Part 48

FIG. 354.—LARYNGEAL MUSCLES OF HORSE, SEEN FROM LEFT SIDE AFTER REMOVAL OF MAJOR PART OF LAMINA OF THYROID CARTILAGE.

_a_, Kerato-hyoideus; _b_, thyro-hyoideus; _c_, crico-thyroideus; _d_, sterno-thyroideus; _e_, ventricularis; _f_, vocalis; _g_, crico-arytenoideus lateralis; _h_, crico-arytenoideus post. _s_, dorsalis; _i_, arytenoideus transversus; _1_, lingual process; _2_, small cornu; _3_, thyroid cornu, of hyoid bone; _4_, thyroid cartilage; _5_, cricoid cartilage; _6_, epiglottis; _7_, apex of arytenoid cartilage; _8_, trachea; _9_, muscular process of arytenoid; _10_, lateral lobe of thyroid gland; _10′_, isthmus of thyroid gland; _11_, laryngeal saccule. (Ellenberger-Baum, Anat. d. Haustiere.) ]

B. INTRINSIC MUSCLES

The =crico-thyroideus= (M. crico-thyreoideus) is a short muscle which fills the groove on the lateral surface of the cricoid cartilage; its fibers are directed dorsally and somewhat forward. It arises on the lower half of the lateral surface and posterior edge of the cricoid cartilage, and is inserted into the posterior border and adjacent part of the surfaces of the lamina of the thyroid cartilage. Its action is to draw the thyroid cartilage and the ventral part of the cricoid cartilage together. In this action the cricoid cartilage is probably rotated, carrying the bases of the arytenoid cartilages with it and thus tensing the vocal cords.

The =crico-arytenoideus dorsalis s. posterior= (M. crico-arytænoideus dorsalis) is a strong, somewhat fan-shaped muscle, which, with its fellow, covers the dorsal surface of the lamina of the cricoid cartilage. It is partially divisible into two layers. It has a broad origin on half of the lamina of the cricoid cartilage, and its fibers converge to be inserted into the processus muscularis of the arytenoid cartilage. Its action is to dilate the rima glottidis by rotating the arytenoid cartilage so as to carry the vocal process and cord outward.

The =crico-arytenoideus lateralis= (M. crico-arytænoideus lateralis) lies on the inner face of the thyroid lamina. It arises from the anterior border of the lateral part of the arch of the cricoid cartilage. The fibers pass in a dorsal direction and converge on the processus muscularis of the arytenoid cartilage. It closes the rima glottidis by rotating the arytenoid cartilage inward.

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.

[Illustration:

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.

[Illustration:

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=.[123] 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.[124] 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.[125] 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.

[Illustration:

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

The =thoracic cavity= (Cavum thoracis) is the second in point of size of the body cavities. In form it is somewhat like a truncated cone, much compressed laterally in its anterior part, and with the base cut off very obliquely.

The =dorsal wall= or =roof= is formed by the thoracic vertebræ and the ligaments and muscles connected with them.

The =lateral walls= are formed by the ribs and the intercostal muscles.

The =ventral wall= or =floor= is formed by the sternum, the cartilages of the sternal ribs, and the muscles in connection therewith. It is about one-half as long as the dorsal wall.

The =posterior wall=, formed by the diaphragm, is very oblique and is strongly convex.

The =anterior aperture= or =inlet= (Apertura thoracis cranialis) is relatively small, and of narrow, oval form. It is bounded dorsally by the first thoracic vertebra and laterally by the first pair of ribs. It is occupied by the longus colli muscles, the trachea, œsophagus, vessels, nerves, and the prepectoral lymph glands.

The cavity is lined by the =endothoracic fascia= and by the =pleuræ=.

A longitudinal =septum=, termed the =mediastinum= (Septum mediastinale), extends from the dorsal wall to the ventral and posterior walls, and subdivides the cavity into two lateral chambers which contain the lungs. Each of these chambers is lined by a serous membrane called the pleura, and is called a =pleural cavity= (Cavum pleuræ). The mediastinum is, for the most part, not median in position; this is largely due to the fact that the largest organ contained in it, the heart, is placed more on the left side; consequently the right pleural cavity and lung are larger than the left. Practically all the organs in the thorax are in the mediastinal space with the exception of the lungs, the posterior vena cava, and the right phrenic nerve. The part in which the heart and the pericardium are situated, together with that above it, is usually called the middle mediastinal space; the parts before and behind this are termed the anterior and posterior mediastinal spaces respectively.

THE PLEURÆ

The =pleuræ= are two serous sacs which line the pleural cavities and are reflected at the roots of the lungs to invest those organs. We, therefore, distinguish =parietal= and =visceral= parts of the pleuræ.

[Illustration:

FIG. 358.—CROSS-SECTION OF THE THORAX OF NEW-BORN FOAL.

_Tr._, Trachea; _S_, left brachial artery; _B_, brachiocephalic artery; _l.g._, lymph gland. Pleura indicated by red line. ]

The =parietal pleura= (Pleura parietalis) lines the cavity in which each lung is situated (Cavum pleuræ). On the lateral thoracic wall it is adherent to the ribs and intercostal muscles and is termed the =costal pleura= (Pleura costalis). Behind it is closely attached to the diaphragm, forming the =diaphragmatic pleura= (Pleura diaphragmatica). The part which is in apposition with the opposite sac or covers the mediastinal organs is termed the =mediastinal pleura= (Pleura mediastinalis); where this layer is adherent to the pericardium, it is distinguished as =pericardiac pleura= (Pleura pericardiaca).[126]

The pleura is reflected at the root of the lung, which it covers, constituting the =visceral= or =pulmonary pleura= (Pleura pulmonalis). Behind the root of the lung a considerable triangular area is not covered by the pleura, the two lungs being attached to each other by connective tissue in this situation.

The =ligament of the lung= (Lig. pulmonale) is a fold formed by the reflection of the pleura from the mediastinum and the diaphragm to the lung, behind the triangular area of adhesion just mentioned. It is seen when the base of the lung is drawn outward. It contains elastic tissue, especially in its posterior part.

[Illustration:

FIG. 359.—CROSS-SECTION OF THORAX OF NEW-BORN FOAL.

_A_, Aorta; _Br._, bronchi; _P.A._, pulmonary arteries; _l_, lymph gland; _R.A._, right atrium; _L.A._; left atrium; _R.V._, right ventricle; _L.V._, left ventricle of heart. ]

The right pleura forms a special =sagittal fold= (Plica venæ cavæ) about a handbreadth to the right of the median plane, which incloses the posterior vena cava in its upper edge and gives off a small accessory fold for the right phrenic nerve. The fold arises from the thoracic floor and from the diaphragm below the foramen venæ cavæ, and intervenes between the mediastinal lobe and the body of the right lung. It is delicate and lace-like.

The posterior mediastinum is very delicate below the œsophagus, and usually appears fenestrated; when these apertures are present, the two pleural cavities communicate with each other.[127]

The pleural sacs contain a serous fluid, the =liquor pleuræ=; in health there is only a sufficient amount to moisten the surface, but it accumulates rapidly after death.

[Illustration:

FIG. 360.—CROSS-SECTION OF THORAX OF NEW-BORN FOAL. Ribs are numbered. ]

THE LUNGS

The =lungs= (Pulmones) occupy much the greater part of the thoracic cavity. They are accurately adapted to the walls of the cavity and the other organs contained therein. The two lungs are not alike in form or size, the right one being considerably larger than the left; the difference is chiefly in width, in conformity with the projection of the heart to the left.

The lung is soft, spongy, and highly elastic. It crepitates when pressed between the finger and thumb, and floats in water. When the thoracic cavity of the unpreserved subject is opened, the lung collapses immediately to about one-third of its original size, and loses its proper form; this is due to its highly elastic character and the fact that the tension of the lung tissue caused by the air pressure in its cavities has been relieved by the external air pressure.[128]

The =color= varies according to the amount of blood contained in the lung. During life the lung has a pink color, but in subjects which have been bled for dissection it is light gray or faintly tinged with red. In unbled subjects it is dark red; the depth of color varies, and is often locally accentuated by gravitation of blood to the most dependent parts (hypostasis).

The =fœtal lung=, since it contains no air and has a relatively small blood-supply, differs from that of an animal which has breathed in the following respects: (1) It is much smaller; (2) it is firmer and does not crepitate; (3) it sinks in water; (4) it is pale gray in color.

In =form= the lungs are like casts of the pleural cavities in which they are situated. When well hardened _in situ_, their surfaces present impressions and elevations corresponding exactly to the structures with which they are in contact. Each lung presents two surfaces, two borders, a base, and an apex.

[Illustration:

FIG. 361.—LEFT LUNG OF HORSE, COSTAL SURFACE. Specimen hardened _in situ_. ]

The =costal= (or external) =surface= (Facies costalis) is convex, and lies against the lateral thoracic wall, to which it is accurately adapted.

The =mediastinal= (or internal) =surface= (Facies mediastinalis) is less extensive than the costal surface. It is molded on the mediastinum and its contents. It presents a large cavity adapted to the pericardium and heart; this is termed the =cardiac impression= (Impressio cardiaca), and is larger and deeper on the left lung than on the right. Above and behind this is the =hilus=, at which the bronchus, vessels, and nerves enter the lungs. The bronchial lymph glands are also found here. Behind this the two lungs are adherent to each other over a triangular area. Above this is a groove for the œsophagus (Sulcus œsophageus), which is deepest on the left lung. A groove for the aorta (Sulcus aortæ) curves upward and backward over the hilus, and passes backward near the dorsal border of the lung; the curved part of the groove for the aortic arch is absent on the right lung, on which there is a groove for the vena azygos. Anteriorly there are grooves for the trachea, the anterior aorta, the anterior vena cava, and other vessels.

[Illustration:

FIG. 362.—LEFT LUNG OF HORSE, MEDIASTINAL AND DIAPHRAGMATIC SURFACES.

Organ hardened _in situ_. (Venous impression for common dorso-cervico-vertebral stem.) ]

[Illustration:

FIG. 363.—RIGHT LUNG OF HORSE, MEDIASTINAL AND DIAPHRAGMATIC SURFACES.

Organ hardened _in situ_. _c_, _d_, Grooves for superior cervical and dorsal veins. Arrows indicate canal for posterior vena cava between mediastinal lobe and main part of lung. ]

The =dorsal= (or superior) =border= (Margo obtusus) is long, thick, and rounded; it lies in the groove alongside of the bodies of the thoracic vertebræ.

The =ventral= (or inferior) =border= (Margo acutus) is thin and short. It presents, opposite to the heart, the =cardiac notch= (Incisura cardiaca). On the left lung this notch is opposite to the third, fourth, and fifth ribs, so that a considerable area of the pericardium here lies in direct contact with the chest-wall. On the right lung the notch is less extensive, and is an intercostal space further forward.

The =base= of the lung (Basis pulmonis) is oval in outline; its surface (Facies diaphragmatica) is deeply concave in adaptation to the thoracic surface of the diaphragm. Laterally and posteriorly it is limited by a thin =basal border= which fits into the narrow recess (Sinus phrenico-costalis) between the diaphragm and the chest-wall. The position of this border, of course, varies during respiration. In the deepest inspiration it may reach the bottom of this recess; in ordinary inspiration it lies about four or five inches (ca. 10 to 12 cm.) from the costal arch, to which it is nearly parallel; in ordinary expiration it lies about twice as far from the costal arch.

The =apex= of the lung (Apex pulmonis) is prismatic, narrow, and flattened transversely. It is partially marked off from the rest of the lung by the cardiac notch. It curves downward, and is related internally to the anterior mediastinum and behind to the anterior part of the pericardium.

In the horse the lungs are not divided into distinct =lobes= by deep fissures, as in most mammals. In the case of the left lung there is no lobation, but the right lung presents a =mediastinal lobe=, separated from the body of the lung by a fissure which forms in its dorsal part a canal for the posterior vena cava and the right phrenic nerve, inclosed in a special fold of the right pleura.[129]

The =root= of the lung (Radix pulmonis) is composed of the structures which enter or leave the lung at the hilus on the mediastinal surface. These are: (1) The =bronchus=; (2) the =pulmonary artery=; (3) the =pulmonary veins=; (4) the =bronchial artery=; (5) the =pulmonary nerves=; (6) the =lymph vessels=, which open into the bronchial lymph glands. The bronchus is situated dorsally, with the bronchial artery on its upper surface and the pulmonary artery immediately below it. The pulmonary veins lie chiefly below and behind the artery.

The =lobulation= of the lungs is not very evident on account of the small amount of interlobular tissue. The lobules appear on the surface or on sections as irregular polygonal areas of different sizes.[130]

=Bronchial Branches.=—Each bronchus at its entrance into the lung gives off a branch to the apex of the lung. It is then continued backward as the main stem bronchus, parallel with the dorsal border of the lung, giving off branches (Rami bronchiales) in monopodic fashion; these branch similarly and reach all parts of the lung. The right bronchus gives off a special branch to the mediastinal lobe.