Part 44

The =anus= is the terminal orifice of the alimentary canal. It is situated below the root of the tail, where it forms a round projection, with a central depression when contracted. It is covered externally by an integument which is thin, hairless, and provided with numerous sebaceous and sweat glands. The mucous lining is pale, glandless, and covered with a thick, squamous, stratified epithelium.

There are three muscles of the anus.

1. The =sphincter ani internus= is merely a terminal thickening of the circular coat of the bowel.

2. The =sphincter ani externus= is a broad band of striped muscle-fibers outside the internal sphincter. Some fibers are attached to the coccygeal fascia above, others to the perineal fascia below. Its action is to close the anus.

3. The =retractor ani= (M. levator ani) is a flat muscle which lies between the rectum and the sacro-sciatic ligament. It arises from the superior ischiatic spine and the sacro-sciatic ligament, and ends under the external sphincter. Its action is to reduce the partial prolapse which the anus undergoes during defecation.

The suspensory ligament of the anus is a band of unstriped muscle which arises from the first coccygeal vertebra, passes downward over the retractor, and unites with its fellow below the anus. In the male it is largely continued by the retractor penis muscle; in the female it blends with the constrictor vulvæ. It may act as an accessory sphincter of the anus.

=Blood-supply.=—Internal pudic artery.

=Nerve-supply.=—Hæmorrhoidal and perineal nerves (for the sphincter ani externus and retractor ani).

THE PANCREAS

The =pancreas= is situated transversely on the dorsal wall of the abdomen, the greater part being to the right of the median plane. Its central part lies under the sixteenth and seventeenth thoracic vertebræ.

When fresh it has a reddish cream color, but if left in the unpreserved cadaver it rapidly decomposes and becomes dark. It resembles the salivary glands in appearance, but is softer, and its lobules are more loosely united. Its average weight is about twelve ounces (ca. 350 g.).

When hardened in situ its shape is very irregular. It is triangular in outline, and presents for description two surfaces, three borders, and three angles.[97]

The =dorsal surface= faces upward and forward. It is partially covered by peritoneum. It is related chiefly to the ventral surface of the right kidney and adrenal, the posterior vena cava, the portal vein, the cœliac artery and its divisions, the gastro-phrenic ligament and the saccus cæcus of the stomach, the right and caudate lobes of the liver, and the gastro-pancreatic fold. There are grooves for the divisions of the cœliac artery, and a large one for the splenic vein.

The =ventral surface= looks downward and backward; it is in general concave. It presents two impressions, separated by an oblique ridge. The smaller of these (Impressio cæcalis) lies to the right, and is caused by the pressure of the base of the cæcum; the larger one (Impressio colica) indicates the area of contact with the terminal part of the great colon and its junction with the small colon. It has usually no peritoneal covering except over a small area at the anterior angle.

The =right border= is nearly straight; it is related to the second part of the duodenum and the caudate lobe of the liver.

The =left border= is slightly concave, and is related to the first part of the duodenum, the left sac of the stomach, and the splenic vessels.

The =posterior border= presents a deep notch to the right of the median plane for the portal vein, which passes through the gland very obliquely. There is a thin bridge of gland tissue dorsal to the vein, thus forming the =portal ring= (Annulus portarum). The root of the great mesentery is in contact with the border just to the left of the median plane.

The =anterior= or =duodenal angle= (Caput pancreatis) is attached to the concavity of the second curve of the duodenum, and the adjacent part of the right lobe of the liver. The ducts leave at this extremity.

The =left= or =splenic angle= (Cauda pancreatis) corresponds to the tail of the pancreas in man. It is in contact with the base of the spleen, the left kidney and adrenal body, and the saccus cæcus of the stomach.

The =right angle= is rounded, and lies on the ventral surface of the right kidney and adrenal body.

The pancreas is attached dorsally by connective tissue to the kidneys and adrenal bodies, the gastro-phrenic ligament and the suspensory ligament of the spleen, the posterior vena cava, the portal fissure, and the gastro-pancreatic fold. The ventral surface is mainly attached by areolar tissue to the base of the cæcum and the terminal part of the great colon.

There are almost invariably two =ducts=. The large one is termed the =pancreatic duct= (Ductus pancreaticus [Wirsungi]). It is formed by the union of two radicles which come from the right and left extremities, and passes through the duodenal angle to end at the duodenal diverticulum alongside of the bile-duct. The duct is nearly half an inch (ca. 1 cm.) wide, and is very thin-walled. It is situated in the substance of the gland near its dorsal surface; none of it is free. The =accessory pancreatic duct= (Ductus pancreaticus accessorius [Santorini]) arises either from the chief duct or its left radicle, and ends on a papilla in the duodenum opposite the chief duct.

=Structure.=—The pancreas belongs to the class of tubulo-alveolar glands, the alveoli being long, like those of the duodenal glands; in other respects it resembles the serous salivary glands very closely. It has no proper capsule and the lobules are rather loosely united.

=Vessels and Nerves.=—The arteries of the pancreas come from the branches of the cœliac and anterior mesenteric arteries. The nerves are derived from the cœliac and mesenteric plexuses of the sympathetic.

THE LIVER

The =liver= (Hepar) is the largest gland in the body. It is situated obliquely on the abdominal surface of the diaphragm. Its highest point is at the level of the right kidney, its lowest on the left side, usually about three or four inches (ca. 8 to 10 cm.) from the abdominal floor, opposite the lower end of the seventh or eighth rib. The greater part of it lies to the right of the median plane.

It is red-brown in color and is rather friable. Its average weight is about ten to twelve pounds (ca. 5 kg.). When in the body, or if hardened _in situ_, it is strongly curved and accurately adapted to the abdominal surface of the diaphragm. When removed in the soft state, it flattens out into a cake-like form quite different from its natural configuration. It presents for description two surfaces and a circumference, which may be divided into four borders.

The =parietal surface= (Facies diaphragmatica) is strongly convex, and lies against the diaphragm. It faces chiefly upward and forward. It presents, just to the right of the median plane, a sagittal groove for the posterior vena cava (Fossa venæ cavæ). The vein is partially embedded in the substance of the gland, and receives the hepatic veins.

The =visceral surface= (Facies visceralis) faces in general downward and backward; it is concave and irregular, being moulded on the organs which lie against it. It presents, a little to the right of the median plane, the =portal fissure= (Porta hepatis). Through this the portal vein, hepatic artery, and hepatic plexus of nerves enter, and the hepatic duct and lymph vessels leave the liver. The portal or hepatic lymph glands are also found here. The pancreas is attached at and to the right of the fissure, and the gastro-hepatic omentum to the left of it. Above the fissure is a ridge which represents the caudate lobe (Lobus caudatus Spigelii), and is continued to the right by the pointed caudate process (Processus caudatus). Further to the left there is a large depression (Impressio gastrica) for the stomach. To the right of this may be seen a groove passing to the right and dorsally; this is the duodenal impression (Impressio duodenalis). Ventral to these is a large depression for the great colon (Impressio colica). Dorsal to this is a smaller depression for the blind end of the base of the cæcum.[98] Coils of the small intestine may also lie on this surface, and the apex of the spleen may reach to it when the stomach is empty.

The =dorsal border= (Margo obtusus) is thick for the most part. It presents from right to left: (1) a depression for the right kidney (Impressio renalis); (2) a notch, which is the dorsal end of the fossa venæ cavæ; (3) a deep notch (Impressio œsophagea) which is mainly occupied by the thick margin of the œsophageal opening of the diaphragm.

The =ventral border= is thin, and is marked by two deep =interlobar fissures= or =incisures= (Incisuræ interlobares), which partially divide the organ into three principal =lobes=—=right=, =middle=, and =left=. The right lobe is the largest, except in old subjects, in which it is frequently much atrophied. The middle lobe is the smallest. It is marked by several small fissures, and by the =umbilical fissure= (Incisura umbilicalis); the latter contains the umbilical vein in the fœtus, which is transformed into the round ligament after birth.

The =right border= is thin and long. It is nearly vertical, and extends backward to about the middle of the sixteenth rib.

The =left border= is thin and short. It extends backward to a point opposite the lower part of the ninth or tenth rib. The ventral and lateral borders together constitute the margo acutus.

The liver is held in position largely by the pressure of the other viscera and by its close application to the diaphragm. It has six ligaments.

1. The =coronary ligament= (Lig. coronarium hepatis) attaches it closely to the diaphragm. It consists of two laminæ. The right one is attached to the right of the fossa venæ cavæ; the left one begins to the left of the vena cava and passes upward and outward, becoming continuous with the left lateral ligament at the left margin of the œsophageal notch; it detaches a middle fold which extends to the notch and is continuous with the small omentum. The two laminæ unite below the vena cava to form the next ligament.

2. The =falciform ligament= (Lig. falciforme hepatis) is a crescentic fold which attaches the middle lobe to the sternal part of the diaphragm and to the abdominal floor for a variable distance. In its concave free edge is found.

3. The =round ligament= (Lig. teres hepatis), a fibrous cord which extends from the umbilical fissure to the umbilicus; it is the vestige of the umbilical vein, which in the fœtus carries the blood from the placenta to the liver.

4. The =right lateral ligament= (Lig. triangulare dextrum) is a wide fold which attaches the dorsal border of the right lobe to the costal part of the diaphragm.

5. The =left lateral ligament= (Lig. triangulare sinistrum) attaches the dorsal edge of the left lobe to the tendinous center of the diaphragm.

6. The =hepato-renal= or =caudate ligament= (Lig. hepatorenale) attaches the caudate process to the right kidney and the base of the cæcum. The gastro-hepatic omentum and the mesoduodenum have been described.

As stated above, the liver is divided by fissures into three principal lobes—right, middle, and left. The right lobe is the largest in the young subject and is irregularly quadrilateral in form. On its dorsal part is the caudate lobe, which ends in a pointed process directed outward, and assists in forming the cavity for the right kidney. The middle lobe is normally much the smallest. The left lobe is oval in outline and thickest centrally. In old or middle-aged subjects it often exceeds the right one in size. In some cases the atrophy of the right lobe is so extreme that the middle lobe may exceed it in size.[99]

The =hepatic duct= (Ductus hepaticus) is formed at the ventral part of the portal fissure by the union of right and left chief lobar ducts. It is two or three inches (ca. 5 to 8 cm.) long and about half an inch (ca. 1 to 1.5 cm.) wide. It passes between the two layers of the lesser omentum, and pierces the wall of the duodenum about five or six inches (ca. 12 to 15 cm.) from the pylorus, alongside of the pancreatic duct. The ducts pass obliquely through the wall of the duodenum for about half an inch (ca. 1 cm.) before opening into the diverticulum duodeni. The arrangement forms an effective valve, which prevents regurgitation from the intestine. There is no gall-bladder.

=Structure.=—The liver is covered by an outer serous, and an inner fibrous coat. The =serous coat= covers the gland except at the attachment of the pancreas and at the portal fossa; it is reflected from it to form the ligaments and the lesser omentum. The =fibrous capsule= is in general thin; it sends laminæ into the ligaments, and also delicate strands into the gland substance. At the portal fissure it is abundant and surrounds the vessels and ducts, which it accompanies in the portal canals of the gland substance.

The gland substance is composed of the parenchyma and the interstitial tissue. The =parenchyma= is made up of lobules, 1 to 2 mm. in diameter, which are held together by a small amount of interlobular connective tissue. On account of the very small amount of the latter, the lobulation of the horse’s liver is not usually at all distinct to the naked eye; for the same reason the organ is also quite friable.[100]

=Vessels and Nerves.=—The =portal vein= enters at the portal fissure. It conveys blood from the digestive tract and the spleen, which contains various products of digestion and numerous white blood-cells. The =hepatic artery= also enters at the portal fissure; it may be termed the nutrient vessel. All the blood is returned from the liver to the posterior vena cava by the hepatic veins. The portal vein and the hepatic artery both divide into interlobular branches, which run together in the portal canals of the interlobular tissue. The branches of the portal vein (Venæ interlobulares) give off intralobular branches which form plexuses of capillaries in the lobules and give rise to a central vein (Vena centralis). The branches of the hepatic artery (Rami arteriosi interlobulares) are of relatively small size. They supply mainly (if not exclusively) the interlobular tissue, the capsule, and the walls of the vessels and ducts. The =hepatic veins=[101] (Venæ hepaticæ) empty into the vena cava as it lies in the fossa of the gland. Their ultimate radicles are the central lobular veins, which emerge from the bases of the lobules and join the sublobular veins (Venæ sublobulares); the latter unite to form the hepatic veins. The largest hepatic veins, three or four in number, join the posterior vena cava just before it leaves the liver to pass through the diaphragm.

The nerve-supply comes from the =hepatic plexus=, composed of branches from the vagus and sympathetic nerves.

THE SPLEEN

The =spleen= (Lien) is the largest of the ductless glands.[102] It is situated chiefly in the left parachondrium, in close relation to the left part of the great curvature of the stomach, to which its long axis corresponds. Its size and weight vary greatly in different subjects, and also in the same subject under different conditions, depending chiefly on the great variability of the amount of blood contained in it. The average weight is about 35 ounces (ca. 1 kg.), its length about 20 inches (ca. 50 cm.), and its greatest width about 8 to 10 inches (ca. 20 to 25 cm.). It is usually bluish-red or somewhat purple in color. In the natural state it is soft and yielding, but not friable.

The weight appears to vary ordinarily from about one to eight pounds, although in large horses the latter figure even may be exceeded without any apparent evidence of disease. There does not seem to be any constant relation to the body-weight. For example, the spleen of a colt about ten months old weighed three and a half pounds, while it often weighs less than two pounds in horses weighing 1000 to 1200 pounds. The chief variation in outline consists of increase of width, especially of the dorsal part.

It extends obliquely in a curved direction from the left crus of the diaphragm and the saccus cæcus of the stomach to the ventral third of the ninth or tenth rib. It presents for description two surfaces, two borders, and two extremities.

The =parietal= or =external surface= (Facies parietalis) is convex, and lies chiefly against the diaphragm, but is in direct contact with the upper parts of the last two ribs and to a small extent with the flank at the lumbo-costal angle.

The =visceral= or =internal surface= (Facies visceralis) is in general concave. It is divided into two unequal portions by a longitudinal ridge; on this is a groove, the =hilus=, in which the vessels and nerves are situated. The area in front of the ridge (Facies gastrica) is moulded on the great curvature of the stomach; it is about two inches (ca. 5 cm.) wide. The area behind the ridge (Facies intestinalis) is much more extensive; it is related chiefly to the small colon, the small intestine, and the great omentum. It may be marked by one or two fissures.

The =anterior border= (Margo anterior) is concave and thin.

The =posterior border= (Margo posterior) is convex and thin.

The =base= or =dorsal extremity= is beveled, and fits into the interval between the left kidney and the left crus of the diaphragm and the psoas major. When hardened _in situ_, it shows an impression (Facies renalis) where it lies against the kidney. The left extremity of the pancreas touches it also. The anterior basal angle fits in between the saccus cæcus of the stomach and the left kidney; the posterior basal angle usually lies against the flank just behind the last rib.

The =apex= or =ventral extremity= is small; it lies—when the stomach is not full—between the left lobe of the liver and the left dorsal portion of the colon; when the stomach is full, the spleen is pushed further back and loses contact with the liver.

The spleen is attached by two peritoneal folds, the suspensory ligament and the gastro-splenic omentum. The =suspensory ligament= (Lig. suspensorium lienis) attaches the base to the left crus of the diaphragm and the left kidney; it contains a quantity of elastic tissue. The part which passes to the diaphragm is the ligamentum phrenico-lienale, and blends with the gastro-phrenic ligament; the part which goes to the kidney is termed the ligamentum renolienale. The =gastro-splenic omentum= (Lig. gastrolienale) passes from the hilus to the left part of the great curvature of the stomach. It is narrow above, where it joins the suspensory ligament; below it becomes much wider and is continuous with the great omentum.

Small globular or lenticular masses of splenic tissue may be found in the gastro-splenic omentum. They are termed =accessory spleens= (Lienes accessoriæ).

=Structure.=—The spleen has an almost complete =serous coat=. Subjacent to this and intimately united with it is a =capsule= of fibrous tissue (Tunica albuginea), which contains many elastic fibers and some unstriped muscular tissue. Numerous =trabeculæ= (Trabeculæ lienis) are given off from the deep face of the capsule and ramify in the substance of the organ to form a supporting network. In the interstices of this framework is the =spleen pulp= (Pulpa lienis), a dark red, soft, grumous material. This is supported by a delicate adenoid =reticulum=, and contains numerous leukocytes, the large splenic cells, red blood-corpuscles, and pigment. The pulp is richly supplied with blood. The branches of the splenic artery enter at the hilus and pass along the trabeculæ. The arteries which enter the pulp have a sheath of lymphoid tissue, which collects on the vessel wall at certain points, forming small =lymph nodules=, the so-called Malpighian corpuscles (Noduli lymphatici lienales). These are visible to the naked eye as white spots, about as large as the head of a pin. The blood passes into cavernous spaces lined by endothelium which is continuous with the cells of the reticulum of the pulp. From these the veins arise. The splenic vein runs in the hilus in company with the artery and nerves, and joins the posterior gastric vein to form a large radicle of the portal vein.

=Blood-supply.=—Splenic artery.

=Nerve-supply.=—Splenic plexus.

THE PERITONEUM

The general disposition of the peritoneum has been described, and other facts in regard to it were mentioned in the description of the viscera. It is now desirable to study it as a continuous whole.[103] (Figs. 256, 257, 278, 279.)

We may consider the peritoneum as consisting of two sacs—a greater and a lesser. The =greater sac= lines the greater part of the abdominal cavity, and covers most of the viscera which have a peritoneal investment. The =lesser sac= is an introversion or recess of the greater sac, formed during the development of the viscera. The two sacs communicate by a relatively narrow passage, termed the =epiploic foramen= of Winslow (Foramen epiploicum). This opening is situated on the visceral surface of the liver above the portal fissure. It can be entered by passing the finger along the caudate lobe of the liver toward its root. Its dorsal (or anterior) wall is formed by the caudate lobe and the posterior vena cava. Its ventral (or posterior) wall consists of the pancreas, the gastro-pancreatic fold, and the portal vein. The walls are normally in contact, and the passage merely a potential one. It is usually about four inches (ca. 10 cm.) in length. It is narrowest at the right extremity, where it is about an inch (ca. 2.5 to 3 cm.) wide.[104] If the finger is passed into the foramen from right to left, it enters the cavity of the lesser sac. If now an opening is made in the great omentum and the other hand introduced through it, the fingers of the two hands touch each other over the lesser curvature of the stomach. The formation and boundaries of the lesser sac should now be examined by spreading out the great omentum. It will be found that the latter now incloses a considerable cavity behind the stomach; this is termed the =omental cavity= (Bursa omentalis). Passing forward over the lesser curvature of the stomach, we enter another space, the =vestibule= of the omental cavity (Vestibulum bursæ omentalis). This space is closed on the left by the gastro-phrenic ligament, below and on the right by the lesser omentum, and dorsally by the gastro-pancreatic fold, which is attached to the dorsal border of the liver and to the posterior vena cava. Above the œsophageal notch the fingers can be passed around the border of the liver and the vena cava till the coronary ligament is encountered. Thus the vestibule is closed except (1) on the right, where it communicates with the cavity of the greater sac by the epiploic foramen; and (2) behind, where it communicates with the cavity of the omentum.