Part 140

a, a, Ridge; b, ventral end; c, dorsal end. (After Ellenberger, in Leisering’s Atlas.) ]

The =gall-bladder= lies in the fossa vesicæ felleæ, between the two parts of the right central lobe; it does not reach to the ventral border of the liver. The cystic duct joins the hepatic duct at the lower part of the portal fissure, forming with it the bile duct (Ductus choledochus); the latter passes to the right and opens into the duodenum, about two or three inches (ca. 5 to 8 cm.) from the pylorus.

Of the ligaments, the coronary and right lateral are well developed, but the left lateral and falciform are small.

THE PANCREAS

The =pancreas= is =V=-shaped, consisting of two long narrow branches, which meet at an acute angle above the pylorus. The =right branch= extends backward above the first part of the duodenum, below the caudate lobe of the liver and the right kidney, and ends usually a short distance behind the latter. The =left branch= passes inward and backward on the visceral surface of the stomach, and ends under the left kidney. There are two =ducts=: the larger one unites with the bile duct, while the smaller one enters the duodenum an inch or two (ca. 3 to 5 cm.) further back.

THE SPLEEN

FIG. 342.—PROJECTION OF VISCERA OF DOG, (MALE) ON BODY-WALL, LEFT SIDE.

A, C, D, Apical, cardiac, and diaphragmatic lobes of lung; P, pericardium; Pr, prostate. Costal attachment and median line of diaphragm are dotted. ]

FIG. 343.—PROJECTION OF VISCERA OF DOG (FEMALE) ON BODY-WALL, RIGHT SIDE.

A, C, D, Apical, cardiac, and diaphragmatic lobes of right lung; P., pericardium; L. K., right kidney; Pan., right branch of pancreas; O, ovary; U, cornu of uterus. Costal attachment and median line of diaphragm are dotted; also posterior contour of stomach. ]

The =spleen= is bright red in color in the fresh state. It is somewhat falciform, long, and narrow; the ventral part is the widest. Its position varies in accordance with the fullness of the stomach. When that organ is moderately full the long axis of the spleen corresponds to the direction of the last rib. Its parietal surface is convex and lies largely against the left flank. The visceral surface is concave in its length, and is marked by a longitudinal ridge, on which the vessels and nerves are situated. The dorsal end lies against the left kidney and left crus of the diaphragm. The ventral end is a little further back, and reaches to the ventral wall of the abdomen. It is so loosely attached by the great omentum as to be regarded as an appendage of the latter.

THE RESPIRATORY SYSTEM

The organs of respiration (Apparatus respiratorius) comprise the =nasal cavity=, the =pharynx=, the =larynx=, the =trachea=, the =bronchi=, and the =lungs=. The lungs are the central organs in which the exchange of gases between the blood and the air takes place; the other parts of the system are passages by which the air passes to and from the lungs. The nasal cavity opens externally at the nostrils, and communicates behind with the pharynx through the posterior nares (Choanæ); it contains the peripheral part of the olfactory apparatus, which mediates the sense of smell. The pharynx is a common passage for the air and food—a remnant of the primitive embryonic arrangement; it has been described as a part of the digestive tube. The larynx is a complex valvular apparatus which regulates the volume of air passing through the tract; it is also the chief organ of voice. The trachea, and the bronchi formed by its bifurcation, are permanently open conducting tubes. The thorax, the pleural sacs which it contains, and the muscles which increase or diminish the size of the cavity are also parts of the system. The bones, joints, and muscles of the thorax have already been described.

For topographic reasons two ductless glands, the =thyroid= and the =thymus=, are usually described in this section, although they are in no sense a part of the respiratory system.

RESPIRATORY SYSTEM OF THE HORSE

THE NASAL CAVITY

The =nasal cavity= (Cavum nasi), the first segment of the respiratory tract, is a long, somewhat cylindrical passage, inclosed by all the facial bones except the mandible and hyoid. It is separated from the mouth ventrally by the palate. It opens externally at the nostrils, and communicates posteriorly with the pharynx through the posterior nares.

The =nostrils= or =anterior nares= (Nares) are somewhat oval in outline, and are placed obliquely, so that they are closer together below than above. They are bounded by two =alæ= or =wings= (Alæ nasi), which meet above and below, forming the =commissures=. The outer ala is concave; the inner one is convex above, concave below. The upper commissure is narrow, the lower one wide and rounded. If the finger is passed into the nostril at the upper commissure, it enters the so-called =false nostril=, which leads backward to a blind cutaneous pouch. This cul-de-sac, the =diverticulum nasi=, extends backward to the angle of junction of the nasal bone and the nasal process of the premaxilla. The posterior part of the diverticulum is blind, but the false nostril communicates below and internally with the vestibule of the nasal cavity. The external opening of the =naso-lacrimal duct= is seen when the nostril is dilated; it is situated on the floor of the vestibule, perforating the skin close to its junction with the mucous membrane, about two inches (ca. 5 cm.) from the lower commissure. (It is not rare to find one or two accessory orifices further back.)

=Structure.=—The =skin= around the nostrils presents long tactile hairs as well as the ordinary ones. It is continued around the alæ and lines the vestibule. In the posterior part of the diverticulum the skin is thin and usually black, and is covered with exceedingly fine hairs; it is provided with numerous sebaceous glands. The inner wing is supported by the =alar cartilage= (Cartilago alaris), which is shaped somewhat like a comma, the convex margin being direct medially. The cartilages are attached by fibrous tissue to the extremity of the septal cartilage. Each consists of an upper, quadrilateral curved plate, the =lamina=, and a narrow =cornu= which curves downward and outward, supporting the inner wing and the lower commissure, but not entering into the formation of the outer wing. The lamina causes the projection of the upper part of the internal wing, from which the thick =alar fold= (Plica alaris) passes backward along the external wall of the nasal cavity to cover the cartilaginous prolongation of the inferior turbinal bone. This fold separates the true from the false nostril. The extremity of the cornu usually causes a slight projection of the skin a short distance behind and below the lower commissure.

FIG. 344.—NOSTRILS OF HORSE.

a, Diverticulum nasi (shown as if distended); b, philtrum; c, false nostril; d, true nostril; e′, prominence caused by lamina of alar cartilage; e″, prominence over cornu of same; f, external wing of nostril; g, internal wing; h, lower commissure; i, orifice of naso-lacrimal duct. (Ellenberger-Baum, Anat. d. Haustiere.) ]

FIG. 345.—CROSS-SECTION OF NARIAL REGION OF HORSE.

1, Nasal bone; 2, tendon of levator labii superioris proprius; 3, external nasal nerve; 4, nasal diverticulum; 5, outer wall of 4; 6, dilatator naris inferior; 7, levator nasolabialis + dilatator naris lateralis; 8, maxilla and premaxilla; 9, naso-lacrimal duct; 10, 11, chief branches of superior labial nerve, with superior labial artery at their inner side; 12, nasal cavity; 13, superior turbinal fold; 14, inferior turbinal fold, inclosing cartilaginous prolongation of inferior turbinal bone; 15, venous plexus; 16, septum nasi; 16′, parietal cartilage; 17, vomero-nasal organ (of Jacobson); 18, venous plexus of palate; 19, buccinator; 20, skin; 23, anterior nasal nerve; 24, palatine process of premaxilla. (After Ellenberger-Baum, Top. Anat. d. Pferdes.) ]

The muscles of the nostrils have been described.

=Blood-supply.=—Palato-labial, superior labial, and lateral nasal arteries.

=Nerve-supply.=—Trigeminal and facial nerves.

The nasal cavity is divided into two symmetrical halves by the median =septum nasi=. The osseous part of the septum (Septum osseum) is formed behind by the perpendicular plate of the ethmoid and below by the vomer. A few ridges on the former correspond to the ethmoidal meatuses. The major part of it, however, is formed by the =septal cartilage= (Cartilago septi nasi). The surfaces of the cartilage are marked by faint grooves for the vessels and nerves which course over it. The =dorsal border= is attached along the frontal and nasal sutures, and extends beyond the apices of the nasal bones about two inches (ca. 5 cm.). From this border a thin, narrow plate, the =parietal cartilage= (Cartilago parietalis) curves outward for a short distance on either side. Near the nostrils these plates are somewhat wider, partially making good the defect in the bony roof of the cavity in this situation. The =ventral border= is thick and rounded; it lies chiefly in the groove of the vomer, but anteriorly it occupies the space between the premaxillary bones. The =posterior border= is continuous with the perpendicular plate of the ethmoid bone. The alar cartilages are attached to the anterior extremity by fibrous tissue in such a manner as to allow very free movement—in fact, an actual joint may be found. The ventral part of the cartilage is about half an inch (ca. 1 cm.), the middle part about one-tenth of an inch (ca. 2.5 mm.), and the dorsal part about a quarter to a third of an inch (ca. 6 to 8 mm.) in thickness.

FIG. 346.—NASAL CARTILAGES OF HORSE, DORSAL VIEW. (After Ellenberger-Baum, Anat. für Künstler.) ]