Part 49

_a_, Ocular muscles; _b_, pterygoideus internus (cut); _c_, stylo-glossus; _d_, digastricus (cut); _e_, genio-glossus; _f_, genio-hyoideus; _g_, hyo-glossus; _h_, thyro-pharyngeus; _i_, crico-pharyngeus; _k_, thyro-hyoideus; _l_, zygomatic process of temporal (sawn off); _1_, parotid gland; _2_, submaxillary gland; _3_, posterior part of sublingual gland; _3′_, anterior part of same; _4_, submaxillary duct; _5_, ductus sublingualis major; _6_, palatine glands; _7_, orbital or zygomatic gland; _8_, ducts of _7_; _9_, lacrimal gland. (Ducts colored red.) (After Ellenberger, in Leisering’s Atlas.) ]

The average periods of eruption are given below.

Tooth Eruption │ Change │ I_{1} 4 to 5 weeks │4 to 5 months I_{2} 4 to 5 weeks │ „ I_{3} 4 weeks │ „ C 3 to 4 weeks │4 to 5 months P_{1} 4 to 5 months│5 to 6 months P_{2} 4 to 5 weeks │ „ P_{3} 3 to 4 weeks │ „ P_{4} 3 to 4 weeks │ „ M_{1} 4 months │ upper 5 to 6 │ M_{2} months │ lower 4 ½ to │ 5 months │ M_{3} 6 to 7 months│

THE SALIVARY GLANDS

The =parotid gland= is small. Its upper part is wide and is divided into two lobes by a deep notch into which the base of the ear is received. The lower end is small and overlaps the submaxillary gland. The =duct= leaves the gland at its anterior border, crosses the masseter muscle, and opens into the mouth opposite the third upper cheek tooth. Small accessory glands (Glandulæ parotidæ accessoriæ) are sometimes found along the course of the duct.

The =submaxillary gland= is usually larger than the parotid. In large dogs it is about two inches (ca. 5 cm.) long and an inch or more (ca. 3 cm.) wide. It is rounded in outline and of a pale yellow color. Its upper part is covered by the parotid, but it is otherwise superficial, and is palpable in the angle of junction of the jugular and external maxillary veins. The =duct= leaves the deep face of the gland, passes along the surface of the digastricus and stylo-glossus, and opens into the mouth near the frenum linguæ on a very indistinct papilla.

The =sublingual gland= is divided into two parts. The posterior part (Glandula sublingualis grandicanalaris) lies on the digastricus muscle in intimate relation with the submaxillary gland, but clearly separable from it after removal of the common fibrous capsule. It has a pointed anterior process. The =duct= (Ductus sublingualis major) accompanies the submaxillary duct, and usually opens beside it, but in some cases joins it. The anterior part (Glandula sublingualis parvicanalaris) is long and narrow; it lies on the stylo-glossus muscle. It has a number (8 to 12) of small ducts (Ductus sublinguales minores), some of which open directly into the mouth, while others join the large duct.

The =orbital= or =zygomatic gland= (Glandula zygomatica) represents the superior molar glands of herbivora. It is prismatic and is situated in the orbital region, between the zygomatic arch and the internal pterygoid muscle, in contact with the periorbita. It has four or five =ducts= (Ductus zygomatici) which open near the last upper cheek tooth; one of them (canal of Nuck) is almost as large as the parotid duct; the others are small.

THE PHARYNX

The fornix is narrow. The orifices of the Eustachian tubes are small and slit-like; the end of the tube causes a rounded projection of the mucous membrane (Torus tubarius). The œsophageal opening is relatively small and is encircled by a fold of the mucous membrane. The hyo-pharyngeus muscle is clearly divided into a kerato-pharyngeus and a chondro-pharyngeus.

THE ŒSOPHAGUS

The =œsophagus= is relatively wide and dilatable except at its origin. The constriction at its origin is termed the =isthmus œsophagi=. The cardiac end is slightly dilated; it joins the stomach under the tenth or eleventh thoracic vertebra, slightly to the left of the median plane. The muscular tissue is striated and consists mainly of two layers of spiral fibers which cross each other; near the cardia, however, the fibers are longitudinal and circular. There are mucous glands and lymph glands in the submucosa throughout.

THE STOMACH

The =stomach= is relatively large. Its capacity in a dog weighing about 40 pounds is about six to seven pints.

Colin estimates the average capacity at about 3 liters (ca. 6½ pints), with a range between 0.6 and 8 liters (ca. 1⅓ to 17½ pints). Neumayer gives the capacity as 100 to 250 c.c. per kilogram of body-weight (ca. 2.7 ounces per pound). The average capacity of the human stomach is estimated at 35 to 40 ounces—only about one-half of that of a dog of medium size.

When full it is irregularly pyriform. The left or cardiac part (Corpus ventriculi) is large and rounded, while the right or pyloric part (Pars pylorica) is small and cylindrical. When empty, or nearly so, the left sac is strongly contracted; the pyloric part is not materially affected by variations in the amount of ingesta.

The parietal surface of the full stomach is very extensive, strongly convex, and faces partly forward, but largely to the left. It is related to the liver, the left part of the diaphragm, and the left and ventral abdominal wall as far back as a transverse section through the second or third lumbar vertebra.

The visceral surface is much less extensive and is considerably flattened; it faces chiefly to the right, and is related to the intestine, pancreas, and left kidney.

The upper part of the lesser curvature is nearly straight and vertical, but the lower part forms a deep, narrow angle, due to the fact that the pyloric part is directed sharply forward and upward.

The greater curvature is nearly four times as long as the lesser curvature. In the full stomach it extends considerably behind the left costal arch (having carried the spleen backward); ventrally, it lies on the abdominal wall, almost midway from the xiphoid cartilage to the pubis.

The left extremity or fundus is large and rounded; it is the most dorsal part of the organ, and lies under the dorsal ends of the eleventh and twelfth ribs.

The pyloric extremity is small and is directed forward and somewhat upward; it lies usually about an inch or two (ca. 3.5 to 5 cm.) below the cardia, a little to the right of the median plane. It is related to the portal fissure of the liver and to the pancreas.

The cardia is situated about two to three inches (ca. 5 to 7 cm.) from the left extremity, and is oval; it lies a little to the left of the median plane, below the tenth or eleventh thoracic vertebra.

When empty, or nearly so, the stomach is separated from the ventral abdominal wall by the liver and small intestine, and the greater curvature extends back to the eleventh or twelfth rib. In this state there is not rarely a marked constriction between the right and left portions.

The longitudinal muscular fibers are found on the curvatures and on the pyloric part. The oblique fibers are arranged in two layers: the external layer is a continuation of the longitudinal fibers of the œsophagus onto the body and fundus. The circular layer covers the whole stomach except the fundus, and forms a pyloric sphincter and an antral sphincter. The internal oblique layer is arranged as in the horse, and forms a similar loop-like, cardiac sphincter.

Three regions of the mucous membrane exist. Cardiac glands are found in a very narrow pale zone around the cardiac opening, and also scattered along the lesser curvature. The fundus gland region has a thick, reddish-brown mucous membrane, which lines about two-thirds of the organ. The pyloric mucous membrane is thinner and pale; in the dead subject it is often stained by regurgitated bile.

THE INTESTINE

The intestine is short—about five or six times the length of the body.

The =small intestine= has an average length of about thirteen feet (ca. 4 meters). It occupies most of the abdominal cavity behind the liver and stomach. The duodenum begins at the portal fissure and passes backward and upward, at first on the visceral surface of the liver, then in contact with the right flank. Near the pelvis it turns inward and sharply forward, passes along the inner border of the left kidney, and joins the jejunum to the left of the root of the mesentery. The mesoduodenum is given off from the common mesentery; it is a relatively wide fold, except at the terminal part. Here there are two short folds, one of which joins the mesocolon; the other passes on to the terminal part of the colon. The first part of the mesoduodenum contains the right branch of the pancreas. Its root is blended with the mesocolon to form a common mesentery. The remainder of the small intestine forms numerous coils, and is attached by a wide mesentery to the sublumbar region. The =pancreatic= and =bile-ducts= open into the duodenum about two or three inches (ca. 5 to 8 cm.) from the pylorus; the =accessory pancreatic duct= opens an inch or two (ca. 2.5 to 5 cm.) further back. The mucous membrane has very long villi. The duodenal (Brunner’s) glands occur only near the pylorus. Peyer’s patches are numerous (about twenty in young subjects), and begin in the duodenum. They are usually elliptical in outline, but the last one is band-like, reaches to the end of the ileum and is four to sixteen inches (ca. 10 to 40 cm.) long in young dogs (Ellenberger). There is an ileo-cæcal valve.[117] The muscular coat is relatively thick.

The =large intestine= is two to two and a half feet (ca. 60 to 75 cm.) long. Its caliber is about the same as that of the small intestine, and it has neither longitudinal bands nor sacculations.

The =cæcum= is about three or four inches (ca. 8 to 10 cm.) long, and is twisted in a spiral fashion. The twist is maintained by the peritoneum, which attaches it to the ileum. It is situated usually against the middle of the upper part of the right flank, below the duodenum and the right branch of the pancreas.[118]

The =colon= is attached to the sublumbar region by a mesentery, the mesocolon. It presents three parts, which correspond to the ascending, transverse, and descending colon of man. The first or right part (Colon ascendens) is very short. It begins at the junction of the ileum and cæcum, and passes forward along the inner surface of the duodenum and the right branch of the pancreas till it reaches the pyloric part of the stomach; here it turns inward and crosses the median plane, forming the transverse part (Colon transversum). The third or left part (Colon descendens) passes backward in the sublumbar region along the inner border or ventral surface of the left kidney; then inclines toward the median plane and joins the rectum.[119] The caliber of the colon is about the same throughout. It has no bands nor sacculations.

The mucous membrane of the cæcum contains numerous solitary glands; they are found also in the first part of the colon.

The =rectum= is almost completely covered with peritoneum, the line of peritoneal reflection being under the second or third coccygeal vertebra. At the junction of the rectum and anus the mucous membrane has a stratified, squamous epithelium, and contains the =anal glands=. A small opening on either side leads into two lateral =anal sacs= (Sinus paranales); these are usually about the size of a hazelnut, and contain a dirty gray, fatty substance, which has a peculiar and very unpleasant odor. The skin which lines these pouches contains coil glands. Further back the skin contains large sebaceous glands and peculiar =circumanal glands=.

The retractor ani muscle is extensive. It arises from the shaft of the ilium, the pubis, and the symphysis pelvis, and passes upward and backward to end on the first coccygeal vertebræ and the sphincter ani externus. The retractors, together with the coccygei, form a sort of pelvic diaphragm, analogous to that of man.

THE LIVER

The liver is relatively large, weighing usually about 5 per cent. of the body-weight. It is divided into six or seven =lobes= by fissures which converge at the portal fissure. When the gland is examined in the soft condition, all the lobes are visible on the visceral surface, but usually only four on the parietal surface.

The =left lateral lobe= is the largest, and is oval in outline. The =left central lobe= is much smaller and is prismatic. The =right central lobe= is second in size, and presents a somewhat tongue-shaped quadrate lobe, marked off by the deep fissure in which the gall-bladder lies. The =right lateral lobe= is third in size, and is oval in outline. On its visceral surface is the large =caudate lobe=; this consists of two parts—on the right, the caudate process; on the left, the papillary process or lobe, both often being subdivided by fissures.

When hardened _in situ_ (with the stomach full), the gland presents the following characters:

The visceral surface is marked by a somewhat oblique ridge (corresponding to the lesser curvature of the stomach), which separates two deep cavities. The depression to the left of the ridge is occupied by the body and fundus of the stomach. The depression on the right is smaller, and is occupied by the pyloric part of the stomach, the origin of the duodenum, and the right branch of the pancreas. The gall-bladder is not visible.

When the stomach is empty and contracted, the visceral surface of the liver is strikingly different. There is then a shallow impression for the left part of the stomach on the left lobe, and a large convex area, related to the small intestine and a mass of omentum. The pyloric and duodenal impressions are not much changed. The gall-bladder is visible.

The portal fissure is very deep and might well be termed a fossa. It is bounded on the right (externally) by the caudate process, on the left (internally) by the papillary process or lobe, which is directed ventrally. The hepatic artery enters at the dorsal end and the portal vein centrally, while the bile-duct leaves at the ventral part. The left central lobe is not visible, and the right central one is largely concealed.

The parietal surface is extremely convex, being adapted to the diaphragm and the adjacent part of the ventral wall of the abdomen. On it all the lobes except the papillary are completely or partially visible. The gall-bladder is visible in the fissure between the quadrate and chief portions of the right central lobe.

The dorsal border presents a renal impression on its right portion. The posterior vena cava passes downward and forward at first in a deep groove on the caudate lobe, then largely embedded in the parietal surface of the right lateral lobe; it receives two large hepatic veins just before piercing the diaphragm.

The œsophageal notch is large, and is occupied on the right by the thick margin of the hiatus œsophageus. The left border is opposite to the ninth and tenth ribs; it is thin and is marked by numerous indentations. The caudate process extends usually a little beyond the last right rib, between the right kidney and the right branch of the pancreas. The ventral border lies on the ventral abdominal wall a variable distance behind the xiphoid cartilage.

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

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=,[120] 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.)