maxilla Anterior wall posterior maxilla and inferior orbital fissure Posterior wall carotid sheath The infratemporal fossa contains: Muscles superior and inferior heads of lateral pterygoid (horizontal fibres) superficial and deep heads of medial pterygoid (vertical fibres) buccinator, pierced by the duct of the parotid gland Nerves Vc and its branches, including the lingual nerve anteriorly, which is joined by the chorda tympani and the inferior alveolar nerve Vessels maxillary artery and its branches, and the pterygoid venous plexus. The otic ganglion lies in the fossa immediately below the foramen ovale. It receives preganglionic parasympathetic fibres from the lesser petrosal nerve, a branch of CN IX which arises from the tympanic plexus. Postganglionic fibres are conveyed to the parotid gland on the auriculotemporal nerve. The Pterygopalatine Fossa This is a small pyramidal-shaped space that lies between the back of the orbit, the maxilla, and the sphenoid. Its position means that it is ideally placed to connect the eyes, nose, mouth and face as well as the infratemporal fossa and brain. The main structures that run through the fossa are the maxillary nerve, which enters the fossa via the foramen rotundum, and the terminal branches of the maxillary artery. The pterygopalatine ganglion and its branches lie within it. 34 Preganglionic parasympathetic fibres leave the facial (geniculate) ganglion in the middle ear as the greater petrosal nerve, which is joined by the deep petrosal nerve (carrying sympathetic fibres from the internal carotid plexus) in the foramen lacerum. They form the nerve of the pterygoid canal which joins the pterygopalatine ganglion. After synapsing in the ganglion, the postganglionic parasympathetic fibres are distributed to the lacrimal gland (via the zygomaticotemporal nerve of Vb to the orbit and then the lacrimal branch of the ophthalmic nerve Va) and all the glands of the nose, palate and paranasal sinuses. This ganglion is therefore known as the ganglion of hay fever. FIG.4.2 The Pterygopalatine Fossa Key: S = greater wing of sphenoid; L = foramen lacerum; M = maxillary sinus Anterior wall = posterior surface of maxilla; Posterior wall = pterygoid process of sphenoid (P) Medial wall (grey) = perpendicular plate of palatine bone 1 = Vb through foramen rotundum 2 = nerve of the pterygoid canal (carries autonomic fibres to pterygopalatine ganglion) The arrow indicates the inferior orbital fissure (IOF) leading to the orbit. THE ORAL CAVITY 4. Read Oral Cavity and Palate. 5. Read Tongue and Sublingual Region. 6. Read Suprahyoid Muscles. 7. Read Muscles of the Tongue. 8. Read Salivary Glands and Ducts. The lateral walls of the oral cavity are formed by buccinator on each side, which is continuous with the superior constrictor at the pterygomandibular raphé. This runs between the medial pterygoid plate of the sphenoid and the mandible posterior to the mylohyoid ridge. Two muscles lie medial to the raphé and dominate the posterior border of the oral cavity. These are the palatoglossus and palatopharyngeus, which connect the tongue and pharynx to the palate and are covered by mucous membrane. As the palatine tonsils lie between these two muscles, they are sometimes known as the tonsillar pillars. The Tongue The tongue occupies the floor of the oral cavity. It is divided into an anterior two-thirds (the oral part) and a posterior third (the pharyngeal part) by a prominent sulcus terminalis. The oral part has a velvety appearance, being covered by small projections called papillae. Most of these are pointed filiform papillae, with numerous, mushroom-shaped fungiform papillae interspersed among them. The fungiform papillae contain taste buds at their bases. The pharyngeal part lies posterior to a row of very large circumvallate papillae which are located anterior to the sulcus terminalis. They also contain taste buds. The posterior third of the tongue is nodular from underlying masses of lymphoid tissue, the lingual tonsils. 35 Muscles These can be divided into intrinsic and extrinsic muscles. The intrinsic muscles are arranged as longitudinal, transverse, and vertical fibres. They change the shape of the tongue. The extrinsic muscles move the position of the tongue. Genioglossus attaches to the anterior part of the mandible, and pulls the tongue forwards (protrusion). Hyoglossus attaches to the hyoid bone, and pulls the tongue down and posteriorly (retraction and depression). Styloglossus attaches to the styloid process, and retracts and elevates the tongue. Palatoglossus attaches to the palate, and has a stronger effect in depressing the palate than its effect on the tongue. Nerve supply All the muscles are supplied by the hypoglossal nerve (CN XII), except palatoglossus, which is supplied by the pharyngeal plexus. The anterior two-thirds is supplied by the lingual nerve for general sensation and the chorda tympani for taste. The posterior third is supplied by the glossopharyngeal nerve. A small part above the epiglottis is supplied by the internal laryngeal branch of the vagus nerve. FIG.4.3 Muscles and Nerves of the Tongue (Lateral view; mandibular ramus removed) Key: T = tongue; M = mandible S = styloid process; H = hyoid 1 = sublingual gland; 2 = submandibular duct 3 = genioglossus; 4 = hyoglossus 5 = geniohyoid; 6 = lingual artery 7 = hypoglossal nerve; 8 = stylohyoid 9 = glossopharyngeal nerve; 10 = styloglossus 11 = lingual nerve, joined by 12 = chorda tympani The Floor of the Mouth The mylohyoid muscles are attached to the inner surfaces of the mandible and the hyoid bone, but are more extensively attached to each other at a midline raphé. Together they form a muscular sling which marks the floor of the oral cavity. Each has a free posterior border. They are supplied by the nerve to mylohyoid, a branch of the inferior alveolar nerve. The Salivary Glands The sublingual glands lie on the superior surface of mylohyoid and along the inner surface of the mandible. They open into the sublingual region via several small, short ducts. The submandibular gland lies in the submandibular fossa under cover of the body of the mandible. Its superficial lobe is separated from the parotid gland by the stylomandibular ligament, and is grooved by the facial artery. The deep lobe passes with its duct around the posterior free border of mylohyoid. It passes upwards and forwards to open on the papilla beside the lingual frenulum. Parasympathetic secretomotor supply to these two glands comes from the facial nerve via its chorda tympani branch and the submandibular ganglion. The parotid gland is the largest salivary gland and lies mainly in the retromandibular region. It also extends onto the lateral surface of the mandibular ramus and masseter. Its duct crosses masseter and pierces buccinator to enter the mouth opposite the upper second molar tooth. Parasympathetic secretomotor supply comes from the glossopharyngeal nerve (CN IX). Its tympanic branch gives the lesser petrosal nerve, which synapses in the otic ganglion. Postganglionic branches join the auriculotemporal nerve to supply the gland. 36  FIG.4.4 The Parotid Gland (Transverse section) Key: P = parotid gland; M = mastoid process; R = ramus of mandible; F = facial nerve 1 = pharyngeal wall; 2 = external carotid artery; 3 = retromandibular vein; 4 = medial pterygoid 5 = masseter; 6 = sternocleidomastoid; 7 = posterior belly of digastric; 8 = styloid process 9 = internal carotid artery; 10 = internal jugular vein; 11 = vagus nerve in 12 = carotid sheath. The Palate The palate forms the roof of the oral cavity. The hard palate is formed by the palatine process of the maxilla and the horizontal plate of the palatine bone. It is covered with mucous membrane and is continuous with the soft palate posteriorly. The soft palate contains five muscles which can change its shape. These are palatoglossus, palatopharyngeus, tensor palati, levator palati, and musculus uvulae. The palatoglossus and palatopharyngeus function mainly to raise the tongue and oropharynx to approximate the palate, i.e. to control the opening of the oropharynx. Tensor palati attaches to the medial pterygoid plate and courses around its hamulus to attach to the sides of the soft palate. It tenses the soft palate and is innervated by the mandibular nerve (Vc). Levator palati attaches to the superior surface of the soft palate, and elevates it to close the nasopharynx. The musculus uvulae stiffens the uvula. THE TEMPOROMANDIBULAR JOINT 9. Read The Mandible. 10. Read Temporomandibular Joint: Bony Surfaces. 11. Read Temporomandibular Joint: Soft Tissues. 12. Read Muscles of Mastication. 13. Review Muscles of Facial Expression. Each temporomandibular joint (TMJ) is a synovial condyloid joint between the head of the mandible, and the articular tubercle on the inferior surface of the temporal bone. An intra-articular fibrocartilaginous disc divides the joint into upper and lower cavities. It is also atypical in that its articular surfaces are covered by fibrocartilage instead of hyaline cartilage. Few other joints are moved as often as the TMJ. In addition to the movements of eating or chewing, which can create great force within the joint, speaking and swallowing also require TMJ motion that is fully controlled but requires little force. The TMJ exhibits a combination of complexity, close-to- continuous use and a capacity for force and finesse. 37  FIG.4.5 The Temporomandibular Joint (Lateral view) Key: D = intra-articular disc E = external auditory meatus M = mastoid process S = styloid process H = head of the mandible L = two heads of lateral pterygoid The Muscles of Mastication Masseter arises from the zygomatic process of the maxilla and the zygomatic arch to insert into the lateral aspect of ramus and angle of the mandible. It elevates the mandible. Temporalis arises from the inferior temporal line and temporal fossa to insert into the coronoid process and anterior border of the mandibular ramus. It primarily elevates the mandible but its posterior fibres can also retract. FIG.4.6 Muscles of Mastication Key: 1 = temporalis; 2 = zygomatic arch; 3 = articular tubercle; 4 = external auditory meatus 5 = mandibular condyle; 6 = angle of mandible; 7 = masseter; 8 = superior head of lateral pterygoid 9 = inferior head of lateral pterygoid; 10 = medial pterygoid; 11 = superior constrictor of pharynx 12 = pterygomandibular raphé; 13 = buccinator Medial pterygoid arises from the pterygoid fossa and medial surface of the lateral pterygoid plate of the sphenoid to insert into the medial surface of the mandibular ramus. It elevates the mandible and produces lateral movement as with chewing with the mouth closed. Lateral pterygoid has two heads. Its superior head arises from the infratemporal fossa of the sphenoid, and the inferior head arises from the lateral surface of the lateral pterygoid plate. The superior fibres attach to the articular capsule and disc of the TMJ, while its inferior fibres are directed posteriorly to the pterygoid fovea or pit on the neck of the mandible. It is the only muscle of mastication that opens the mouth by protruding and depressing the mandible. Its superior fibres also assist in stabilising the joint. Movements The mandible may be depressed, elevated, protruded and retracted. It can also be moved from side to side as in grinding movements. Movement at the TMJ has two components: 1. Gliding occurs in the upper compartment. The mandibular condyle and the intra-articular fibrocartilaginous disc move together over the articular surface. 2. Hinge movement occurs in the lower compartment, where the condyle of the mandible articulates with the inferior surface of the fibrocartilaginous disc. 38 However, these components are never entirely independent. Both are equally involved in depression and elevation, which occur around an axis passing through the lingulae. Protrusion, retraction and grinding movements mainly involve the gliding component. In depression of the mandible, the condyles are pulled forwards by the lateral pterygoid and the body of the mandible is pulled downwards by the digastric and infrahyoid muscles. Passive depression is assisted by gravity. Elevation is produced by masseter, temporalis, and medial pterygoid. Protrusion is produced by both pterygoids, while retraction is performed by the posterior fibres of temporalis. In grinding movements, there is alternate protrusion and retraction on both sides. In chewing, the mandible is moved from side to side, and is also elevated by masseter and temporalis to keep the mouth closed. 39 Clinical Applications 1. Trigeminal neuralgia Trigeminal neuralgia is a neuropathic disorder characterised by episodes of severe facial pain, in the region of distribution of the trigeminal nerve. The pain may last from a few seconds or minutes to hours. Patients may describe trigger areas on the face which, when touched, may result in an episode. Eating, talking or brushing the teeth may also trigger an attack. Anticonvulsant medication may be used to alleviate pain. 2. Fractures of the mandible The mandible is the second most common facial bone to be fractured, with the first being the nasal bone. As the mandible forms part of a bony ring with the temporomandibular joints and base of the skull, the force of trauma may be transmitted round the ring to cause fractures which may be removed from the point of impact. Multiple fractures are more frequently seen than fracture at a single site. If the force is strong enough to cause a mandibular fracture, associated severe cervical spine injuries may also be sustained. The airway must be secured as the highest priority. 3. Dislocation of the temporomandibular joint The temporomandibular joint may dislocate anteriorly when the mouth is opened wide as in an excessive yawn or during a boxing match. Under a light anaesthetic, the mandible is pressed downwards and backwards to click it back into place. 4. Hypoglossal nerve palsy Paralysis of the hypoglossal nerve may be caused by tumours, penetrating injuries and various neurological conditions. The tongue may show wasting and fasciculations, which are spontaneous quivering movements caused by firing of muscle motor units. Unopposed action of genioglossus on the unaffected side will lead to deviation of the tongue towards the side of the lesion. 5. Conditions of the parotid gland A specific viral infection of the parotid glands produces mumps. This may spread to the other salivary glands and also to the brain, pancreas, and testes. In adult males, orchitis may result in sterility. The combined MMR vaccine provides effective immunity against mumps, measles and rubella. Tumours of the parotid gland may be benign or malignant. The most common benign tumour is a pleomorphic adenoma which is often asymptomatic, only noted incidentally while washing or shaving. Malignant parotid tumours are usually highly invasive and may involve the facial nerve, causing unilateral paralysis. 6. Conditions of the submandibular gland The concentration of mucus is higher in the submandibular gland, which accounts for the viscous nature of its secretions compared to the other salivary glands. This increased viscosity, and subsequent slower salivary flow, contributes to the development of salivary gland calculi (sialolithiasis) and stasis in certain disease states. When the submandibular duct becomes blocked by a calculus, the gland swells up and becomes painful when saliva is produced. Transoral extraction of a calculus impacted at the orifice of the duct may be performed under local anaesthesia in the outpatient clinic. 40  Unit 5 The Pharynx, Larynx and Trachea Intended Learning Outcomes On completing this Unit and after attending the prosections session in the Dissection Room along with the applied anatomy seminar, you should be able to: 1. Describe the parts of the pharynx and their main features. 2. Understand the lymphatic tissue associated with the pharynx. 3. Describe the main cartilages of the larynx and the muscles which move them, and understand the roles of these muscles in protecting the airway and in phonation. 4. Describe the spaces and membranes of the larynx. 5. Describe the sensory and motor innervation of the pharynx and larynx. 6. Describe the appearances of the larynx on endoscopy. 7. Understand the basic mechanisms involved in speech and swallowing. 8. Understand the clinical applications associated with the pharynx and larynx. 41 Summary 1. The pharynx is a fibromuscular tube which extends from the base of the skull to the lower border of the cricoid cartilage, where it becomes the oesophagus. The lymphatic tissue of the pharynx (the pharyngeal, lingual, tubal, and palatine tonsils) forms the so-called Waldeyer’s ring at the shared entrance to the digestive and respiratory tracts. 2. The three parts of the pharynx are open anteriorly to the nasal cavity (nasopharynx), oral cavity (oropharynx), and larynx (laryngopharynx). The outer muscular walls of the pharynx comprise three circularly disposed muscles, the superior, middle and inferior constrictors. 3. Innervation of the pharynx is by the pharyngeal plexus. The glossopharyngeal nerve (CN IX) mediates sensation and the vagus nerve (CN X) provides motor supply, with fibres from the cranial accessory (CN XI). There is also a rich sympathetic plexus. 4. The larynx is made up of cartilage, ligaments, muscles, and mucous membrane. It guards the entrance to the lower respiratory passages and houses the vocal cords. 5. The boundaries formed by the vestibular and vocal folds divide the laryngeal cavity into three parts: the vestibule, ventricle, and infraglottic cavity. 6. The main cartilages of the larynx are the cricoid, thyroid, arytenoids and epiglottis. The laryngeal muscles may be considered in three groups, dependent upon their actions: muscles which close off the vestibule during swallowing; muscles which abduct or adduct the vocal cords, and muscles which affect the tension of the vocal cords. The posterior cricoarytenoid muscles are the sole abductors of the vocal cords. 7. The internal branch of the superior laryngeal nerve gives sensory supply to the mucous membrane above and down to the vocal cords, while the recurrent laryngeal nerve supplies the mucous membrane below the vocal cords. 42 Self-Directed Learning THE PHARYNX 1. Read Site of Airway. 2. Read Nasopharynx and Oropharynx. 3. Read Muscles of the Pharynx. 4. Read Tonsils and Lymphatics of the Head. The pharynx is a fibromuscular tube which extends from the base of the skull to the lower border of the cricoid cartilage. It has three parts which lie posterior to the nasal cavity (nasopharynx), oral cavity (oropharynx), and larynx (laryngopharynx). Muscles The outer layer comprises three circularly disposed muscles. The superior constrictor arises from the pterygomandibular raphé between the pterygoid hamulus and the mandible. The middle constrictor arises from the stylohyoid ligament, and the greater and lesser horns of the hyoid bone. The inferior constrictor arises from the thyroid and cricoid cartilages. The three constrictors interdigitate posteriorly at the midline pharyngeal raphé, except for the cricoid fibres of the inferior constrictor (cricopharyngeus) which are sphincteric. The inner layer comprises three small longitudinally oriented muscles: the stylopharyngeus, palatopharyngeus, and salpingopharyngeus. They arise from the styloid process, palatine apo- neurosis, and the medial end of the cartilaginous auditory tube respectively. They pull the larynx and pharynx upwards in swallowing. FIG.5.1 The Pharynx (Sagittal view) Key: 1 = nasal cavity; 2 = nasopharynx; 3 = auditory tube opening; 4 = oral cavity 5 = oropharynx; 6 = epiglottis; 7 = laryngopharynx 43  FIG.5.2 The Pharynx (Posterior view) Key: 1 = styloid process; 2, 3, 4 = superior, middle and inferior constrictors 5 = cricopharyngeus; 6 = median raphé; 7 = hyoid bone; 8 = stylopharyngeus Nerve Supply The pharyngeal muscles are supplied by the pharyngeal branch of the vagus nerve in the pharyngeal plexus. Stylopharyngeus is supplied by a branch of the glossopharyngeal nerve. The accessory nerve is said to contribute to the plexus, as its cranial root gives motor fibres to the vagus nerve. The inferior constrictor receives some additional nerve fibres travelling in the external laryngeal and recurrent laryngeal branches of the vagus. Sensory innervation in the nasopharynx, oropharynx and laryngopharynx is mediated by branches of the maxillary, glossopharyngeal and vagus nerves respectively. These nerves also convey parasympathetic preganglionic fibres for the pharyngeal glands. Lymphatic Tissue Waldeyer’s ring consists of four groups of specialised lymphatic tissue at the entrance of the digestive and respiratory tracts. These are the: pharyngeal tonsil (“adenoids”) tubal tonsils at the medial end of the Eustachian or auditory tube palatine tonsils (“the tonsils”) between the palatoglossal and palatopharyngeal arches in the oropharynx; and lingual tonsil on the posterior surface of the tongue. THE LARYNX 5. Read Laryngopharynx and Laryngeal Inlet. 6. Read Larynx: Structural Framework. 7. Read Laryngeal wall and cavity. 8. Read Larynx: Intrinsic Muscles. The larynx is made up of cartilages, ligaments, and muscles, and is lined by mucous membrane. It guards the entrance to the lower respiratory tract and houses the vocal cords. 44 The Cartilages The epiglottis This is a leaf-shaped cartilage lying deep to the body of the hyoid. The stem of the “leaf” is directed inferiorly, and the tip rises approximately 1cm above the body of the hyoid at the back of the tongue. It is composed of yellow elastic cartilage which never calcifies. The mucous membrane is reflected from the anterior surface of the epiglottis to the back of the tongue in three longitudinal ridges: the glosso-epiglottic folds. The depressions on either side of the median fold are termed valleculae. Inferior to each lateral glosso-epiglottic fold are depressions known as the piriform fossae. The arytenoid cartilages Each arytenoid cartilage resembles a three-sided pyramid, with the base inferior and the apex superior. The anterior-most point on the base is elongated to form the vocal process, to which the vocal ligament attaches. The muscular process is a lateral projection for the attachment of the posterior and lateral cricoarytenoid muscles. The base of each cartilage articulates with a sloping shoulder on the upper border of the cricoid lamina. This is a synovial joint that allows both rotary and gliding movements. The cricoid cartilage The cricoid cartilage is shaped like a signet ring, in that the posterior part, the lamina, is much taller than the remaining part, the arch. It is the only complete cartilaginous ring in the entire respiratory tract. On the external surface of the arch are facets for articulation with the inferior horns of the thyroid cartilage. There are facets on the superior rim of the cricoid cartilage for articulation with the arytenoid cartilages. The thyroid cartilage The thyroid cartilage lies below the hyoid bone and consists of two flat pentagonal plates known as the laminae, which are fused anteriorly. The posterior borders of the laminae project upwards and downwards as the superior and inferior horns (cornua). The point of junction of the two laminae anteriorly constitutes the laryngeal prominence (“Adam’s apple”). The superior border is connected to the hyoid via the thyrohyoid membrane. FIG.5.3 The Laryngeal Cartilages (Sagittal view) Key: 1 = hyoid bone; 2 = epiglottis 3 = thyroid cartilage; 4 = vestibular fold 5 = vocal fold; 6 = cricothyroid membrane 7 = cricoid cartilage; 8 = arytenoid cartilage 9 = quadrangular membrane; 10 = aryepiglottic fold 45 The Membranes The cartilages of the larynx are connected by several membranes. The thyrohyoid membrane This membrane passes from the upper border of the thyroid cartilage to the upper border of the posterior surface of the hyoid bone. In the midline, it is thickened as the median thyrohyoid ligament. At the posterior edges are the lateral thyrohyoid ligaments, which extend from the superior horn of the thyroid cartilage to the tip of the greater horn of the hyoid bone. It is pierced by the superior laryngeal vessels and the internal laryngeal branch of the superior laryngeal nerve, a branch of the vagus nerve. The quadrangular membrane This broad fibroelastic membrane extends between the epiglottis and arytenoid cartilages. Its upper margin forms the aryepiglottic fold, while its lower margin is the vestibular fold, also known as the false cord. The cricothyroid membrane The cricothyroid membrane, also known as the conus elasticus or cricovocal membrane, is a highly elastic membrane whose fibres run forward from the superior rim of the cricoid arch and vocal processes of the arytenoids to the inner surface of the thyroid angle. The most anterior fibres are thickened to form the median cricothyroid ligament. The superior edge of the membrane forms the vocal ligaments. Together with their overlying epithelium they form the vocal folds. The Interior The laryngeal inlet is the communication of the larynx with the pharynx. Its boundaries are the epiglottis anteriorly, the arytenoid cartilages posteriorly, and the aryepiglottic folds laterally. The cavity is divided by the vocal folds into supraglottic and infraglottic parts. The two vocal folds form the glottis, with the space in between being known as the rima glottidis. The supraglottic part consists of the vestibule, which extends from the laryngeal opening to the vestibular folds. A small portion of the mucous membrane anteriorly within the ventricle herniates between the vestibular and vocal folds through an opening into the saccule of the larynx, which contains mucous glands for lubrication of the vocal folds. The infraglottic part is narrow at the vocal folds, and widens towards the cricoid cartilage. It is continuous with the trachea inferiorly. FIG.5.4 The Interior of the Larynx (Coronal view) Key: 1 = aryepiglottic fold; 2 = vestibule 3 = vestibular fold; 4 = ventricle 5 = vocal fold; 6 = infraglottic cavity 7 = rima glottidis 46 The Muscles The intrinsic muscles of the larynx can be divided into three different groups depending on their actions and effects. Opening and closing the laryngeal inlet The inlet of the larynx is closed by aryepiglotticus, which runs in the aryepiglottic folds and is an extension of the oblique arytenoids. Its action is opposed by thyroepiglotticus. Abduction and adduction of the vocal folds Abduction (opening) and adduction (closing) of the vocal folds are produced by movements at the cricoarytenoid joints. The posterior cricoarytenoid, which arises from the posterior surface of the cricoid lamina and inserts on the muscular process of the ipsilateral arytenoid, is the sole abductor of the vocal cords. The lateral arytenoids and interarytenoids act as adductors. Changing tension in the vocal folds The cricothyroid muscle causes the thyroid cartilage to move forwards or the cricoid cartilage upwards and backwards around