The PHARYNX is a fibromuscular tube which extends from the base of the skull to the lower border of the cricoid cartilage (at which point it becomes the esophagus). Portions of the pharynx lie posterior to the nasal cavity (nasal pharynx), oral cavity (oral pharynx) and larynx (laryngeal pharynx). The muscular walls of the pharynx are comprised of an outer layer made up of 3 circularly disposed muscles, the CONSTRICTORS. The constrictors are logically named (from superior most to inferior most) the SUPERIOR, MIDDLE and INFERIOR PHARYNGEAL CONSTRICTORS. Figure 1 below shows the arrangement and attachments of the pharyngeal constrictors. Note that the inferior edges of the superior and middle constrictors nestle inside the superior edges of the next lower constrictor. (This arrangement has been described as being similar to 3 stacked flower pots.)
ACTIONS OF THE PHARYNGEAL MUSCLES
During swallowing, successive contraction of the superior, middle, and inferior constrictor muscles helps to propel the bolus (ball) of food down into the esophagus. In addition, contraction of the 3 longitudinal muscles of the pharynx helps to raise the pharynx, effectively aiding it in engulfing the bolus of food.
In between acts of swallowing, the lowest fibers of the inferior constrictor are thought to act as a sphincter, guarding the entrance to the esophagus and preventing the entry of air into the digestive system.
The LARYNX is an apparatus made up of cartilage, ligaments, muscles, and mucous membrane, which guards the entrance to the lower respiratory passages (trachea, bronchi, and lungs) and houses the vocal cords. Figure 4 shows the major cartilaginous and ligamentous structures which make up the larynx. (Note that the hyoid bone is not usually considered as part of the larynx and is included in this figure strictly as an aid to orientation.) Before you consider the muscles associated with the laryngeal apparatus, pay very careful attention to the arrangement of the structural components shown in Figure 4, and realize that three major types of movement are possible:
1. MOVEMENT AFFECTING THE TENSION OF THE VOCAL CORDS. Since the vocal cords are attached to the arytenoid cartilages posteriorly and the thyroid cartilage anteriorly, regulation of vocal cord tension (and therefore pitch of the voice) is accomplished primarily by pivoting the thyroid cartilage forward or backward at the cricothyroid joint (Figure 51.
2. MOVEMENTS WHICH INCREASE OR DECREASE THE SIZE OF THE OPENING BETWEEN THE VOCAL CORDS (ABDUCTION OR ADDUCTION OF THE VOCAL CORDS). This action is best appreciated by imagining the larynx viewed from above, as is shown semi-schematically in Figure 6. Realize: I. That the vocal cords on each side are covered with a mucous membrane, so that when they are abducted, relatively little air can pass either between them or around them. 2. The bases of the two arytenoid cartilages are L-shaped. The medial process of each is called the vocal process, and the vocal cords attach to it. The lateral process of each is called the muscular process, and most of the muscles which act to abduct or adduct the vocal cords attach to it. Abduction of the vocal cords may be accomplished either by externally rotating the arytenoid cartilages on a pivot located at the angle of the "L" (center panel, Figure 6), or by sliding the 2 arytenoid cartilages apart slightly (right panel, Figure 6). Realize that there is only 1 muscle which has an abductor action on the vocal cords, although there are several which act to adduct the cords.
3. MOVEMENTS WHICH CLOSE OFF THE ENTRANCE TO THE LARYNX, AS IN SWALLOWING. Figure 7 below indicates with arrows the movements of the epiglottis and arytenoid cartilages which take place during swallowing to block off the entrance to the larynx, to prevent food from entering the larynx. Basically, the epiglottis is pulled down, and the arytenoid cartilages are pulled forward, toward the epiglottis. Realize that two other mechanisms which are independent of laryngeal movements per se also help to keep food from entering the larynx during swallowing. First, the larynx is raised, along with the pharynx, by the contraction of the longitudinal muscles of the pharynx. Second, the epiglottis also acts partly as a passive barrier to the movement of food, so that food tends to be funneled to either side of the larynx, into the PIRIFORM RECESSES and from there into the esophagus. Refer to Figure 9-56 in your atlas, which shows the pharynx opened up to reveal the interior of the pharynx, as seen from behind. Locate the epiglottis, piriform recesses, and esophagus, and trace the path of a bolus of food from the back of the tongue into the esophagus.
OTHER STRUCTURAL FEATURES OF THE LARYNX
Refer to Figures 9-78 and 9-79 in your atlas to appreciate several additional structural features which have been intentionally omitted from the previous discussion. These features (described below) are not particularly important to understanding movements of the larynx, but you will find it somewhat difficult to understand the appearance of the larynx as seen through a laryngoscope or on dissection, until you appreciate that:
1. Above and parallel to the vocal ligament or vocal cord on each side
is a second ligament called the VESTIBULAR or VENTRICULAR LIGAMENT (Atlas
Figure 9-78, right side). This ligament is not actively moved during vocalization,
and is therefore referred to as a "false vocal cord." Nonetheless, this
ligament (or the fold of mucous membrane which covers it) is a prominent
feature when the larynx is examined with a laryngoscope.
2. The entire larynx is covered with a mucous membrane, which is given different names in its different parts, depending on the structures it covers over or runs between. (See Atlas Figure 9-78, left side.)
3. The membrane running between the epiglottis and the arytenoid cartilages, called the QUADRANGULAR MEMBRANE, is reinforced by a small cartilage which is embedded in it, the CUNEIFORM CARTILAGE.
4. On each side, another small cartilage, called the CORNICULATE CARTHAGE, caps the arytenoid cartilage superiorly.
N.B. Do not waste any time during the dissection seeking the cuneiform
and corniculate cartilages, as finding them grill not significantly advance
your understanding of the larynx.
PARTS OF THE LARYNGEAL CAVITY
The boundaries formed by the vestibular and vocal folds (see left side
of Atlas Figure 9-78) divide the cavity of the larynx into three parts.
These are illustrated schematically in Figure 8 below:
1. The VESTIBULE, which is the area above the vestibular folds.
2. The VENTRICLE, which is a pocket-like recess between the vestibular and vocal folds on both sides.
3. The INFRAGLOTTIC CAVITY, which is the area below the vocal folds.
There are two other terms you must know:
1. The GLOTTIS is the aperture formed by the vocal folds (analogous to the iris of the eye).
2. The RIMA GLOTTIDIS is the opening between the vocal folds (analogous
to the pupil of the eye). Realize that the size of the rime glottidis is
a major factor in determining how much air can enter the trachea and lungs.
MUSCLES OF THE LARYNX
Since you presumably already understand the possible movements of the larynx, understanding the actions of the laryngeal muscles should now be a relatively straightforward matter. The muscles will be presented in functional groups.
1 . MUSCLES WHICH AFFECT THE TENSION OF THE VOCAL CORDS. The CRICOTHYROID MUSCLES (illustrated in Figure 5) are the muscles primarily responsible for regulating the tension of the vocal cords. The small VOCALES muscles, which lie parallel and inferior to the vocal cords (see Atlas Figure 9-74) control fine adjustment of vocal cord tension.
2. MUSCLES WHICH OPEN AND CLOSE THE RIMA GLOTTIDIS (ABDUCT AND ADDUCT
THE VOCAL CORDS).
BLOOD SUPPLY TO THE LARYNX AND PHARYNX
Blood is supplied to this region by the ascending pharyngea] arteries
(pharynx only) and branches of the superior and inferior thyroid arteries
(pharynx and larynx).
NERVE SUPPLY TO THE LARYNX AND PHARYNX
1. MOTOR INNERVATION OF THE MUSCLES OF THE PHARYNX AND LARYNX arises mainly from the CRANIAL DIVISION of the ACCESSORY NERVE (XI). Note, however, that these axons TRAVEL with those of the VAGUS NERVE, and are indistinguishable from it (see Figure 1 1).
Exception: The STYLOPHARYNGEUS MUSCLE receives its motor innervation
from the GLOSSOPHARYNGEAL NERVE (TX) (see Figure 12). Note that this is
the ONLY muscle innervated by this nerve.
Most of the muscles of the larynx receive their innervation via the RECURRENT LARYNGEAL BRANCH of the vagus nerve (see Figure 11 ).
Exception: The CRICOTHYROID MUSCLE, which receives its innervation via the EXTERNAL LARYNGEAL NERVE.
2. SENSORY INNERVATION OF THE LARYNX AND PHARYNX.
Pharynx: via the GLOSSOPHARYNGEAL NERVE (see figure 12).
Larynx: via branches of the VAGUS NERVE (see Figure 11). Above the vocal folds the sensory innervation of the larynx is via the INTERNAL LARYNGEAL NERVE. Below the vocal folds it is by way of branches of the RECURRENT LARYNGEAL NERVE.
3. PARASYMPATHETIC INNERVATION OF THE LARYNX AND PHARYNX is mainly by way of branches of the vagus nerve.
SPECIAL LYMPHATIC TISSUE OF THE PHARYNX
Realize that the pharynx contains a "ring" of specialized lymphatic
tissue designed to prevent the entry of pathogens into the digestive and
respiratory tracts. This specialized lymphatic tissue is known as "tonsils"
and is organized into three groups: 1. nasopharyngeal tonsils (adenoids),
located in the nasal pharynx; 2. palatine tonsils (tonsils), located between
the palatoglossal and palatopharyngeal folds in the oral pharynx; and 3.
lingual tonsils, located on the posterior surface of the tongue. The approximate
location of these three groups of lymphoid tissue is indicated in Figure