Chapter V: Muscles of the Leg and Foot (7)

The =hyo-epiglotticus= (M. hyo-epiglotticus) is a feeble muscle which occupies a median position above the central part of the thyro-hyoid membrane, inclosed by an elastic sheath, the hyo-epiglottic ligament. It arises from the body of the hyoid bone by two branches which unite and are inserted into the front of the base of the epiglottis. Its action is to draw the epiglottis toward the root of the tongue.

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.

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=.[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.

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.

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.

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.

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.

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.

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

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

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.

The arrangement of the larger bronchi is not the same in the two
lungs. The right bronchus, after detaching the apical branch, gives
off a large ventral branch which shortly bifurcates. One of the
divisions passes ventrally into the part of the lung behind the
cardiac notch (homologue of cardiac lobe), while the other passes
downward and backward into the lower part of the base. The stem
bronchus then gives off the branch to the mediastinal lobe, and
divides, about six inches from the hilus of the lung, into two nearly
equal branches; the dorsal branch is a direct continuation of the
stem, the ventral one passes into the middle part of the base. The
left bronchus, after a short course, divides into two nearly equal
branches. The dorsal branch may be regarded as the continuation of the
stem; it passes backward parallel with the dorsal border of the lung,
giving off a ventral branch much further back than in the right lung.
The ventral division of the stem bronchus divides, after a very short
course, into two branches; the anterior one passes ventrally into the
homologue of the cardiac lobe, while the posterior one passes downward
and backward into the base of the lung.

The =structure= of the larger bronchial tubes is, in general, similar to that of the trachea. Their walls contain irregular plates of cartilage instead of rings. There is a continuous layer of unstriped muscle, composed of circularly arranged bundles. The mucous membrane presents numerous longitudinal folds; it contains many elastic fibers, mucous glands, and lymph nodules, and is lined by ciliated columnar epithelium. As the tubes diminish in size the coats become thinner and the cartilages smaller; in tubes about 1 mm. in diameter the cartilages and mucous glands are absent.

By repeated branching the =interlobular bronchi= are formed, and from these arise the =lobular bronchioles=. The latter enter a lobule and branch within it, forming the =respiratory bronchioles=; these give off the =alveolar ducts=, the walls of which are pouched out to form hemispherical diverticula, the =alveoli= or =air-cells=.

A =lobule= of the lung, the unit of lung structure, is made up of a lobular bronchiole with its branches, and their air-cells, blood- and lymph vessels, and nerves. Between the lobules is the =interlobular tissue=, which forms the supporting framework of the lung.

=Vessels of the Lungs.=—The branches of the =pulmonary artery= carry venous blood to the lungs. They accompany the bronchi, and form rich capillary plexuses on the walls of the alveoli. Here the blood is arterialized, and is returned to the heart by the =pulmonary veins=. The =bronchial arteries= are relatively small vessels which carry arterial blood for the nutrition of the lungs. The branches of these arteries accompany the bronchial ramifications as far as the alveolar ducts, but do not extend to the alveoli. The œsophageal artery also supplies pulmonary branches (in the horse) which reach the lung by way of the ligament of the latter. These branches vary much in size, and ramify chiefly in the subpleural tissue of the basal part of the lung.[131] The =lymph vessels= are numerous, and are arranged in two sets. The superficial set forms close networks in and under the pleura, while the deep set accompanies the bronchi and pulmonary vessels. All converge to the root of the lung and enter the =bronchial lymph glands=.

=Nerve-supply.=—The pulmonary nerves come from the vagus and sympathetic nerves. They enter at the hilus and supply branches to the bronchial arteries and the air-tubes.

THE THYROID GLAND

The =thyroid gland= (Glandula thyreoidea) is a very vascular ductless gland, situated on the trachea close to the larynx. It is red brown in color, and consists of two =lateral lobes= and a connecting =isthmus= (Fig. 354).

The =lateral lobes= are situated on either side of the first and second or second and third rings of the trachea, to which they are loosely attached. Each is about the size of a walnut, and has a convex superficial face which is related to the parotid gland and the omo-hyoid muscle, and a slightly concave, deep face, applied to the trachea.

The =isthmus= in the adult horse is usually very rudimentary. It may occur as a very narrow glandular band which connects the posterior extremities of the lateral lobes, extending across the ventral surface of the trachea, but it is frequently only a small strand of connective tissue, and sometimes is entirely absent. It may be represented by a small tail-like process of one lobe.

The lateral lobes vary in size and position, and are often dissimilar
on the two sides. The anterior extremity is the larger; it is rounded
and usually lies about a finger’s breadth behind the larynx, but may
be in contact with it. The posterior extremity is often produced to
form a narrow tail-like process, which is continuous with the isthmus
when one is present. In the ass and mule the isthmus is usually well
developed. There is sometimes a narrow band of thyroid tissue which
extends forward from the isthmus for a variable distance; this is
termed the pyramidal lobe. The position of the lateral lobe is
indicated approximately by the junction of the external maxillary and
jugular veins.

=Structure.=—The gland is enveloped by a thin, fibrous =capsule=, from which =trabeculæ= pass into the gland, dividing it into =lobules=. The lobules consist of non-communicating =alveoli= or =follicles= of varying form and size. The alveoli are lined by cubical epithelium, and contain a viscid colloid substance.

=Blood-supply.=—The arteries are relatively very large; there are usually two thyroid arteries, which arise from the carotid and enter each extremity of the gland.

=Nerve-supply.=—The nerves are derived from the sympathetic system.

Nodules of thyroid tissue of variable size, sometimes as large as a pea, may be found near the anterior extremity of the lateral lobes, on the course of the anterior thyroid artery or on the trachea, even at a considerable distance from the thyroid region. They are termed =accessory thyroids= (Glandulæ thyreoidæ accessoriæ).

The =parathyroids= are small glandular bodies found in the thyroid region. They are paler and not so dense as the thyroid tissue, but often cannot be differentiated by their naked-eye appearance. In the horse there is usually only one on each side in the connective tissue over the anterior extremity of the lateral lobe of the thyroid. They resemble in structure embryonic thyroid tissue.

THE THYMUS GLAND

The =thymus= is a ductless gland which has a close resemblance to the lymphoid tissues. It is well developed only in late fœtal life and for a few months after birth. After this it undergoes rapid atrophy, fatty infiltration, and amyloid degeneration, so that in the adult it is represented by a thin remnant in the lower part of the anterior mediastinum or has entirely disappeared. In the new-born foal it is of a grayish-pink color, and consists of two =lateral lobes= (Fig. 358). The greater part of the gland is situated in the anterior mediastinum, but the two lobes are continued into the neck by a chain of lobules which lie along the course of the carotid artery, extending sometimes as far as the thyroid gland.[132]

=Structure.=—The thymus is inclosed in a thin, connective-tissue =capsule=, and consists of secondary =lobules= (Lobuli thymi), of varying shape and size, held together by areolar tissue and fat, and all attached to a cord of adenoid tissue termed the =tractus centralis=. These lobules are subdivided into polyhedral primary lobules, which are composed of lymphoid tissue, and present a dark vascular =cortex=, and a lighter =medulla= that contains Hassal’s corpuscles.

=Blood-supply.=—Internal thoracic and carotid arteries.

=Nerve-supply.=—Sympathetic and vagus nerve.

RESPIRATORY SYSTEM OF THE OX

THE NASAL CAVITY

The =nostrils=, situated on either side of the muzzle, are relatively small, and are much less dilatable than those of the horse. The alæ are thick and firm. The superior commissure is narrow. There is no false nostril or diverticulum nasi. The skin is smooth, bare, and moist, and there is no clear line of demarcation between it and the nasal mucosa. The opening of the naso-lacrimal duct is not visible, as it is on the outer wall of the nostril, and is concealed by the cartilaginous prolongation of the inferior turbinal. There are two =parietal= or =lateral cartilages= on either side, united by fibrous tissue. The dorsal cartilages are thin laminæ which curve outward and downward from the dorsal margin of the septal cartilage; they are prolongations of the nasal bones, and carry the alar cartilages on their anterior extremities. The ventral pair are lateral continuations of the basal lamellæ of the inferior turbinals. They lie along the nasal processes of the premaxillæ; anteriorly, they become thicker, turn a little upward, and each blends with the cartilaginous prolongation of the upper coil of the inferior turbinal. The =alar cartilages= are of peculiar form. The lamina is oblong, and curves outward and downward from the anterior extremity of the dorsal parietal cartilage, with which it is connected. The cornu springs from the outer part of the lamina and curves outward and upward into the outer wing of the nostril; it carries on its extremity a small transverse bar, thus having some resemblance to the fluke of an anchor.

The =nasal cavity= is short, wide anteriorly, narrow behind. It is not completely divided by the septum, which in its posterior third is separated from the floor of the cavity by an interval that increases from before backward. The middle meatus is very narrow, and divides posteriorly into two branches; the upper division leads to the ethmoidal meatuses, and communicates with the frontal sinus and the cavity of the superior turbinal. Anterior to the division are communications with the upper cavity of the ventral turbinal and the maxillary sinus. The main facts in regard to the sinuses were stated in the osteology.

FIG. 364.—MUZZLE OF OX.

_49_, Naso-labial region; _45_, lower lip. (After Ellenberger-Baum,
Anat. für Künstler.)
]

THE LARYNX

The =larynx= is more compact than in the horse, and presents numerous differential features. The =cricoid cartilage= is compressed laterally. The lamina is not distinctly marked off from the arch; it slopes downward and backward and has a large median ridge. The =thyroid cartilage= is complete ventrally, _i. e._, the laminæ are united throughout to form a long plate which is notched in front and behind; the laryngeal prominence is small and is situated posteriorly. The height and thickness of the cartilage increase from before backward. The posterior cornua are about an inch long, and form syndesmoses with the cricoid cartilage; the short anterior cornua unite similarly with the hyoid bone. The =epiglottis= is oval in outline and its apex is rounded; its base rests on the thyro-hyoid membrane, to which it is rather loosely attached. The cuneiform cartilages are absent. The =arytenoid cartilages= present only slight differences. The muscular process is well developed, and the vocal process is narrow and long. The =vocal cords= project very little from the wall, so that the rima glottidis is wide; the vocal ligament is attached ventrally to the crico-thyroid membrane; it has the form of a half tube, open behind. The =lateral ventricles= are extremely shallow, and the saccules are absent. The thyro-arytenoideus muscle is not divided; it is fan-shaped, thick, and narrow dorsally; ventrally, it is attached to the base of the epiglottis, the angle of the union of the thyroid laminæ, and the crico-thyroid membrane. The hyo-epiglotticus muscle is large and bifid.

THE TRACHEA

The caliber of the trachea is relatively small, its width being about an inch and a half (ca. 4 cm.) and its height one and a half to two inches (ca. 4 to 5 cm.). The rings are smaller, and number about fifty. In the neck their free ends are separated by a varying interval, so that the tube is flattened and membranous dorsally. In the thorax the ends are in apposition, so that they form a ridge dorsally. The trachea is adherent to the right lung from the second rib backward. The bifurcation is opposite the fourth intercostal space.

FIG. 365.—SAGITTAL SECTION OF HEAD OF COW.

_1_, Cerebral hemisphere; _2_, lateral ventricle; _3_, thalamus; _4_,
corpora quadrigemina; _5_, optic chiasma; _6_, pituitary body; _7_,
pons; _8_, medulla oblongata; _9_, cerebellum; _10_, spinal cord;
_11_, ventral straight muscles; _12_, pharyngeal lymph gland; _13_,
arytenoid cartilage; _14_, epiglottis; _15_, cricoid cartilage;
_16_, vocal cord; _17_, palatine sinus; _18_, soft palate; _19_,
Eustachian opening; _C_{1}_, _C_{2}_, _C_{3}_, first, second, and
third cervical vertebræ; _B.o._, basioccipital; _Sp._, presphenoid.
]

THE BRONCHI

There are three chief bronchi. The bronchus for the apical and the anterior cardiac lobe of the right lung is detached from the trachea at a considerable distance anterior to the bifurcation.

THE THORACIC CAVITY AND PLEURA

The thoracic cavity is relatively small; it is especially short dorsally, and is diminished laterally by the mode of attachment of the diaphragm to the ribs (_vide_ Myology). The endothoracic fascia is better developed than in the horse, and is distinctly elastic. The pleura is also thick, and there are no perforations of the mediastinum. The pleural sacs are more unequal in extent than in the horse, and the ventral part of the mediastinum is further to the left; in front of the pericardium it lies for the most part on the left wall of the chest.

FIG. 366.—CROSS-SECTION OF THORAX OF SHEEP, PASSING THROUGH MIDDLE OF
FIFTH PAIR OF RIBS, POSTERIOR VIEW.

_1_, _1′_, Lungs; _2_, _2′_, bronchi; _3_, _3′_, pulmonary arteries;
_4_, thoracic aorta; _5_, œsophagus; _6_, lymph gland; _7_, vena
hemiazygos; _8_, posterior vena cava; _9_, large pulmonary vein;
_10_, left atrium, _11_, vena hemiazygos; _12_, left ventricle;
_13_, right ventricle; _14_, sternum; _15_, internal thoracic
vessels; _16_, sixth thoracic vertebra.
]

THE LUNGS

The difference in size between the two lungs is greater than in the horse, the right lung weighing about half as much again as the left one. The average weight of the lungs is about 7½ pounds (ca. 3 to 4 kg.); they form about ¹⁄₁₇₀ of the body-weight.

FIG. 367.—RIGHT LUNG OF OX, COSTAL SURFACE.

Organ hardened _in situ_.
]

FIG. 368.—RIGHT LUNG OF OX, MEDIASTINAL ASPECT.

Hardened _in situ_. _1_, Left bronchus; _2_, pulmonary veins; _3_,
pulmonary artery.
]

They are divided into lobes by deep =fissures= (Incisuræ interlobares). The =left lung= is divided into =three lobes=, named from before backward =apical=, =cardiac=, and =diaphragmatic=. The =right lung= has =four= or =five lobes=; the =apical lobe= is much larger than that of the left lung, and occupies the space in front of the pericardium, pushing the mediastinum against the left wall;[133] the =mediastinal lobe= resembles that of the horse, while the =cardiac lobe= is often divided by a fissure into two parts. The apical lobe of the right lung receives a special bronchus from the trachea opposite the third rib. The right lung is adherent to the trachea from the second rib backward.

FIG. 369.—LUNGS AND HEART OF SHEEP, VENTRAL VIEW.

Specimen hardened _in situ_. Space between heart and lungs was
occupied by pericardium and fat.
]

The interlobar fissures begin at the ventral margin of the lung and
pass toward the root. Those of the left lung lie opposite to the
fourth and sixth ribs. The diaphragmatic lobe is the largest, and has
the form of a three-sided pyramid with its base resting on the
diaphragm. The cardiac lobe is prismatic and forms the posterior
margin of the cardiac notch; its long axis corresponds to the fifth
rib. The apical lobe of the left lung is small and pointed; its
ventral margin lies on the pulmonary artery and the anterior aorta;
below this the space in front of the heart is occupied by the apical
lobe of the right lung. The cardiac lobe of the right lung is much
larger than that of the left lung, and covers the right face of the
pericardium, so that the latter has no contact with the lateral wall
of the thorax.

The =lobulation= is very distinct on account of the extremely large amount of interlobular tissue.

In the =sheep= the costal attachment of the diaphragm resembles that of the horse, and the basal part of the lung corresponds thereto. The lobation of the lungs resembles that of the ox, but the lobulation is very indistinct. The pleural sacs extend under the first lumbar vertebra.

THE THYROID GLAND

The lobes of the thyroid gland are more extensive and paler in color than in the horse; they extend further forward, overlapping the crico-pharyngeus muscle to a small extent, and come in contact dorsally with the œsophagus. They are flattened, and have an irregular, lobulated surface. In the young subject the isthmus is well developed, being about half an inch (ca. 1 to 1.5 cm.) in width.

Accessory thyroids may be found, and parathyroids also occur near the posterior extremity or inner surface of the lateral lobes of the thyroid.

FIG. 370.—RIGHT LUNG OF SHEEP, MEDIASTINAL ASPECT.

Hardened _in situ_. _B_, Bronchus; _P.a._, pulmonary artery; _P.v._,
pulmonary vein; _V.c._, posterior vena cava; _L.g._, bronchial lymph
gland.
]

THE THYMUS GLAND

The thymus is pale and distinctly lobulated. It is much larger in the calf than in the foal, weighing at five or six weeks about 15 to 25 ounces (ca. 425 to 600 grams). The cervical part is specially large, thick, and compact; it extends to the thyroid gland. It undergoes atrophy slowly and remnants of the thoracic part often remain even in advanced age.

RESPIRATORY SYSTEM OF THE PIG

THE NASAL CAVITY

The =nostrils= are small, and are situated on the flat anterior surface of the =rostrum= or snout (Rostrum suis). The latter is a short cylindrical projection, with which the upper lip is fused, and is circumscribed by a prominent circular margin. The skin on the snout is thin and highly sensitive; it presents small pores, and scattered over it are fine short hairs. In the snout between the nostrils is the =os rostri=, which is to be regarded as a special development of the extremity of the septum nasi in adaptation to the habit of rooting. A plate of cartilage, representing the lamina of the alar cartilage of the horse, curves outward and downward from the upper part of the os rostri and a pointed bar of cartilage curves upward from the lower part of the bone in the outer wing of the nostril. The notch between the nasal bone and the premaxilla is closed in by parietal cartilages which resemble those of the ox.

The =nasal cavity= is long and narrow. It is divided behind by a horizontal plate into an upper olfactory part, which leads to the ethmoidal meatuses, and a lower respiratory part, which is a direct continuation of the inferior meatus. The posterior part of the septum is membranous.

The =turbinal bones= resemble in general those of the ox. The =superior meatus= is exceedingly small. The =middle meatus= is a deep fissure between the two turbinals: it divides posteriorly into two branches; one of these extends upward and backward between the lateral mass of the ethmoid bone and the superior turbinal; the other widens and joins the inferior meatus. The opening between the maxillary sinus and the middle meatus lies in a plane through the last cheek teeth. The middle meatus is continuous with the space inclosed by the dorsal part of the inferior turbinal and with the cavity of the superior turbinal. The dorsal division of the middle meatus presents several openings into the frontal sinus. The =inferior meatus= is relatively roomy; it communicates with the space inclosed by the ventral coil of the inferior turbinal. The opening of the naso-lacrimal duct is found in the posterior part of the inferior meatus. The naso-palatine duct and the vomero-nasal organ (of Jacobson) resemble those of the ox.

THE LARYNX

The larynx is remarkable for its great length and mobility. The cartilages are more loosely attached to each other than in the other animals. The =cricoid cartilage= is thick and compressed laterally; its lamina is long and narrow; its arch is directed obliquely downward and backward. The =thyroid cartilage= is very long; its laminæ are united ventrally and form a median ridge. The anterior cornua being absent, there are no joints formed with the hyoid bone. The posterior cornua are broad, bent inward, and articulate with the cricoid cartilage. The =epiglottis= is relatively very large, and is more closely attached to the hyoid bone than to the rest of the larynx. The middle part of its base is turned forward, and rests on the thyro-hyoid membrane; it is closely connected with the body of the hyoid bone by the hyo-epiglottic ligament and the strong hyo-epiglottic muscle.[134] The apex of the =arytenoid cartilage= is very large, and is divided into two parts at its extremity; the inner part is fused with that of the opposite cartilage. There is a small =interarytenoid cartilage= in the transverse ligament. The =rima glottidis= is very narrow. The =vocal cords= are directed obliquely downward and backward; and each is pierced by a long, slit-like opening, which leads into the large laryngeal saccule. The thyro-arytenoid or vocal ligament is similarly divided into a larger anterior and a smaller posterior part. There is a middle ventricle near the base of the epiglottis. The thyro-arytenoid muscle is very strong and is undivided. The arytenoideus is very small. The crico-thyroid consists of two strata; the superficial layer corresponds to the muscle of the other animals; the deep part consists of transverse fibers.

TRACHEA AND BRONCHI

The trachea is circular in cross-section, and contains thirty or more rings which almost meet dorsally. A special bronchus is detached for the apical lobe of the right lung, as in the ox.

THE LUNGS (Figs. 324, 325)

The =right lung= has =four= or =five lobes=, according to whether the apical is subdivided or not. The =left lung= resembles that of the ox in form and lobation. The lobulation is distinct, but the interlobular septa are thinner than in the ox.

FIG. 371.—RIGHT LUNG OF PIG, MEDIASTINAL ASPECT.

Hardened _in situ_. _B_, Left bronchus; _V._, pulmonary veins; _A._,
pulmonary artery; _l_, lymph glands.
]

THE THYROID GLAND

The two lobes of the thyroid are long and flattened; they are situated close together, on the ventral surface of the trachea, extending as far forward as the cricoid cartilage. The gland is dark red in color.

THE THYMUS

The thymus is very large, extending to the larynx or even to the submaxillary space in young subjects.

RESPIRATORY SYSTEM OF THE DOG

THE NASAL CAVITY

The =nostrils= are situated on the muzzle, with which the upper lip blends. They are shaped somewhat like a comma, with the broad part next to the septum and the narrow part directed backward and outward. The skin around the nostrils is bare, usually black, and in health moist and cool. The muzzle is marked by a median furrow (philtrum) or a deep fissure in some breeds. The cartilaginous framework is formed essentially by the septal cartilage and the parietal cartilages which proceed from it. The septal cartilage projects beyond the premaxilla, and is much thickened at its extremity; it gives off from its upper and lower margins the two parietal cartilages, which curve laterally and toward each other. A grooved plate extends outward from the septal cartilage in the floor of the nostril, and another lamina supports the alar fold of the inferior turbinal bone.

The length of the =nasal cavity= varies greatly in different breeds, corresponding, of course, to the length of the face. The cavity is roomy, but is very largely occupied by the turbinals and the lateral masses of the ethmoid bone. The =middle meatus= is short and narrow, and divides posteriorly into two branches; the upper branch leads to the ethmoidal meatuses; the lower branch joins the inferior meatus. The =inferior meatus= is very small in its middle part, owing to the great development here of the inferior turbinal. The posterior part of the nasal cavity is divided by a horizontal plate, the lamina transversalis, into an upper olfactory part and a lower respiratory part. The sinuses have been described (_vide_ Osteology).

FIG. 372.—NASAL CARTILAGES OF DOG.

_a_, Upper parietal cartilage; _b_, lower parietal cartilage; _c_,
alar cartilage. (After Ellenberger, in Leisering’s Atlas.)
]

THE LARYNX

The larynx is relatively short. The lamina of the =cricoid cartilage= is wide; the arch is grooved laterally. The laminæ of the =thyroid cartilage= are high, but short; they unite ventrally to form the body, on which there is anteriorly a marked prominence, and posteriorly a deep notch. The oblique line on the outer surface of the lamina is prominent. There is a rounded notch (Fissura thyreoidea) below the short anterior cornua for the passage of the superior laryngeal nerve. The posterior cornu is strong, and has a rounded surface for articulation with the cricoid cartilage. The =arytenoid cartilages= are relatively small, and have between them a small =interarytenoid cartilage=. The =epiglottis= is quadrilateral; its lower part or stalk is narrow, fitting into the angle of the thyroid cartilage. The =cuneiform cartilages= are large and somewhat crescent-shaped; they are not blended with the epiglottis.

FIG. 373.—SAGITTAL SECTION OF NASAL REGION OF DOG.

_1_, Skin; _2_, parietal cartilage; _3_, floor of nasal cavity; _4_,
lamina transversalis; _5_, mucous membrane of hard palate; _6_,
upper lip; _7_, frontal bone; _8_, frontal sinus; _9_, cranial plate
of frontal bone; _10_, cranial cavity; _11_, dotted line indicating
anterior limit of olfactory region; _a_, superior turbinal; _b_,
inferior turbinal; _b′_, _b″_, inferior turbinal fold; _c_, _e_,
ethmoturbinals; _d_, cribriform plate of ethmoid; _f_, superior
meatus; _g_, middle meatus; _h_, inferior meatus; _i_,
naso-pharyngeal meatus. (After Ellenberger, in Leisering’s Atlas.)
]

The =false vocal cords= extend from the cuneiform cartilages to the thyroid. The =true vocal cords= are large and prominent. The entrance to the large lateral ventricle and saccule is a long slit parallel with the anterior margin of the true vocal cord.

The crico-thyroid muscle is thick. The hyo-epiglotticus is well developed, and is double at its hyoid attachment. The anterior part of the thyro-arytenoideus arises on the cuneiform cartilage. Hence Lesbre has suggested the name cuneo-arytenoideus for it.

THE TRACHEA AND BRONCHI

The trachea is very slightly flattened dorsally. It contains forty to forty-five =U=-shaped rings; the ends of the rings do not meet dorsally, so that here the wall of the trachea is membranous and is composed of a layer of transverse smooth muscle-fibers outside of the rings, the fibrous membrane, and the mucous membrane.

The stem bronchi diverge at an obtuse angle, and each divides into two branches before entering the lung. The anterior branch goes to the apical and cardiac lobes, the posterior one to the diaphragmatic lobe, supplying also the mediastinal lobe of the right lung. The anterior branch of the left bronchus crosses under the pulmonary artery.

THE LUNGS

The lungs (Figs. 342, 343) differ in shape from those of the horse and ox in conformity with the shape of the thorax, which is relatively very wide in the dog; the lateral thoracic walls are strongly curved, and the costal surface of the lungs is correspondingly convex. There is no cardiac notch on the left lung, although a small part of the pericardium is not covered by the lung at the lowest part of the fifth and sixth intercostal spaces. On the right side the cardiac notch is opposite the fourth and fifth intercostal spaces. Each lung is divided by deep fissures into =three lobes=—=apical=, =cardiac=, and =diaphragmatic=; the right lung has in addition a =mediastinal lobe=. The apical lobe of the right lung is considerably larger than that of the left. The latter is often more or less fused with the cardiac lobe. Variable accessory lobes may be found. On account of the small amount of interlobular tissue the lobulation is not distinct.

THE THYROID GLAND

The =lateral lobes= of the thyroid gland are long and narrow, and have a flattened, ellipsoidal form; they are placed on the lateral surfaces of the trachea near the larynx. The extremities are small, the posterior one often being pointed. The =isthmus= is inconstant and variable; in large dogs it usually has the form of a glandular band which may be nearly half an inch (ca. 1 cm.) wide; in small dogs it is usually absent.

=Accessory thyroids= are frequently present; three or four may be found on either side, as well as a median one near the hyoid bone.

The =parathyroids=, about the size of millet or hemp seed, are four in number usually. Two are placed on the deep face of the thyroid lobes; the others are placed laterally near the anterior extremity of the thyroid.

THE THYMUS

The thymus is relatively small, and is situated almost entirely in the thorax. The left lobe is much larger than the right, and extends back as far as the sixth rib at the time of its greatest size. According to Baum the gland increases in size during the first two weeks after birth, and atrophies rapidly in the next two or three months. Traces of it are present at two or three years of age, and may be found even in old subjects.

THE UROGENITAL SYSTEM

This system (Apparatus urogenitalis) includes two groups of organs, the urinary and the genital. The =urinary organs= elaborate and remove the chief excretory fluid, the urine. The =genital organs= serve for the formation, development, and expulsion of the products of the reproductive glands. In the higher vertebrates the two apparatus are independent except at the terminal part, which constitutes a urogenital tract, and includes the vulva in the female and the greater part of the urethra in the male.

THE URINARY ORGANS