Part 143

The =arytenoideus transversus= (M. arytænoideus transversus) is an unpaired muscle which stretches across the concave dorsal surface of the arytenoid cartilages. Its fibers are attached on either side to the processus muscularis and the ridge which extends forward from it. The right and left parts of the muscle meet at a fibrous raphé which is connected with the transverse arytenoid ligament. Filters of the thyro-arytenoideus muscle overlap its anterior part. It narrows the rima by drawing the arytenoid cartilages together.

FIG. 355.—ADITUS LARYNGIS OF HORSE, EXPOSED BY OPENING PHARYNX AND BEGINNING OF ŒSOPHAGUS ALONG MEDIAN DORSAL LINE.

1, False vocal cord; 2, lateral ventricle; 3, true vocal cords; 4, rima glottidis. ]

The =thyro-arytenoideus= (Mm. ventricularis et vocalis) is situated in the lateral wall of the larynx, covered by the lamina of the thyroid cartilage. It consists of two parts, between which the mucous membrane of the lateral ventricle pouches to form the saccule. The =anterior part= (M. ventricularis) arises from the anterior part of the crico-thyroid membrane and the ventral border of the thyroid lamina. Its fibers pass upward and backward to end partly on the processus muscularis, partly on the arytenoideus transversus, meeting its fellow. The =posterior part= (M. vocalis) has a more extensive origin on the crico-thyroid membrane. Its direction corresponds with that of the true vocal cord. It is inserted into the outer surface of the arytenoid cartilage below the processus muscularis. The muscle closes the rima and slackens the vocal cords. With the arytenoideus transversus and crico-arytenoideus lateralis it forms a sphincter which closes the entrance to the larynx.

CAVITY OF THE LARYNX

The =cavity= of the larynx (Cavum laryngis) is smaller than one would expect from its external appearance. On looking into it through the pharyngeal end two folds are seen projecting from its lateral walls. These are termed the =vocal cords=, and they divide the cavity into two portions.

The =aditus laryngis=, or pharyngeal aperture, is a large, oblique, oval opening, which faces forward and upward into the ventral part of the pharynx. It is bounded in front by the epiglottis, behind by the arytenoid cartilages, and laterally by the aryteno-epiglottic folds of mucous membrane (Plicæ aryepiglotticæ), which stretch between the edges of the epiglottis and the arytenoid cartilages.

FIG. 356.—SECTION OF LARYNX OF HORSE.

The plane of the section is parallel with the vocal cords. ]

The =vestibule= of the larynx (Vestibulum laryngis) is that part of the cavity which extends from the aditus to the vocal cords. On its lateral walls are the =false vocal cords= (Plicæ ventriculares), each of which consists of a fold of mucous membrane covering the anterior part of the thyro-arytenoid ligament and the cuneiform process. Between the false and true vocal cord there is a pocket-like depression termed the =lateral ventricle= of the larynx (Ventriculus lateralis laryngis). This is the entrance to the =laryngeal saccule= (Appendix ventriculi laryngis), a cul-de-sac of the mucous membrane which is about one inch (ca. 2 to 3 cm.) long and extends upward and backward on the inner surface of the thyroid lamina. There is a small =middle ventricle= (Ventriculus laryngis medianus) at the base of the epiglottis.

The middle, narrow part of the cavity is termed the =glottis= or =rima glottidis=. It is bounded on either side by the true vocal cord and the inner surface of the arytenoid cartilage. The =true vocal cords= (Labia vocales) are situated behind the false cords and the lateral ventricles. They extend from the angle between the body and laminæ of the thyroid cartilage to the vocal processes of the arytenoid cartilages. They are prismatic in cross-section, and their free edges look forward and somewhat upward. The mucous membrane of the cord (Plica vocalis) is very thin and smooth, and is intimately attached to the underlying ligament. In ordinary breathing the rima is somewhat lanceolate in form; when dilated, it is diamond-shaped, the widest part being between the vocal processes. The narrow part of the glottis between the vocal cords is termed the glottis vocalis (Pars intermembranacea), while the wider part between the arytenoid cartilages is the glottis respiratoria (Pars intercartilaginea).

The =posterior compartment= of the laryngeal cavity is directly continuous with the trachea. It is inclosed by the cricoid cartilage and the crico-thyroid membrane. It is oval in form, the transverse diameter being an inch and a half to two inches (ca. 4 to 5 cm.), and the dorso-ventral diameter two to two and a half inches (ca. 5 to 6 cm.).

The =mucous membrane= which lines the larynx (Tunica mucosa laryngis) is reflected around the margin of the aditus to become continuous with that of the pharynx, and behind it is continuous with that which lines the trachea. It is closely attached to the pharyngeal surface of the epiglottis, but elsewhere in the aditus and vestibule it is loosely attached by submucous tissue which contains many elastic fibers. It is thin and very closely adherent over the vocal cords and the inner surfaces of the arytenoid cartilages. The epithelium is of the stratified squamous type from the aditus to the glottis, beyond which it is columnar ciliated in character. There are numerous =mucous glands= (Glandulæ laryngeæ), except over the glottis and the pharyngeal surface of the epiglottis, in which situations they are scanty.

=Blood-supply.=—Thyro-laryngeal artery.

=Nerve-supply.=—Superior laryngeal and recurrent laryngeal nerves (from the vagus).

THE TRACHEA

The =trachea= extends from the larynx to the roots of the lungs, where it divides into the right and left bronchi. It is kept permanently open by a series of about fifty to fifty-five incomplete cartilaginous rings embedded in its wall. It occupies a median position, except near its termination, where it is pushed a little to the right by the arch of the aorta. It is approximately cylindrical, but its cervical portion is for the most part depressed dorso-ventrally by contact with the longus colli muscle, so that the dorsal surface is flattened. The average caliber is about two to two and one-half inches (ca. 5 to 6 cm.), but in the greater part of the neck the transverse diameter is greater and the dorso-ventral smaller. It is inclosed by a fascia propria.

In its =cervical part= the trachea is related dorsally to the œsophagus for a short distance, but chiefly to the longus colli muscle. Laterally it is related to the thyroid gland, the carotid artery, the jugular vein, the vagus, sympathetic, and recurrent laryngeal nerves, and the tracheal lymph ducts and cervical lymph glands. The œsophagus lies on its left face from the third cervical vertebra backward. The sterno-cephalicus muscles cross it very obliquely, passing from the ventral surface forward over its sides, and diverging to reach the angles of the jaw. The omo-hyoidei also cross it very obliquely, passing over the lateral surfaces of the tube, and converging ventrally to the body of the hyoid bone. The sterno-thyrohyoideus lies on the ventral surface. The scaleni lie on either side near the entrance to the thorax.

The =thoracic part= of the trachea passes backward between the pleural sacs and divides into the two bronchi over the left atrium of the heart. It is related dorsally to the longus colli for a short distance, and beyond this to the œsophagus. Its left face is crossed by the aortic arch, the left brachial artery, and the thoracic duct. Its right face is crossed by the vena azygos, the dorso-cervical and vertebral vessels, and the right vagus nerve. Ventrally it is related to the anterior vena cava, the brachiocephalic and common carotid trunks, and the left recurrent nerve.

FIG. 357.—CROSS-SECTION OF VENTRAL PART OF NECK OF HORSE.

The section is cut at right angles to the long axis of the neck, passing through the junction of the second and third cervical vertebræ. S.c., Spinal cord; S.v., spinal vein; V.v., V.a., vertebral vein and artery; T.p., transverse process (tip); F, intervertebral fibro-cartilage; T, atlantal tendon common to mastoido-humeralis, splenius, and lower part of trachelo-mastoideus; M, digitation of mastoido-humeralis inserted by T. ]

The =bifurcation= of the trachea (Bifurcatio tracheæ) is situated opposite to the fifth intercostal space, and about five or six inches (ca. 12 to 15 cm.) below the seventh and eighth thoracic vertebræ.

The trachea is composed of—(1) A fibro-elastic membrane in which are embedded (2) the cartilaginous rings; (3) a muscular layer; (4) the mucous membrane. The =elastic membrane= is intimately attached to the perichondrium of the rings. In the intervals between the latter it constitutes the annular ligaments (Ligamenta annularia). The =rings= of the trachea (Cartilagines tracheales) are composed of hyaline cartilage. They are incomplete dorsally, and when their free ends are drawn apart, resemble somewhat the letter C. In the cervical part the thin wide free ends overlap, while in the thoracic part they do not meet; here the deficiency is made up by thin plates of variable size and form, embedded in a membrana transversa. Ventrally the rings are about one-half inch (ca. 1 to 1.2 cm.) wide and much thicker than dorsally. The first ring is attached to the cricoid cartilage by the crico-tracheal membrane, and is usually fused dorsally with the second ring. Various irregularities, such as partial bifurcation or partial or complete fusion with an adjacent ring, are common. The arrangement in the terminal part is very irregular. The =muscular layer= (Musculus trachealis) consists of unstriped fibers which stretch across the dorsal part of the tube. It is separated from the ends of the rings and the membrana transversa by a quantity of areolar tissue. When it contracts the caliber of the tube is diminished. The =mucous membrane= is pale, normally, and presents numerous fine longitudinal folds, in which are bundles of elastic fibers. The epithelium is stratified columnar ciliated. Numerous tubular =mucous glands= (Glandulæ tracheales) are present.

=Blood-supply.=—Common carotid arteries.

=Nerve-supply.=—Vagus and sympathetic nerves.

THE BRONCHI

The two =bronchi=, right and left (Bronchus dexter, sinister), are formed by the bifurcation of the trachea. Each passes backward and outward to the hilus of the corresponding lung. The right bronchus is a little wider and less oblique in direction than the left. They are related ventrally to the divisions of the pulmonary artery, and dorsally to the branches of the bronchial artery and the bronchial lymph glands. Their structure is similar to that of the trachea.

THE THORACIC CAVITY