HBF-III LEC 25 Gross Anatomy Neck Triangles Notes 2025 PDF

Organization & Neck Triangles Page 1 of 21 Dr. Shunbin Xu Session Learning Objectives By the end of this session, students should be able to accurately: 1. Use anatomical terminology to describe the basic organization and functions of the...

Organization & Neck Triangles Page 1 of 21 Dr. Shunbin Xu Session Learning Objectives By the end of this session, students should be able to accurately: 1. Use anatomical terminology to describe the basic organization and functions of the neck. A. Describe the skeletal framework of the neck region and how it relates to the functions. B. List of fascial layers and how they define the fascial compartment and fascial spaces. C. Understand the clinical significance of the fascial spaces. 2. Describe the structures in the superficial fascia. A. List of the structures in the superficial fascia. B. Describe the anatomical relationships of the structures in the superficial fascia and their relationships to other fascial compartments. C. Understand the functions of the structures in the superficial fascia in relation to their clinical significance. 3. Describe the boundaries and contents of the posterior and anterior triangles of the neck. A. Describe the boundaries of posterior and anterior triangles and subtriangles and their relationships. B. Describe the origin and insertion of all muscles in the neck region, their innervation and functions in correlation to the skeletal framework. C. Describe the major arteries and veins and their branches and the areas which they supply in the neck region. Understand their anatomical relationships in correlation with other neural and muscular structures in the neck region. D. Describe major nerves and their branches in the neck region and their anatomical relationship in correlation to vascular and muscular structures and functions. Organization & Neck Triangles Page 2 of 21 Dr. Shunbin Xu Session Outline I. Introduction of the neck A. Borders B. Functions C. Skeletal Framework D. Fascial Compartments/Spaces II. Structures in the superficial fascia A. Platysma muscle B. Superficial veins III. Subdivisions of the neck region Posterior triangle A. Boundaries B. Muscles forming the boundaries C. Nerves - Cutaneous nerves of the cervical plexus and the accessory nerve Anterior triangle A. Subtriangles and Boundaries B. Relevant landmarks C. Muscular triangle D. Submental and submandibular triangles E. Carotid triangle IV. Lymphatic drainage of the neck Supplemental Reading Gray’s Anatomy for Students (4th Ed) https://www.clinicalkey.com/#!/content/book/3-s2.0-B9780323393041000087?scrollTo=%23hl0008943 Organization & Neck Triangles Page 3 of 21 Dr. Shunbin Xu I. Introduction of the neck The neck is a region of transition between the head and the thorax (Fig.1). A. Borders Superior border (Fig.1): Fig.1 (Gray’ Fig.8.4) Anterior: the inferior margin of the mandible; Posterior: bone features on the posterior aspect of the skull (mastoid process of the temporal bone; the superior nuchal line of the occipital bone). The posterior neck is higher than the anterior neck to connect cervical viscera with the posterior openings of the nasal and oral cavities. Inferior border (Fig.1): Anterior: extends from the top of the sternum, along the clavicle, and onto the adjacent acromion. Posterior: less well defined, but can be approximated by a line between the acromion and the spinous process of vertebra CVII, which is prominent and easily palpable. B. Functions of the neck – a passage between head and the rest of the body - Positioning and supporting the head - Passage for major vessels that supply the head - Passage for the nerves that innervate the organs within the thorax and abdomen and the extremities - Passage connecting the oral cavity to the upper part of digestive tract, esophagus – pharynx - Passage connecting the nasal cavity to the upper part of the respiratory tract, the trachea – larynx. C. Skeleton Framework Fig. 2 (Netter’s Plate 15) 1) 7 cervical vertebrae form the bony framework of the neck (Fig.2, 3): C1: atlas; C2: axis; C3-C7. Organization & Neck Triangles Page 4 of 21 Dr. Shunbin Xu Unique features of the cervical vertebrae to fulfill the functions of the neck: 1) Small body (except C1, which does not have a body)(Fig.3); 2) Bifid spinous processes (except C1, which does not have a spinous process)(Fig.3B-E) 3) Transverse processes with a foramen (foramen transversarium), which form a longitudinal passage for vertebral arteries and veins (Fig.3 & 4). 4) C1 (atlas) (Fig.3A-C) does not have a body and bifid spinous process. It forms unique articulation with C2 (Fig.3E-G) and the occipital bone of the skull (Fig. 4 & 5). Fig.3B Fig.3A (Netter’s Plate 21) Fig.3C Anterior Fig.3D (Netter’s Plate 19) Fig.3E Fig.3G Fig.3F Organization & Neck Triangles Page 5 of 21 Dr. Shunbin Xu Fig.4 (Netter’s plate 22) Fig.5A (Netter’s plate 23) Fig.5B (Netter’s plate 23) Organization & Neck Triangles Page 6 of 21 Dr. Shunbin Xu 2) Hyoid bone (Figs.2,6): At the interface between three very dynamic compartments: - Superiorly: the floor of the oral cavity; - Inferiorly: the larynx; - Posteriorly: the pharynx. Fig.6 (Gray’s Fig.8.9) Does not articulate with another bone; Highly movable; Serves as a strong anchor for a number of muscles and soft tissue structures in the head and neck. D. Fascial Compartments/Spaces (Fig.7) Fig.7 (Gray’s Fig.8.5) The vertebral compartment: encloses the cervical vertebrae, spinal cord, cervical nerves and associated postural muscles. The visceral compartment: wraps around glands (thyroid and parathyroid), and parts of the respiratory and digestive tracts that pass between the head and thorax- larynx and pharynx. Two vascular compartments: one on each side, contain the major blood vessels and the vagus nerve (Cranial Nerve X or CN X). Fascia of the neck: The superficial fascia (Fig.8): Loose connective tissue. Unique feature: contains a thin sheet of Superficial facial muscle, the platysma m., which begins in the superficial fascia of the thorax, runs upward to attach to the mandible and blend with the muscles on the face. The deep cervical fascia (Fig.9-12): Wrap around and delineate different Fig. 8 (Netters Plate 25) compartments: 1) Investing layer: surrounds all Fig.9 (Gray’s Fig.8.162) structures in the neck. - The external and anterior jugular veins, and all cutaneous branches of the cervical plexus pierce the investing fascia. - it splits to enclose sternocleido- mastoid (SCM), trapezius, and infra- and supra-hyoid muscles. Organization & Neck Triangles Page 7 of 21 Dr. Shunbin Xu 2) Prevertebral layer: surrounds the vertebral column and the deep muscles associated with the back. The prevertebral fascia passing between the attachment points on the transverse processes split into two layers; the anterior layer is also called Alar fascia. 3) Pretracheal layer: encloses the viscera of the neck; the posterior Fig.10 (Netter Plate 26) part enclosing the pharynx and esophagus Fig.11 (Netter Plate26) is called buccopharyngeal fascia. 4) Carotid sheaths: very thick; surround the major neurovascular bundles in Prevertebral space the vascular compartments on either side of the neck. Fascial spaces (Figs.9- 12): 1) Pre-tracheal space: between the investing and the pretracheal layers of the deep cervical fascia; it passes between the neck Fig.12 (Gray’s Fig.8.163) and the anterior part of the superior mediastinum. 2) Retropharyngeal space: between the buccopharyngeal fascia and the prevertebral fascia, which extends from the base of the skull to the upper part of the posterior mediastinum. 3) Prevertebral space: the two layers of the prevertebral fascia where it passes between the attachment points on the transverse processes create a longitudinal fascial space containing loose connective tissue that extends from the base of the skull through the posterior mediastinum to the diaphragm. Clinical importance: these fascial spaces provide a conduit for the spread of infections from the neck to the mediastinum. Organization & Neck Triangles Page 8 of 21 Dr. Shunbin Xu II. Structures in the superficial fascia: A. Platysma muscle (Fig.13): a large, thin sheet of muscle in the superficial fascia of the neck. Inferior attachment: the superficial fascia of the upper thorax. Superior attachment: mandible (medial fibers), muscles around the mouth (lateral fibers). Innervation: CN VII (facial nerve) Function: tense the skin of the neck and move the Fig.13 (Grant’s Fig.7.5) lower lip and corners of the mouth down. Fig.14 (Gray’s Fig.8.160) B. Superficial veins (Fig.14&15): The external jugular: formed posterior to the angle of the mandible by the posterior auricular vein and the retro- mandibular vein; in the superficial fascia and deep to the platysma but superficial to the SCM throughout its course, crossing it diagonally as it descends. About 3 cm superior to the clavicle and immediately posterior to the SCM, it pierces the investing layer of cervical fascia, passes deep to the clavicle, and enters the subclavian vein. Fig.15 (Grant’s Fig.7.6) Anterior jugular vein: Variable and inconsistent; begin as small veins, come together at or just superior to the hyoid bone. Descends on either side of the midline of the neck. Inferiorly, near the medial attachment of the SCM, it pierces the investing layer of deep cervical fascia to enter the subclavian vein or join the external jugular vein immediately before the external jugular vein enters the subclavian vein. Anterior jugular veins communicate through jugular venous arch (Fig.14) Organization & Neck Triangles Page 9 of 21 Dr. Shunbin Xu III. Subdivisions of the neck regions Posterior triangle: A. Boundaries (Fig.16,17): 1) Anterior: the posterior border of the SCM; 2) Posterior: the anterior border of the trapezius m; 3) Inferior: the middle one-third of the clavicle. 4) Superficial (roof): the investing layer of the deep cervical fascia; 5) Deep boundary (floor): the muscles of the neck covered by prevertebral fascia. Fig.16 (Gray’s Fig.8.161) The posterior triangle can be further divided into the occipital and the omoclavicular or subclavian triangles. B. Muscles forming the boundaries of the posterior triangle: SCM (Fig.18): Origin (inferior attachment): upper part of anterior surface of manubrium of the sternum (sternal head) and superior surface of the medial one-third of the clavicle (clavicular head). Insertion: lateral surface of the mastoid process and lateral ½ of the superior nuchal line. Innervation: Motor: CN XI (accesory nerve); Fig.17 (Gray’s Fig.8.183) Proprioception: branches from anterior Fig.18 (Netter’s Plate 29) rami of C2 to C3 (C4). Function: Individually: tilt head toward shoulder on the same side; rotating head to turn face to opposite side; Together: draw head forward. Trapezius (Fig.18): Origin: Superior nuchal line; external occipital protuberance; ligamentum nuchae; spinous processes of vertebrae CVII to TXII Insertion: Lateral one-third of clavicle; acromion; spine of scapula Innervation: Motor—accessory nerve [CN XI]; proprioception—C3 and C4. Function: Assists in rotating the scapula during abduction of humerus above horizontal; Organization & Neck Triangles Page 10 of 21 Dr. Shunbin Xu upper fibers—elevate; middle fibers—adduct; lower fibers—depress scapula. C. Nerves in the posterior triangle: Cutaneous (superficial) branches of the cervical plexus – sensory nerves (Fig.19): The lesser occipital nerve (C2): ascends Fig.19 (Gray’s Fig.8.187) along the posterior border of the SCM, and distributes to the skin of the neck and scalp posterior to the ear. The great auricular nerve (C2 and C3): emerges from the posterior border of the SCM, and ascends across the muscle to the base of the ear, supplying the skin of the parotid region, the ear, and the mastoid area. The transverse cervical nerve (C2 and C3): passes around the midpart of the SCM, and continues horizontally across the muscle to supply the lateral and anterior parts of the neck. The supraclavicular nerves (C3 and C4): a group of cutaneous nerves; after emerging from beneath the posterior border of SCM, descend and supply the skin over the clavicle and shoulder as far inferiorly as rib II. Dissection tip: These cutaneous branches enter the superficial fascia near the midpoint of the SCM. They travel deep to the platysma and superficial to the investing fascia. Pay extra attention when dissecting them. Avoid sharp tools, e.g. scalpel and scissor; use blunt dissection. Cranial nerve XI (the accessory nerve)(Fig.19): innervates SCM and trapezius muscle; - exits the cranial cavity through the jugular foramen; - passes deep to or through and innervating the SCM; crosses the posterior triangle in an obliquely downward direction, within the investing layer of the deep cervical fascia between the SCM and trapezius muscles; continues to the deep surface of the trapezius and innervates it. Clinical correlation: The superficial location of the accessory nerve as it crosses the posterior triangle makes it susceptible to injury. Fig.20 (Grant’s Fig.7.3) Anterior Triangle: A. Boundaries (Fig.16): Posterior: the anterior border of the SCM; Superior: the inferior border of the mandible; Medial: the midline of the neck. Subtriangles of the Anterior Triangle (Fig.20,21): 1) The Submandibular Triangle: Organization & Neck Triangles Page 11 of 21 Dr. Shunbin Xu Superior: the inferior border of the Fig.21 (Netter’s Plate 27) mandible; Anterior and posterior: the anterior and posterior bellies of the digastric muscle; The superficial (roof): investing layer of the deep cervical fascia; The deep (floor): mylohyoid muscle and hyoglossus muscle (which will be introduced in Oral Cavity section). 2) The Submental Triangle: Inferior: the hyoid bone; Lateral: the anterior belly of the digastric muscle; Medial: the midline. 3) The muscular triangle: Superior: the hyoid bone; Lateral: the superior belly of the omohyoid muscle, and the anterior border of the SCM; Medial: the midline. 4) The Carotid Triangle Anteroinferior: the superior belly of the omohyoid muscle; Superior: the stylohyoid muscle and posterior belly of the digastric muscles; Posterior: the anterior border of the SCM. B. Relevant important reference landmarks: 1) Hyoid bone (See Fig.6). 2) The mandible (Fig 22 A&B): mylohyoid line; inferior mental spine (genial tubercles); the digastric fossa. (More details will be introduced in later lectures) Fig.22 (Netter’s Plate 17) A B Organization & Neck Triangles Page 12 of 21 Dr. Shunbin Xu 3) Thyroid cartilage: Important landmark: oblique line. Fig.23 (Gray’s Fig.8.207) Fig.24 (Netter’s Plate 79) 4) Temporal bone (Figs.1 and 25): The mastoid process; the styloid process Styloid process Fig.25 (Netter’s Plate 6) C. Contents of the muscular triangle Hosts 4 infrahyoid muscles, also called strap muscles, include the omohyoid, sternohyoid, thyrohyoid, and sternothyroid muscles. - Arrangement of these muscles: the shorter muscles (thyrohyoid and sternothyroid) are deeper to the longer muscles (omohyoid and sternohyoid) 1) The omohyoid muscle (Fig.26-28): Consists of two bellies and an intermediate tendon; Pass through both the posterior and anterior triangles. The inferior belly: passes through the posterior triangle Origin: the superior border of the scapula, medial to the suprascapular notch; Insertion: the intermediate tendon. The superior belly: in the anterior triangle; it divides the muscular and carotid triangles; serves as the lateral boundary of the muscular triangle and the medial boundary of the carotid triangle. Origin: the intermediate tendon; Insertion: the inferior border of the body of the hyoid bone just lateral to the attachment of the sternohyoid muscle. The intermediate tendon is attached to the clavicle, near its medial end, by a fascial sling. Organization & Neck Triangles Page 13 of 21 Dr. Shunbin Xu Function of the omohyoid Fig.26 (Netter’s Plate 27) muscle: Depress the hyoid bone. 2) The sternohyoid muscle: Origin: the posterior aspect of the sternoclavicular joint and adjacent manubrium of the sternum. Insertion: the body of the hyoid bone; medial to the attachment of omohyoid muscle. Function: depresses the hyoid bone after swallowing. 3) The thyrohyoid muscle: Deep to the superior parts of the omohyoid and sternohyoid. Origin: the oblique line on the lamina of the thyroid cartilage; Insertion: the greater horn and adjacent aspect of the body of the hyoid bone. Functions: Depresses the hyoid, when the thyroid cartilage is fixed; When the hyoid is fixed, it raises the Fig.27 (Gray’s Fig.8.164) larynx (e.g., when high notes are sung). (more detail will be introduced in later lectures on larynx) 4) Sternothyroid muscle: Beneath the sternohyoid and in continuity with the thyrohyoid. Origin: the posterior surface of the manubrium of the sternum Insertion: the oblique line on the lamina of the thyroid cartilage. Functions: draws the larynx (thyroid cartilage) downward. Innervation of infrahyoid muscles (Fig.28): All infrahyoid muscles, except thyrohyoid, are innervated by the anterior rami of C1 to C3 through the ansa cervicalis (will be introduced below). The thyrohyoid muscle is innervated by fibers from the anterior ramus of C1 that travels with the hypoglossal nerve [XII]. Organization & Neck Triangles Page 14 of 21 Dr. Shunbin Xu D. Submental and Submandible Fig.28 (Gray’s Fig.8.177) Triangles (Figs.26-30): One of the prominent features of these two triangles is the suprahyoid muscles. They form the boundaries between these two and other two subtriangles of the anterior triangle. The suprahyoid muscles run from the hyoid bone to the skull or mandible; collectively have a function to raise the hyoid, as occurs during swallowing. 1) The digastric muscle: has two bellies connected by a tendon, which attaches to the body of the hyoid bone. Origins: For the posterior belly: the mastoid notch on the medial side of the mastoid process of the temporal bone. For the anterior belly: the digastric fossa on the lower inside of the mandible. Insertion: The intermediate tendon, which is attached to Fig.29 (Netter’s Plate 58) the body of the hyoid bone. Innervation: The posterior belly: facial nerve [CN VII]; The anterior belly: the mandibular division [V3 ] of the trigeminal nerve [CN V]. Functions: When the mandible is fixed, the digastric muscle raises the hyoid bone; When the hyoid bone is fixed, the digastric muscle opens the mouth by lowering the mandible. 2) The stylohyoid muscle: Origin: the base of the styloid process Insertion: the lateral area of the body of the hyoid bone by straddling the intermediate tendon of the digastric muscle. Innervation: the facial nerve [CN VII]. Organization & Neck Triangles Page 15 of 21 Dr. Shunbin Xu Functions: pulls the hyoid bone postero-superiorly during swallowing 3) The mylohyoid muscle: Superior (or deeper) to the anterior belly of the digastric and, with its partner from the opposite side, forms the floor of the mouth. Origin: the mylohyoid line on the medial surface of the body of the mandible; Insertion: the hyoid bone and muscle on the opposite side. Fig.30 (Gray’s Fig.8.163) Innervation: inferior alveolar branch of the trigeminal nerve (V3). Function: supports and elevates the floor of the mouth and elevates the hyoid bone. 4) Geniohyoid muscle: superior (or deeper) to the mylohyoid muscle. It is not generally considered a muscle of the anterior triangle of the neck; however, it is regarded as a suprahyoid muscle. The muscles from each side are next to each other in the midline. Origin: the inferior mental spine on the inner surface of the mandible. Insertion: anterior surface of the body of the hyoid bone. Innervation: a branch from the anterior ramus of C1 carried along the hypoglossal nerve [CN XII]. Functions: When the mandible is fixed, it elevates and pulls the hyoid bone forward. When the hyoid bone is fixed, it pulls the mandible downward and inward. Other contents of the submandibular and submental triangles (Fig.31) The submandibular gland: Fig.31 (Netter’s Plate 31) a large salivary gland, “hooked” around the free posterior margin of the mylohyoid muscle. The superficial part of the gland (superficial to the mylohyoid muscle) fills most of the space in the submandibular triangle. Facial artery and vein pass deep to the platysma. The vein runs superficial; the artery is more tortuous and courses through or deep to the submandibular gland and emerges more anteriorly than the vein. E. The Carotid Triangle The most prominent structure in the carotid triangle: carotid sheath, which encloses the vascular compartment of the neck. Dissection tip: Carotid sheath is a thick fascia tissue; many important vessels and nerves run in or through it. Blunt tools are recommended during the dissection. Organization & Neck Triangles Page 16 of 21 Dr. Shunbin Xu The contents of the carotid sheath (Figs. 32&33): (1) the superior part of the common carotid artery and its divisions into internal and external carotid arteries. The bifurcation of the common carotid into internal and external carotid occurs at the level Common carotid artery Internal jugular vein between vertebra C-III and C-IV, about the superior border of the thyroid cartilage (Fig.34). (2) the internal jugular vein; (3) the vagus nerve. Spatial relationships in the Fig.32 (Netter’s Plate 32a) sheath: The internal jugular vein is posterolateral to the common or the internal Fig.33 (Netter’s Plate 32b) carotid arteries; the vagus nerve is posterior to the common or internal carotid arteries and medial to the internal jugular vein, runs between the common carotid Fig.34 Common artery Fig.8.12) (Gray’s carotid or internal carotid and the Internal jugular vein internal jugular vein (Fig.33). The internal carotid artery gives off no branches in the neck, runs posterior-medial to the external carotid artery, enters the cranial cavity through the carotid canal in the petrous part of the temporal bone (will be Fig.34 (Gray’s Fig.8.12) introduced later). The external carotid artery begins to branch immediately after the bifurcation to supplies most of tissues of the head and neck region. Eight branches: Common carotid artery (1) The superior thyroid artery: the first branch — arises from the anterior surface near or at the bifurcation and passes downward and forward to reach the superior pole of the thyroid gland. Organization & Neck Triangles Page 17 of 21 Dr. Shunbin Xu Supplies: the thyroid gland, thyrohyoid muscle, internal structures of the larynx, SCM, and cricothyroid muscles. (2) The ascending pharyngeal artery: the second and smallest branch - arises from the posterior aspect and ascends between the internal carotid artery and the pharynx; meningeal branches enter the posterior cranial cavity through the condylar canals. Supplies: Pharyngeal constrictors and stylo- pharyngeus muscle, palate, tonsil, pharyngotympanic tube, meninges in posterior cranial fossa. (3) The lingual artery arises from the anterior surface, just above the superior thyroid artery at the level of the hyoid Fig.35 (Gray’s Fig.8.171) bone, passes deep to the hypoglossal nerve [CN XII] (Fig.36), and passes between the middle constrictor and hyoglossus muscles (which will be introduced in later lectures). Supplies: Muscles of the tongue, palatine tonsil, soft palate, epiglottis, floor of mouth, sublingual gland. (4) The facial artery: the third anterior branch— arises just above the lingual artery, passes deep to the stylohyoid and posterior belly of the digastric muscles, continues deep between the submandibular gland and Facial a. mandible, and emerges over the edge of the mandible anterior to the masseter muscle, to enter the face (Fig.36). Supplies: All structures in the face from the inferior border of the mandible anterior to the masseter Lingual a. muscle to the medial corner of the eye, the soft palate, palatine tonsil, pharyngotympanic tube, and submandibular gland. Fig.36 (Gray’s Fig.8.175B) Note: in ~ 20% of cases, the lingual and facial arteries arise from a common trunk! (5) The occipital artery (Figs.35 & 37) arises from the posterior surface, near the level of origin of the facial artery, passes upward and posteriorly deep to the posterior belly of the digastric muscle, and emerges on the posterior aspect of the scalp. Supplies: SCM, meninges in posterior cranial fossa, mastoid cells, deep muscles of the back, posterior scalp Organization & Neck Triangles Page 18 of 21 Dr. Shunbin Xu (6) The posterior auricular artery (Figs. 35 & 37): a small branch arising from the posterior surface and passing upward and posteriorly. Supplies: Parotid gland and nearby Fig.37 (Netter’s Plate 137) muscles, external ear and scalp posterior to ear, middle and inner ear structures (7) The superficial temporal artery (Figs. 35 & 37): one of the terminal branches and appears as an upward continuation of the external carotid artery. (This branch won’t be seen in the dissection of the neck; will be introduced in parotid and temporal region) (8) The maxillary artery (Figs. 35 & 37): the larger of the two terminal branches. (This branch won’t be seen in the dissection of the neck; will be introduced later in the infratemporal (IT) fossa. Come back to revisit after those lectures). Internal jugular vein (Fig.38): - The main drainage of the head and neck region, collecting blood from the skull, brain, superficial face and parts of the neck. - Starts inside the skull as a dilation of the sigmoid Fig.38 (Grant’s Fig.7.9) sinus – a dural sinus, joined by another venous dural sinus – inferior petrosal sinus. It exits the skull through the jugular foramen (which will be introduced in detail in next week) and enters the carotid sheath. It runs first posterior to the internal carotid arteries, then, a more lateral position, and is lateral to the common carotid artery. Eventually it joins the subclavian vein to form right and left brachiocephalic veins. Major branches of the internal jugular vein include: facial, lingual, pharyngeal, occipital, superior thyroid and middle thyroid veins. Clinical Correlation: Central venous access – the internal jugular vein is often used as central venous access to place large-bore catheters for dialysis, parenteral nutrition, and the administration of drugs that have tendency to produce phlebitis. Puncture of Organization & Neck Triangles Page 19 of 21 Dr. Shunbin Xu the internal jugular vein carries fewer risks (than the subclavian vein); however, hematoma and damage to the carotid artery could be important complications (Gray’s anatomy for students). Jugular venous pulse –The internal jugular venous pulse is an important clinical sign to assess the venous pressure and waveform. It reflects the function of the right side of the heart. Nerves in the anterior triangle 1) Hypoglossal nerve (Cranial nerve CN XII)(Fig.39): It appears in the anterior triangle as it passes forward, hooking around the occipital artery, across the lateral surfaces of the internal and external carotid arteries and the lingual artery, and then continues deep to the posterior belly of the digastric and stylohyoid muscles. It passes over the surface of the hyoglossus muscle and disappears deep to the mylohyoid muscle. CN XII supplies the tongue, does not give off any branches in Fig.39 (Gray’s Fig.8.175) the anterior triangle of the neck. Fig.40 (Gray’s Fig.8.177) 2) Nerve to the thyrohyoid muscle (Fig.40): from spinal nerve C1; however, it travels with CN XII, before CN XII disappears deep to the mylohyoid muscle; but it is not a branch of CN XII. 3) Ansa cervicalis (Fig.40): a loop of nerves; a part of the cervical plexus; composed by a superior root, and, continuously, an inferior root. - The superior root: mainly Nerve to the composed by fibers from thyrohyoid muscle the anterior rami of the C1; travels with the CN XII for short distance. The superior root runs downward in the anterior side of the carotid sheath between common/internal carotid a. and internal jugular vein. Organization & Neck Triangles Page 20 of 21 Dr. Shunbin Xu - The inferior root: from anterior rami of C2 and C3. It passes around the lateral side of the carotid sheath and joins the superior root to form a loop. The ansa cervicalis gives out branches to innervate the other three strap muscles, omohyoid, sternohyoid, and sternothyroid muscles. 4) Sensor of blood pressure - Carotid sinus (Fig.41): located at the bifurcation of the common carotid artery and Fig.41 (Netter’s Plate 131) the beginning of the internal carotid artery, as a subtle dilation in these areas. It is innervated by a branch of the glossopharyngeal n. (CN IX). Clinical correlation: An increase in intrasinus tension, can result in a decreased systemic blood pressure, bradycardia and slowing of respiration; the opposite effects follow a decreased tension within the sinus. Deliberate carotid artery compression can induce loss of consciousness. Hypersensitive carotid sinus is a rare cause of spontaneous syncopal attacks. A hypersensitive carotid sinus is defined as a sinus that stimulation by a gentle neck massage results in asystole for more than two seconds, a bradycardia of more than 30% of control or a drop in systolic blood pressure of more than 30 mm Hg. It can be treated by unilateral or bilateral carotid sinus denervation (Trout et al. 1979 Ann Surg. 189: 575-80). 5) Sensor of blood chemistry, primarily oxygen content – Carotid body (Fig.41): also located in the area of the bifurcation and is innervated by branches from both the glossopharyngeal (CN IX) and vagus (CN X)] nerves. Clinical correlation: Chronic hypoxic conditions, such as patients living at high altitudes or those who have chronic obstructive pulmonary disease (COPD) or cyanotic heart problems, can overburden the carotid bodies and subsequently lead to hypertrophy, hyperplasia, and neoplasia of the chief cells (Baysal BE & Myers EN. Microsc Res Tech. 2002 59:25661). This condition is seen in the hyperplastic type of carotid body tumors (CBTs). Dissection tips: The branches of CN IX and CN X to the Fig.42 (Gray’s carotid sinus and carotid body are thin nerve fibers, run Fig. 8.173) within the carotid sheath, are difficult to find and dissect out. Keep these branches in mind while doing the dissection with blunt tools. Organization & Neck Triangles Page 21 of 21 Dr. Shunbin Xu 6) Other branches of the Vagus nerve: (1) The superior laryngeal nerve (Fig.42): divides into external and internal laryngeal branches; travels medial to the carotid sheath. The internal laryngeal nerve passes through the thyrohyoid membrane, supplies sensory innervation to the mucosa of larynx above the level of the vocal cord. The external laryngeal nerve runs inferiorly to give out motor branches innervating the cricothyroid muscle and part of the inferior pharyngeal constrictor muscle (will be discussed further in Pharynx and Larynx later) (2) Pharyngeal branch (Fig.42): a motor branch to the pharynx. It could be too superior to be revealed in the anterior triangle dissection. You may see this in later pharynx dissection. 7) Facial nerve [VII]: innervate two muscles associated with the anterior triangle of the neck - the posterior belly of the digastric, and the stylohyoid. Also innervates the platysma muscle that overlies the anterior triangle and part of the posterior triangle of the neck. Details of the facial nerve will be introduced in later sections. 8) Glossopharyngeal nerve [IX]: innervates the carotid body and sinus. The details of the glossopharyngeal nerve will be introduced in later sections. 9) Transverse cervical nerve: has been introduced earlier; a branch of the cervical plexus from the anterior rami of cervical nerves C2 and C3; provides cutaneous innervation to this area. IV. Lymphatic Drainage of the Neck (Fig.43): Superficial nodes around the head: Occipital nodes Mastoid nodes (retro- auricular/posterior auricular) Pre-auricular and parotid nodes Submandibular nodes Submental nodes Superficial cervical nodes: along the external jugular vein. Deep cervical nodes: form a chain along the internal jugular vein, including: jugulaodigastic node, and jugulo-omohyoid mode. Clinical correlation: Enlargement of neck lymph nodes - cervical lymphadenopathy is a common manifestation of disease processes Fig.43 (Gray’s 8.193) that occur in the head and neck. It may also be a common manifestation of diffuse diseases of the body, including lymphoma, sarcoidosis, and certain types of viral infection, e.g. glandular fever and HIV infection. Evaluation of cervical lymph nodes is extremely important in determining the nature and etiology of the primary disease process that has produced nodal enlargement. Soft, tender, and inflamed lymph nodes suggest an acute inflammatory process, which is most likely to be infective. Firm multinodular large-volume rubbery nodes often suggest a diagnosis of lymphoma. Most cervical lymph nodes are easily palpable and suitable for biopsy to establish a tissue diagnosis. Biopsy can be performed using an ultrasound for guidance and good samples of lymph nodes may be obtained (Gray’s Anatomy for Students).