3D Model larynx.docx

3D Model larynx The laryngeal prominence is produced by the meeting of the laminae of the thyroid cartilage at an acute angle in the anterior midline. This thyroid angle, most acute in postpubertal males, forms the laryngeal prominence (“Adam’s apple”), which is palpable and frequently visible. Duri...

3D Model larynx The laryngeal prominence is produced by the meeting of the laminae of the thyroid cartilage at an acute angle in the anterior midline. This thyroid angle, most acute in postpubertal males, forms the laryngeal prominence (“Adam’s apple”), which is palpable and frequently visible. During palpation of the prominence, it can be felt to recede on swallowing. The vocal folds are at the level of the middle of the laryngeal prominence. The cricoid cartilage can be felt inferior to the laryngeal prominence at the level of the C6 vertebra. Extend your neck as far as possible and run your finger over the laryngeal prominence. As your finger passes inferiorly from the prominence, feel the cricothyroid ligament, the site for a needle cricothyrotomy or coniotomy (see the Clinical Box “Aspiration of Foreign Bodies and Heimlich Maneuver” in this chapter). After your finger passes over the arch of the cricoid cartilage, note that your fingertip sinks in because the arch of the cartilage projects farther anteriorly than the rings of the trachea. The cricoid cartilage, a key landmark in the neck, indicates the level of the C6 vertebra site where the carotid artery can be compressed against the transverse process of the C6 vertebra junction of the larynx and trachea joining of the pharynx and esophagus point where the recurrent laryngeal nerve enters the larynx site that is approximately 3 cm superior to the isthmus of the thyroid gland The first tracheal cartilage is broader than the others and is palpable (see Fig. 9.33A). The second through fourth cartilages cannot be felt because the thyroid isthmus connecting the right and left lobes of the thyroid covers them. The thyroid gland may be palpated by anterior or posterior approaches (i.e., standing in front of or behind the person). Place your fingertips anterior (for the isthmus) or immediately lateral (for the lobes) to the trachea and then direct the person to swallow (see Bickley, 2021, for details). Although both approaches to examining the thyroid are performed, the posterior approach usually allows better palpation, but the anterior approach allows observation. A perfectly normal thyroid gland may not be visible or distinctly palpable in some females, except during menstruation or pregnancy. The normal gland has the consistency of muscle tissue. The isthmus of the thyroid gland lies immediately inferior to the cricoid cartilage; it extends approximately 1.25 cm on either side of the midline. It can usually be felt by placing the fingertips of one hand on the midline below the cricoid arch and then asking the person to swallow. The isthmus will be felt moving up and then down. The apex of each lobe of the thyroid gland extends superiorly to the middle of the lamina of the thyroid cartilage (Fig. 9.50). The surface anatomy of the posterior aspect of the neck is described in Chapter 2, Back. Key points are the following: The spinous processes of the C6 and C7 vertebrae are palpable and visible, especially when the neck is flexed. The transverse processes of the C1, C6, and C7 vertebrae are palpable. The tubercles of the C1 vertebra can be palpated by deep pressure postero-inferior to the tips of the mastoid processes. Cricothyrotomy In extreme emergency cases (e.g., severe airway obstruction, major facial or cervical trauma or angio-edema) where intubation isn’t possible, experienced persons (e.g., physicians or EMT [Emergency Medical Technician] personnel) insert a large-bore needle through the cricothyroid membrane/ligament (see Fig. 9.29) (needle cricothyrotomy, or “coniotomy”) to permit fast entry of air. Later, a surgical cricothyrotomy (inferior laryngotomy) may be performed, which involves an incision through the skin and cricothyroid membrane and insertion of a small tracheostomy tube into the trachea (Fig. B9.13). Cricothyrotomy is a more expedient procedure than tracheostomy and manipulation of the cervical spine usually unnecessary. If long-term intubation is anticipated, the cricothyrotomy may be replaced by a formal tracheostomy to avoid the complication of cricoid stenosis. Tracheostomy  A transverse incision through the skin of the neck and anterior wall of the trachea, tracheostomy, establishes an airway in patients with upper airway obstruction or respiratory failure (Fig. B9.13). The infrahyoid muscles are retracted laterally, and the isthmus of the thyroid gland is either divided or retracted superiorly. An opening is made in the trachea between the first and second tracheal rings or through the second through fourth rings. A tracheostomy tube is then inserted into the trachea and secured. To avoid complications during a tracheostomy, the following anatomical relationships are important: The inferior thyroid veins arise from a venous plexus on the thyroid gland and descend anterior to the trachea. A small thyroid ima artery is present in approximately 10% of people; it ascends from the brachiocephalic trunk or the arch of the aorta to the isthmus of the thyroid gland. The left brachiocephalic vein, jugular venous arch, and pleurae may be encountered, particularly in infants and children. The thymus covers the inferior part of the trachea in infants and children. The trachea is small, mobile, and soft in infants, making it easy to cut through its posterior wall and damage the esophagus. The thyroid gland lies deep to the sternothyroid and sternohyoid muscles, located anteriorly in the neck at the level of the C5–T1 vertebrae (Fig. 9.27). It consists primarily of right and left lobes, anterolateral to the larynx and trachea. A relatively thin isthmus unites the lobes over the trachea, usually anterior to the second and third tracheal rings. The thyroid gland is surrounded by a thin fibrous capsule, which sends septa deeply into the gland. Dense connective tissue attaches the capsule to the cricoid cartilage and superior tracheal rings. External to the capsule is a loose sheath formed by the visceral portion of the pretracheal layer of deep cervical fascia. Arteries of Thyroid Gland The highly vascular thyroid gland is supplied by the superior and inferior thyroid arteries (Figs. 9.27B and 9.28). These vessels lie between the fibrous capsule and the loose fascial sheath. Usually, the first branches of the external carotid arteries, the superior thyroid arteries, descend to the superior poles of the gland, pierce the pretracheal layer of deep cervical fascia, and divide into anterior and posterior branches supplying mainly the anterosuperior aspect of the gland. The inferior thyroid arteries, the largest branches of the thyrocervical trunks arising from the subclavian arteries, run superomedially posterior to the carotid sheaths to reach the posterior aspect of the thyroid gland. They divide into several branches that pierce the pretracheal layer of the deep cervical fascia and supply the postero-inferior aspect, including the inferior poles of the gland. The right and left superior and inferior thyroid arteries anastomose extensively within the gland, ensuring its supply while providing potential collateral circulation between the subclavian and external carotid arteries. In approximately 10% of people, a small, unpaired thyroid ima artery (L. arteria thyroidea ima) arises from the brachiocephalic trunk (see the Clinical Box “Thyroid Ima Artery” in this chapter); however, it may arise from the arch of the aorta or from the right common carotid, subclavian, or internal thoracic arteries. When present, this small artery ascends on the anterior surface of the trachea, supplying small branches to it. The artery then continues to the isthmus of the thyroid gland, where it divides and supplies it. Veins of Thyroid Gland Three pairs of thyroid veins usually form a thyroid plexus of veins on the anterior surface of the thyroid gland and anterior to the trachea (Figs. 9.28 and 9.29). The superior thyroid veins accompany the superior thyroid arteries; they drain the superior poles of the thyroid gland; the middle thyroid veins do not accompany but run essentially parallel courses with the inferior thyroid arteries; they drain the middle of the lobes. The usually independent inferior thyroid veins drain the inferior poles. The superior and middle thyroid veins drain into the IJVs; the inferior thyroid veins drain into the brachiocephalic veins posterior to the manubrium. The posterior wall of the larynx is split in the median plane, and the two sides are spread apart and held in place by a surgical needle. On the left side, the mucous membrane is intact. On the right side, the mucous and submucous coats are peeled off, and the skeletal coat- consisiting of cartilages, ligaments, and the fibro-elastic membrane- is uncovered. The vocal folds control sound production (Figs. 9.36 and 9.37). The apex of each wedge-shaped fold projects medially into the laryngeal cavity. Each vocal fold contains a vocal ligament, consisting of thickened elastic tissue that is the medial free edge of the conus elasticus (Figs. 9.33E, 9.35, and 9.36A) vocalis muscle, composed of exceptionally fine muscle fibers immediately lateral to and terminating at intervals relative to the length of the vocal ligaments (Fig. 9.36A) The vocal folds are the sharp-edged folds of mucous membrane overlying and incorporating the vocal ligaments and the thyro-arytenoid muscles. They are the source of the sounds (tone) that come from the larynx. These folds produce audible vibrations when their free margins are closely (but not tightly) apposed during phonation and air is forcibly expired intermittently (Fig. 9.37C). The vocal folds also serve as the main inspiratory sphincter of the larynx when they are tightly closed. Complete adduction of the folds forms an effective sphincter that prevents entry of air. The glottis (the vocal apparatus of the larynx) makes up the vocal folds and processes, together with the rima glottidis, the aperture between the vocal folds (Fig. 9.36C). The shape of the rima (L., slit) varies according to the position of the vocal folds (Fig. 9.37). During ordinary breathing, the rima is narrow and wedge shaped; during forced respiration, it is wide and trapezoidal in shape. The rima glottidis is slit-like when the vocal folds are closely approximated during phonation. Variation in the tension and length of the vocal folds, in the width of the rima glottidis, and in the intensity of the expiratory effort produces changes in the pitch of the voice. The lower range of pitch of the voice of postpubertal males results from the greater length of the vocal folds. The vestibular folds, extending between the thyroid and the arytenoid cartilages (Figs. 9.35 and 9.36), play little or no part in voice production; they are protective in function. They consist of two thick folds of mucous membrane enclosing the vestibular ligaments. The space between these ligaments is the rima vestibuli. The lateral recesses between the vocal and the vestibular folds are the laryngeal ventricles.   The risk of injury to the recurrent laryngeal nerves is ever present during neck surgery. Near the inferior pole of the thyroid gland, the right recurrent laryngeal nerve is intimately related to the inferior thyroid artery and its branches (Fig.B9.10). This nerve may cross anterior or posterior to branches of the artery, or it may pass between them. Because of this close relationship, the inferior thyroid artery is ligated some distance lateral to the thyroid gland, where it is not close to the nerve. Although the danger of injuring the left recurrent laryngeal nerve during surgery is not as great, owing to its more vertical ascent from the superior mediastinum, the artery and nerve are also closely associated near the inferior pole of the thyroid gland (see Fig, 9.28). Hoarseness is the usual sign of unilateral recurrent nerve injury; however, temporary aphonia or disturbance of phonation (voice production) and laryngeal spasm may occur. These signs usually result from bruising the recurrent laryngeal nerves during surgery or from the pressure of accumulated blood and serous exudate after the operation.