The Orbit and Eye Anatomy

The Orbit and Eye:

• The layers of the eyelid, from outside to inside, consists of: skin, subcutaneous tissue, orbicularis oculi muscle, tarsus, and conjunctiva.
• The orbicularis oculi muscle is present in the eyelid it is supplied by the facial nerve and its action closes the eyelid.
• Tarsus refers to the dense palate of fibrous tissues in the eyelid.
• Conjunctiva refers to the clear membrane that lines the eyelid.
• The orbit contains: eye ball, extraocular muscles, vessels, nerves, and lacrimal gland.
• Levator palpebrae superioris refers to the extraocular muscle which elevates the upper eyelid.
• The extraocular muscles that move the eye consist of four recti and two oblique muscles.
• The ophthalmic artery supplies the orbit and branches from the internal carotid artery (ICA).
• Ophthalmic veins drain the orbit and drain into the cavernous sinus.
• The nerve supply of the orbit is:
  • Special sensory: optic nerve
  • General sensory: ophthalmic nerve
  • Motor: oculomotor, trochlear, and abducent nerves
• The lacrimal apparatus includes the lacrimal gland, lacrimal sac, and nasolacrimal duct.

The Ear:

• The external ear consists of the auricle and auditory canal.
• The external ear is separated from the middle ear by the tympanic membrane (ear drum).
• The middle ear is present inside the petrous part of the temporal bone.
• The middle ear contains three ossicles: malleus, incus, and stapes.
• The middle ear contains two muscles: tensor tympani and stapedius.
• The middle ear is connected to the nasopharynx by the Eustachian tube.
• The Eustachian tube equalizes air pressure on either side of the eardrum.
• The cochlea is responsible for hearing and the semicircular canals are responsible for equilibrium in the inner ear.
• The auditory (vestibulo-cochlear) nerve provides a special sensory nerve supply to the inner ear.

The Nose and Paranasal Air Sinuses:

• Nostrils (nares) refers to the anterior nasal opening.
• Choana refers to the posterior nasal opening.
• The nasal septum divides the nasal cavity into right and left nasal cavities.
• The lateral wall of the nasal cavity shows three elevations called turbinates (conchae).
• The roof of the nasal cavity contains the olfactory mucosa, from which the olfactory nerve arises to carry smell sensation.
• The floor of the nasal cavity is formed by the palate, which separates the nasal cavity from the oral cavity.
• The paranasal air sinuses consist of the: frontal, ethmoidal, sphenoidal, and maxillary sinuses.
• The maxillary air sinuses are pyramidal in shape and related above to the orbit and below to the alveolar process of the maxilla.
• The maxillary sinus opens into the nasal cavity, with its opening situated high up in the base of the sinus.
• Nerves related to maxillary air sinuses are: infraorbital nerve (runs in the roof of the maxillary sinus), anterior superior alveolar, middle superior alveolar & posterior superior alveolar nerves (run in the lateral wall of the maxillary sinus).

Part 2:

• Platysma origin: from fascia of the front of chest covering pectoralis major & deltoid muscle. Insertion: into lower border of the mandible, Nerve supply: Cervical branch of facial nerve, Action: Wrinkles skin of neck & depress mandible
• Cutaneous nerves present in the superficial fascia of the neck: great auricular, lesser occipital, transverse cervical, supraclavicular nerves
• Superficial veins in the superficial fascia of the neck: anterior jugular vein & external jugular vein
• Anterior jugular vein drains into external jugular vein
• External jugular vein drains into subclavian vein
• Parts of the deep fascia of the neck: general investing fascia, pretracheal fascia, carotid sheath, prevertebral fascia
• Contents of the carotid sheath (both in its upper part and lower part): common carotid artery (CCA), internal carotid artery (ICA), internal jugular vein (IJV), last 4 cranial nerves
• The artery present outside the carotid sheath is the external carotid artery (ECA).
• The artery present inside the upper part of the carotid sheath is ICA.
• The artery present inside the lower part of the carotid sheath is CCA.
• Cranial nerves in the upper part of the carotid sheath: last 4 cranial nerves which are: glossopharyngeal, vagus, accessory, and hypoglossal nerves.
• The only cranial nerve which remains in the lower part of the carotid sheath is the vagus nerve.
• Sternomastoid origin:
  • Sternal head: from manubrium of sternum
  • Clavicular head: from medial 1/3 of clavicle
• Sternomastoid insertion: into lateral surface of mastoid process & lateral 1/3 of superior nuchal line, Nerve supply: Motor from spinal accessory nerve
• If both sternomastoid muscles contract, they bend the neck forwards
• If only one sternomastoid muscle contracts, it bends the neck and turn it to the opposite side
• Supra-hyoid muscles are the: digastric, stylohyoid, mylohyoid, and geniohyoid.
• The Digastric origin
  • Anterior belly: from digastric fossa on the lower border of the mandible
  • Posterior belly: from the medial surface of the mastoid process
• The digastric both bellies are inserted into an intermediate tendon which is connected to hyoid bone by a fibrous loop
  • Anterior belly: nerve to mylohyoid
  • Posterior belly: facial nerve
• The digastric action
  • Anterior belly: depresses the mandible
  • Posterior belly: elevates the hyoid bone
• The mylohoid origin: from mylohyoid line on inner surface of body of mandible
• The mylohoid insertion: anterior fibers into median raphe & posterior fibers into hyoid bone
• The mylohoid nerve supply: Nerve to mylohyoid (branch from inferior alveolar nerve)
• The mylohoid action: Support the floor of mouth, elevates hyoid bone & depresses mandible
• Infra-hyoid muscles are: omohyoid, sternohyoid, sternothyroid, thyrohyoid.
• Muscles present in the side of the neck are: scalene muscles (anterior, middle & posterior).
• A muscle in the back of the neck is the trapezius.
• Subdivisions of the anterior triangle of the neck: digastric triangle, muscular triangle, carotid triangle, submental triangle

Part 3:

• Boundaries and contents of digastric triangle:
  • Above: Lower border of mandible
  • Anteriorly & below: Anterior belly of digastric
  • Posteriorly & below: Posterior belly of digastric
  • Contents: Submandibular gland; Facial A; Common facial V; Lingual N; Hypoglossal N; Lymph nodes
• Boundaries and contents of muscular triangle:
  • Anteriorly: Midline of the neck
  • Posteriorly & above: Superior belly of omohyoid
  • Posteriorly & below: Sternomastoid
  • Contents: Infrahyoid muscles & thyroid gland
• Boundaries and contents of carotid triangle:
  • Anteriorly & above: Posterior belly of digastric
  • Anteriorly & below: Superior belly of omohyoid
  • Posteriorly: Sternomastoid
  • Contents: Carotid sheath and its contents
• The submandibular salivary gland lies in the digastric triangle.
• It extends anteriorly to the mental foramen and posteriorly to the angle of the mandible.
• The submandibular salivary gland extends above to the mylohyoid line and below to intermediate tendon of digastric.
• The gland has two parts: superficial and deep.
• The superficial part of the submandibular salivary gland is related medially to the mylohyoid, laterally to the mandible, and below to skin and fascia.
• The deep part of the submandibular salivary gland is related above to the lingual nerve and below to the hypoglossal nerve.
• The duct of the submandibular salivary gland opens on the summit of the sublingual papilla in the floor of the mouth at the side of the frenulum of the tongue.
• The arterial supply of the submandibular salivary gland is from the facial and lingual arteries.
• The sensory nerve supply is from the lingual nerve, sympathetic supply from plexus around the facial artery, and parasympathetic supply from the facial nerve → chorda tympani nerve → relay in the submandibular ganglion.
• The sublingual salivary gland lies under the mucous membrane of the floor of the mouth, forming an elevation called the sublingual fold.
• The sublingual salivary gland is related above to the mucous membrane, below to the mylohyoid, lateral to the mandible, and medial to the tongue.
• The sublingual salivary gland has 10-20 small ducts which open on the sublingual fold.
• Arterial supply of the sublingual salivary gland is from the facial and lingual arteries.
• The nerve supply consists of the sensory (lingual nerve), sympathetic (plexus around the facial artery), and parasympathetic supply (facial nerve → chorda tympani nerve → relay in the submandibular ganglion).
• The shape of the thyroid gland is butterfly-shaped with an isthmus & two lateral lobes.
• The isthmus of the thyroid gland is in front of the 2nd, 3rd, and 4th tracheal rings.
• The lateral lobes extend above the thyroid cartilage & below to the 5th or 6th tracheal rings.
• The isthmus of the thyroid gland is anteriorly related to skin, fascia, infrahyoid muscles, and posteriorly related to the 2nd, 3rd, and 4th tracheal rings.

Part 4:

• Relations of the lateral lobes of thyroid gland:
  • Anterolateral surface: skin, fascia, infrahyoid & sternomastoid muscles
  • Posterior surface: parathyroid glands & CCA
  • Medial surface: larynx, pharynx, external laryngeal nerve above & trachea, esophagus, recurrent laryngeal nerve below
• Arterial supply of the thyroid gland: superior thyroid artery, inferior thyroid artery & thyroid ima artery
• Venous drainage of the thyroid gland: superior thyroid vein, middle thyroid vein & inferior thyroid vein
• The common carotid artery (CCA) begins on the right side as a branch from the brachiocephalic artery and on the left side as a branch from the arch of the aorta.
• The CCA enters the neck behind the sternoclavicular joint and ascends inside the carotid sheath, lying medial to the internal jugular vein (IJV) and under cover of the sternomastoid.
• The CCA ends at the level of the upper border of the thyroid cartilage, corresponding to the level of the disc between C3 and C4, by dividing into the ICA and external carotid artery (ECA).
• The ICA begins at the upper border of the thyroid cartilage as one of the two terminal branches of the CCA.
• The ICA ascends inside the carotid sheath to the base of the skull, passes through the carotid canal to enter the cranial cavity, and runs inside the cavernous sinus.
• The ICA ends by dividing into the anterior cerebral artery and middle cerebral artery.
• The ECA begins at the upper border of the thyroid cartilage as one of the two terminal branches of the CCA and ascends outside the carotid sheath to ascend in the parotid gland.
• The ECA ends by dividing into the superficial temporal artery and maxillary artery.
• Branches from the anterior aspect of the ECA are the facial, lingual, and superior thyroid arteries, and branches from the posterior aspect are the posterior auricular and occipital arteries.
• The IJV begins at the jugular foramen as a continuation of the sigmoid sinus.
• It descends inside the carotid sheath and ends by joining the subclavian vein to form the brachiocephalic vein.
• The right and left brachiocephalic veins unite to form the superior vena cava (SVC).