Part 120

The =muscles= (Figs. 243, 244) are covered by the pharyngeal fascia, which is attached to the base of the skull, the great cornu of the hyoid bone, and the thyroid cartilage of the larynx. They are as follows:

1. The =stylo-pharyngeus= arises from the inner surface of the dorsal third of the great cornu of the hyoid bone, passes downward and inward, and enters the wall of the pharynx by passing between the pterygo-pharyngeus and palato-pharyngeus. Its fibers radiate, many passing forward, others inward beneath the hyo-pharyngeus. It raises and dilates the pharynx to receive the bolus in swallowing.

FIG. 254.—POSTERIOR PART OF A SAGITTAL SECTION OF HEAD OF HORSE, CUT ABOUT 1 CM. TO THE LEFT OF THE MEDIAN PLANE.

1, Posterior nares; 2, pharyngeal orifice of Eustachian tube; 3, aditus laryngis; 4, entrance to œsophagus; 5, posterior pillar of soft palate; 5′, junction of 5 with its fellow over entrance to œsophagus; 6, epiglottis; 7, body of thyroid cartilage; 8, arytenoid cartilage; 9, 9, cricoid cartilage; 10, true vocal cord; 11, false vocal cord; 12, lateral ventricle of larynx; 13, crico-arytenoideus post. s. dorsalis; 14, œsophagus; 15, external carotid artery; 16, hypoglossal nerve; 17, glosso-pharyngeal nerve; 18, great cornu of hyoid bone; 19, Eustachian tube; 20, body of hyoid bone; 21, hyoideus transversus; 22, ridges of hard palate; 22′, soft palate; 23, septum between frontal sinuses; 24, olfactory mucous membrane; 25, sphenoidal sinus; 26, basilar part of occipital bone; 26′, supraoccipital; 27, body of sphenoid bone; 28, pituitary body; 29, chiasma opticum; 30, corpora quadrigemina; 31, thalamus; 32, arachnoid; 33, odontoid ligament; 34, posterior auricular muscles. ]

2. The =palato-pharyngeus= arises by means of the aponeurosis of the soft palate from the palate and pterygoid bones. Its fibers pass backward on the lateral wall of the pharynx, and are inserted in part into the upper edge of the thyroid cartilage, in part turn inward to end at the median fibrous raphé. Its action is to shorten the pharynx, and to draw the larynx and œsophagus toward the root of the tongue in swallowing.

3. The =pterygo-pharyngeus= is flat and triangular. It lies on the anterior part of the lateral wall of the pharynx. It arises from the pterygoid bone above the preceding muscle—from which it is not distinctly separated—crosses the levator palati, and is inserted into the median raphé. Its action is similar to the preceding.

4. The =hyo-pharyngeus= may consist of two portions:

(a) The =kerato-pharyngeus= is a small and inconstant muscle which arises from the inner surface of the great cornu of the hyoid bone near its lower end. It passes upward and backward, turns inward toward the raphé, and spreads out under the next muscle.

(b) The =chondro-pharyngeus=, broad and fleshy, arises from the thyroid cornu of the hyoid bone and by a thin fasciculus from the wing of the thyroid cartilage and ends at the median raphé.

5. The =thyro-pharyngeus= arises from the lateral surface of the wing of the thyroid cartilage. Its fibers pass forward and inward to the median raphé.

6. The =crico-pharyngeus= arises from the cricoid cartilage and ends at the raphé. The fibers are directed upward, forward, and inward; they blend behind with the longitudinal fibers of the œsophagus.

The last three muscles are constrictors of the pharynx.

The =pharyngeal aponeurosis= is attached to the base of the cranium. It is well developed on the inner face of the palato-pharyngeus muscle and forms a median raphé (Raphé pharyngis) dorsally, which is wide in its posterior part.

The =mucous membrane= of the pharynx is continuous with that of the several cavities which open into it. It is thin and closely adherent to the base of the skull in the vicinity of the posterior nares, where the muscular wall is absent. Behind the Eustachian openings is a median cul-de-sac, the =pharyngeal recess=. The recess is somewhat variable, but is usually about an inch in depth and will admit the end of the finger. In the ass and mule it is much deeper. Here also the muscular wall is absent and the mucous membrane lies against the guttural pouches. From the Eustachian opening a fold of the mucous membrane (Plica salpingo-pharyngea) passes toward but does not reach the laryngeal opening. Below, a horizontal fold, the =posterior pillar of the soft palate= (Arcus pharyngopalatinus), passes along the lateral wall and unites with its fellow over the entrance to the œsophagus. The upper part of the cavity (the naso-pharynx) is lined with a ciliated epithelium, while the lower part (oro-pharynx) has a stratified squamous epithelium. The communication between the two is oval and is bounded by the free edge of the soft palate and its posterior pillars; it is termed the =pharyngeal isthmus=. On either side of the laryngeal opening is a narrow deep depression, the =pyriform sinus= (Recessus piriformis).

The submucous tissue contains numerous mucous glands (Glandulæ pharyngeæ). In the young subject the lymph follicles are numerous and form a collection dorsally and between the Eustachian openings, known as the pharyngeal tonsil.

=Blood-supply.=—External carotid, external maxillary, and thyro-laryngeal arteries.

=Nerve-supply.=—Glosso-pharyngeal, vagus, and sympathetic nerves.

THE ŒSOPHAGUS

The œsophagus is a musculo-membranous tube, about 50 to 60 inches (ca. 125 to 150 cm.) in length, which extends from the pharynx to the stomach. It begins in the median plane above the cricoid cartilage of the larynx. In its course it shows several changes of direction. At the level of the fourth cervical vertebra it inclines to the left side of the trachea, and continues this relation to the level of the third thoracic vertebra. Here it again gains the dorsal surface of the trachea, and passing backward, crosses the left bronchus, being here almost in the median plane. It continues in the mediastinum between the lungs backward, upward, and a little to the left, to reach the hiatus œsophageus of the diaphragm. Passing through this it terminates at once at the cardiac orifice of the stomach, at the level of the fourteenth thoracic vertebra, a little to the left of the median plane, and about four or five inches (ca. 10 to 12 cm.) ventral to the vertebral column.

Viewed with reference to the frontal plane, its course is downward and backward till it enters the thorax and passes upward to gain the dorsal face of the trachea. For a short distance (i. e., to the root of the lung) its direction is almost horizontal; behind this it passes somewhat upward to its termination. The =cervical part= of the tube is about four to six inches (10 to 15 cm.) longer than the =thoracic part=, while the so-called abdominal part is about an inch (2 to 3 cm.) long.

FIG. 255.—CROSS-SECTION OF HEAD OF HORSE.

The section passes through the temporo-mandibular articulation, but is slightly oblique. 1, Corpus callosum; 2, lateral ventricle of brain; 3, caudate nucleus; 4, internal capsule; 5, lenticular nucleus; 6, optic chiasma; 7, middle cerebral artery; 8, sphenoidal sinuses; 9, cavernous sinus; 10, Eustachian tube, inner lamina; 11, 11, guttural pouches; 12, soft palate; 13, epiglottis; 14, hyo-epiglottic muscle; 15, thyro-hyoid muscle. ]

The principal =relations= of the œsophagus at its origin are: to the cricoid cartilage below; to the guttural pouches and the ventral straight muscles above; and to the carotid arteries laterally. In the middle of the neck the relations are: to the left longus colli muscle above; to the trachea internally; to the left carotid artery, vagus, sympathetic, and recurrent nerves externally. At its entrance into the thorax it has the trachea on its inner side; the first rib, the roots of the brachial plexus of nerves and the inferior cervical ganglion externally. After gaining the upper surface of the trachea, it has the aorta on its left and the vena azygos and right vagus nerve on its right side. In its course through the posterior mediastinum the œsophageal trunks of the vagus nerves lie above and below it, and the œsophageal artery is dorsal to it.

=Structure.=—The wall is composed of four coats: (1) A fibrous sheath; (2) the muscular coat; (3) a submucous layer; (4) the mucous membrane. The =muscular coat= is of the striped variety as far as the base of the heart, where it rapidly changes to the unstriped type. In addition to this change, the muscular coat becomes much thicker and firmer, while the lumen is diminished. The outer fibers are arranged longitudinally, beginning in two bundles attached in the interval between the arytenoid and cricoid cartilages. The inner fibers run in two spiral strata to the terminal part of the tube, where the arrangement is an outer longitudinal and an inner circular layer. The =mucous membrane= is pale, and is covered with squamous stratified epithelium. It is loosely attached to the muscular coat by an abundant submucosa, and lies in longitudinal folds which obliterate the lumen except during deglutition.

=Blood-supply.=—Carotid, broncho-œsophageal, and gastric arteries.

=Nerve-supply.=—Vagus, glosso-pharyngeal, and sympathetic nerves.

THE ABDOMINAL CAVITY

The abdominal cavity (Cavum abdominis) is the largest of the body cavities. It is separated from the thoracic cavity by the diaphragm and is continuous behind with the pelvic cavity.