8.) Locate and carefully clean the stylopharyngeus mm. On the posterior surfaces of the stylopharyngeus mm., locate the glossopharyngeal nn. (CN IX).

Note: Stylopharyngeus m. is one of three internal pharyngeal muscles with longitudinally (obliquely)-oriented fibers. Portions of this muscle are visible from an external view due to its attachment site on the styloid process of the temporal bone. Unlike the other pharyngeal muscles, which are innervated by branches of vagus n. (CN X), stylopharyngeus m. is innervated by glossopharyngeal n. The glossopharyngeal n. (CN IX) also afferently (sensory) innervates most of the pharynx. The stylopharyngeus m. attaches the styloid process of the temporal bone to the superior margin of the thyroid cartilage. The muscle runs between the superior and middle pharyngeal constrictor mm., thus it is a helpful landmark of the posterior pharynx. 

9.) Clean and distinguish the pharyngeal constrictor muscles.

Note: There are three (pharyngeal constrictor) muscles in the external layer of the pharynx, all of which have circularly-oriented fibers. The pharyngeal constrictors surround the pharynx, and meet along a posterior midline pharyngeal raphe. 

Note: Distinguishing between the pharyngeal constrictor mm. can be difficult. The superior and middle pharyngeal constrictor mm. are distinguished by the muscle belly of the stylopharyngeus m., which inserts between these muscles. The middle and inferior pharyngeal constrictor mm. are more difficult to visually distinguish. The fibers of the middle pharyngeal constrictor mm. attach to the hyoid, whereas the inferior pharyngeal constrictor mm. attach to the thyroid and cricoid cartilages.

Note: The pharynx is supported by two associated bodies of fascia: pharyngobasilar fascia and buccopharyngeal fascia. The pharyngobasilar fascia (the deep investing/epimysial layer of the external pharyngeal mm.) suspends the superior pharyngeal constrictor m. from the basilar part of the occipital bone and the adjacent temporal bone. The pharyngobasilar fascia is closely associated with the pharyngeal raphe at the pharyngeal tubercle. The buccopharyngeal fascia (the superficial investing/epimysial layer of the external pharyngeal mm.) consists of a thickened epimysium of the superior pharyngeal constrictor that extends anteriorly from the pharyngeal raphe to the superficial surface of the buccinator m. (of the cheek). A condensed band of the buccopharyngeal fascia that extends from the pterygoid hamulus to the posterior end of the mylohyoid line of the mandible is the pterygomandibular raphe. The pterygomandibular raphe connects the buccinator m. to the superior pharyngeal constrictor m.

Note: The superior pharyngeal constrictor m. consists of four parts, each arising from a distinct location (associated by name), and coalescing posteriorly to the superior portion of the pharyngeal raphe. The four distinct parts (and their anterior attachments) include the:

• pterygopharyngeal part (pterygoid hamulus & occasionally the posterior aspect of the medial pterygoid plate),

• buccopharyngeal part (pterygomandibular raphe),

• mylopharyngeal part (posterior portion of mylohyoid line of the mandible), and the

• glossopharyngeal part (posterolateral tongue).

Note: The middle pharyngeal constrictor m. originates on the hyoid bone (greater and lesser horns) and the stylohyoid ligament, and inserts on the pharyngeal raphe.

Note: The inferior pharyngeal constrictor m. consists of two parts. These parts (and their attachments) include the:

• thyropharyngeal part (thyroid cartilage of the larynx), and the

• cricopharyngeal part (cricoid cartilage of the larynx). 

Note: The pharyngeal constrictor mm. are innervated by the pharyngeal (neural) plexus, which receives efferent (motor) fibers from the vagus nn. (CN X) and afferent (sensory) fibers from the glossopharyngeal nn. (CN IX). The cricopharyngeal part of the inferior pharyngeal constrictor m. receives dual innervation from the pharyngeal (neural) plexus and the vagus n. (CN X) via the recurrent laryngeal n. and the superior laryngeal n. (Sakamoto, 2013). When activated, the pharyngeal constrictors serially constrict the lumen of the pharynx. Some fibers of the middle pharyngeal constrictor mm. can also act as an elevator of the pharynx (Sakamoto, 2014). The plexus also receives autonomic fibers from the vagus n. (parasympathetic) and superior cervical ganglion (sympathetic).

Note: The pharyngeal constrictor mm. are primarily supplied by the ascending pharyngeal a., a branch of the external carotid artery. Branches from the inferior thyroid a. also may supply the inferior pharyngeal constrictor m. Both superior and middle pharyngeal constrictor mm. may be partially supplied by tonsillar br. of the facial artery (also a branch of the external carotid a.). The superior pharyngeal constrictor m. may also receive blood from the ascending palatine a. (a br. of the facial a.). 

Note: The pharyngeal constrictor muscles are drained of blood by the pharyngeal venous plexus, which drains into the internal jugular vv. 

12.) Locate the branches of the vagus n. (CN X) in this region, specifically the superior laryngeal n. (external and internal brs.) and recurrent laryngeal n.

Note: Superior laryngeal n. typically branches from vagus n. (CN X) superior to the carotid bifurcation. Internal br. of superior laryngeal n. afferently (sensory) serves the mucosa of the: laryngeal vestibule, middle laryngeal cavity, and superior surface of the vocal folds. This branch is accompanied by the superior laryngeal a. (a branch of superior thyroid a.). The external br. of superior laryngeal n. efferently (motor) serves the inferior pharyngeal constrictor m. and cricothyroid m.

Note: The recurrent laryngeal n. efferently serves all intrinsic mm. of the larynx, except cricothyroid m., and afferently serves (and is secretomotor to) the mucosa of larynx below vocal folds. The portion of the recurrent laryngeal n. superior to the border of the cricoid cartilage is synonymous with the inferior laryngeal n. 

13.) Locate the carotid bifurcation. Differentiate between the external and internal carotid aa. Locate the carotid body and carotid sinus.

Note: The bifurcation of the common carotid a. typically occurs at the level of C3-C4 or the superior border of the thyroid cartilage.

Note: The carotid body is typically located on the deep side of the bifurcation of the common carotid a. and should be slightly darker than the surrounding connective tissue. The carotid body (a chemoreceptor) functions in monitoring oxygen levels in the blood.

Note: The carotid sinus is typically a dilation of the internal carotid a., just superior to the carotid bifurcation, or at the bifurcation. The sinus functions to monitor blood pressure via baroreceptors in its walls.

Note: The carotid branch of the glossopharyngeal n. (CN IX), and the vagus n. (CN X) afferently (sensory) serve both the carotid sinus and body.

14.) Follow the external carotid a. superiorly, and locate the superior thyroid a. (found previously) and its branch, the superior laryngeal a. Superior to the superior thyroid a., you may see more branches of the external carotid a. to be dissected in future labs. 

Note: The superior thyroid a. is typically the first anterior branch of the external carotid a., and descends to the thyroid gland. 

Note: The superior laryngeal a. accompanies the internal br. of superior laryngeal n.

Note: The lingual a. and facial a. are anterior brs. of the external carotid a., and are typically the largest of the external carotid branches. The lingual a. is located near the middle pharyngeal constrictor m. The facial a. branches from the external carotid a. in a common branch with or just superior to the lingual a. Both arteries will be dissected in more detail in subsequent labs.

15.) Locate the thyrocervical trunk (first part of the subclavian a., as seen in previous labs). Find the inferior thyroid a., and then locate its branch to the larynx (the inferior laryngeal a.).

Note: The epiglottis is the superior boundary of the laryngopharynx and is posterior to the root of the tongue.The epiglottis is composed of epiglottic cartilage covered by mucous membrane. During swallowing, the epiglottis closes inferiorly to cover the elevated larynx to close off access into the larynx.

Note: The entrance to the esophagus is located approximately at the level of C6 or the inferior border of the cricoid cartilage.

17.) Looking at the posterior larynx, locate the laryngeal inlet, the ary-epiglottic folds, and the interarytenoid notch.

Note: The laryngeal inlet is the opening to the larynx from the laryngopharynx, and is the proximal-most portion of the laryngeal cavity. The interarytenoid notch is found between the mucosa covering the arytenoid cartilages at the posterosuperior portion of the larynx. The ary-epiglottic folds (the lateral-most border between the oropharynx and laryngopharynx) span the distance between the arytenoid cartilages and the lateral aspects of the epiglottis. The posterior aspect of the ary-epiglottic fold is supported by cuneiform cartilage.

18.) Remove the mucosal layer of the laryngopharynx to expose the posterior aspect of the larynx.

19.) Identify the muscles of the posterior larynx.

Note: The oblique and transverse arytenoid muscles attach posterior surfaces of contralateral arytenoid cartilages. Some sources consider these two muscles parts of one unified arytenoid muscle. Some superior fibers of the oblique arytenoid m. continue and wrap around the arytenoid cartilage into the ary-epiglottic fold. These fibers are the ary-epiglottic part of the oblique arytenoid m. When activated, the oblique and transverse arytenoid mm. adduct the vocal processes of the arytenoid cartilages, which combined with the lateral crico-arytenoid muscles, closes the glottis.

Note: The posterior crico-arytenoid muscles are the sole abductors of the vocal processes of the  arytenoid cartilages, which opens the glottis.

20.) Reflect the inferior pharyngeal constrictor mm. laterally from the thyroid cartilages. Locate the cricothyroid m. The larynx will need to be rotated laterally to view this muscle. 

21.) Remove the pharyngeal mucosa of the ary-epiglottic folds superior to the oblique and transverse arytenoid mm. to reveal the arytenoid cartilages and lateral crico-arytenoid mm. 

Note: The arytenoid cartilages sit atop the superior margin of the lamina of the cricoid cartilage. Each arytenoid cartilage has an anteriorly positioned vocal process and a laterally positioned muscular process. Vocal ligaments attach the laminae of the thyroid cartilage to the vocal processes of the arytenoid cartilages. Muscular processes offer a location for attachment of intrinsic laryngeal muscles. Many of these muscles will rotate (or medially glide) the arytenoid cartilages to either adduct the vocal ligaments (narrow the rima glottidis) or abduct the vocal ligaments (expand the rima glottidis).

22.) Turn the donor supine, and locate the thyrohyoid membrane and cricothyroid m. 

Note: Cricothyroid mm. are located on the anterolateral larynx between the thyroid and cricoid cartilages. These muscles will tilt the thyroid cartilage anteriorly at the cricothyroid joint, which lengthen (and thus tense) the vocal ligaments. The cricothyroid m. is innervated by the external br. of the superior laryngeal n. Recall that all other intrinsic laryngeal muscles are innervated by the recurrent laryngeal n.

23.) Reflect the cricothyroid muscle unilaterally, and locate the cricothyroid joint.

Note: The paired, cricothyroid joints are synovial and important in rotational movements, which will lengthen (adds tension) to the vocal ligaments. These joints are located on the lateral portions of the larynx between the lateral lamina of the cricoid cartilage and inferior horn of the thyroid cartilage.

24.) Using sharp-sharp scissors, mid-sagittally section the thyroid cartilage.

25.) With the donor prone, disarticulate (cut) a single cricothyroid joint and reflect the posterior edge of the ipsilateral thyroid lamina anteriorly (toward the anterior midline) to reveal a closer view of the lateral crico-arytenoid m.  

Note: The lateral crico-arytenoid m. runs obliquely along the lateral surface of the cricoid, ascending toward its attachment to the muscular process of the arytenoid cartilage. 

26.) Make a midsagittal cut from the posterior laryngeal inlet inferiorly through the cricoid cartilage to open the larynx and visualize internal structures. Retract the sides of the larynx laterally. Locate the laryngeal cartilages from an internal view.

27.) Locate the laryngeal vestibule, vestibular folds, rima vestibuli, laryngeal ventricle, vocal folds, rima glottidis, and infraglottic cavity.

Note: The laryngeal vestibule is located between the inlet to the larynx and the vestibular folds.

Note: The vestibular folds are often referred to as the false vocal folds or ventricular folds. The folds are composed of the vestibular ligaments (inferior parts of the quadrangular membranes) covered with mucosa. These folds are located superior and lateral to the vocal folds. These folds are involved in the production of some sounds (e.g. throat singing), but are not associated with speech.

Note: The laryngeal ventricle is the space between the vestibular folds and the vocal folds. 

Note: The vocal folds are often referred to as the true vocal folds/cords. The folds are composed of vocal ligaments and muscles covered with mucosa, and flank the rima glottidis. These folds are important in sound production. The rima glottidis changes width depending on position of the arytenoid cartilages and vocal folds.

Note: The infraglottic cavity is located inferior to the rima glottidis and vocal folds and superior to inferior border of the cricoid cartilage.

28.) On one side, remove the mucosal lining of the larynx to expose the contents of the vocal folds (vocalis m., vocal ligament, and thyro-arytenoid m.) and the remainder of the conus elasticus.

Note: Two sheets of fibroelastic connective tissue help to give shape to the walls of the larynx and laryngeal features: the quadrangular membrane and the conus elasticus. 

Note: The conus elasticus is a fibroelastic sheet, that shapes the walls of the larynx as a funnel between the thyroid and cricoid cartilages. The superior-most aspects of the conus elasticus are the vocal ligaments and the walls of the laryngeal ventricle. The anteromedial aspect of the conus elasticus is the (median) cricothyroid ligament, the connective tissue pierced during a cricothyrotomy. 

Note: The thyro-arytenoid m. and vocalis m. attach the anterior aspects of the arytenoid cartilages to the posterior aspect of the laminae of the thyroid cartilage. The vocalis m. is immediately lateral to (and has fibers inserting on) the vocal ligament. The thyro-arytenoid m. runs lateral and parallel to the vocalis m. Some sources consider the vocalis m. as specialized fibers of the thyro-arytenoid m. The thyro-arytenoid mm. pull the arytenoid cartilages toward the thyroid cartilage, thus decreasing tension on the vocal ligaments (lowers pitch). Vocalis mm. add lateral tension to the vocal ligaments (finely increases pitch and timbre). 

Note: Fibers of the thyro-arytenoid m. extend beyond the arytenoid cartilage into the ary-epiglottic fold. Some of these fibers, known as the thyro-epiglottic part of the muscle, may continue through the ary-epiglottic fold into the margin of the epiglottis. When activated, the thyro-epiglottic parts widen the laryngeal inlet. 

29.) On the ipsilateral side, continue to remove the mucosal lining of the larynx superiorly to uncover the quadrangular membrane, and its vestibular ligament. 

Note: The quadrangular membrane is a poorly defined fibroelastic sheet spanning the space between the lateral margins of the epiglottis and the arytenoid cartilages. The inferior-most extent of the quadrangular membrane forms the vestibular ligaments. 

30.) With the donor supine, investigate the relationships of the larynx to the hyoid bone and the trachea. 

Note: The thyroid cartilage is suspended from the hyoid bone by the thyrohyoid membrane. The superior laryngeal a. & v., as well as the internal branch of the superior laryngeal n., pierce the thyrohyoid membrane through a common opening. 

Note: The cricoid cartilage helps to suspend the trachea inferiorly via the cricotracheal ligament. 

Note: The trachea connects the larynx to the primary bronchi of the lungs. The trachea begins at the inferior margin of the cricoid cartilage (near C6), and ends at the tracheal bifurcation in the superior mediastinum (near T4/5). The trachea sits anterior to the esophagus in the neck and mediastinum, and may be palpated in the neck. The trachea consists of a series of 16-20 ‘C-shaped’ hyaline cartilages connected by fibroelastic connective tissue. The posterior aspect of the trachea (where the cartilages are incomplete) is comprised of a fibromuscular membrane containing trachealis muscle, smooth muscle that can alter the diameter of the lumen of the trachea. 

31.) Just proximal to the tracheal bifurcation, excise (with two transverse cuts) a 4-5 cm. section of the trachea, and examine its features.