THE SPLEEN

The bright red, elongated, and straplike spleen is oriented more or less vertically under the protection of the more caudal ribs on the left side (Figure 34–4/6). It follows the greater curvature of the stomach to which it is loosely attached by a gastrosplenic ligament that is sufficiently generous to make splenic torsion a relatively frequent mishap. Its parietal surface is in contact with the diaphragm. Its visceral surface is divided into two narrow strips by a long hilus: the cranial strip is related to the stomach, the caudal one to the intestines. The dorsal extremity extends into the space between the stomach, left kidney, and pancreas, but it is usually prevented from making direct contact with these organs by the interposition of fat. The ventral extremity may emerge below the left costal arch and, exceptionally, may even cross the abdomen to the right side; although its position is determined by the degree of fullness of the stomach, it never wholly leaves the protection of the ribs. Its sectioned surface is patterned by the presence of very prominent splenic corpuscles.

Figure 34–4 Stomach partially opened, caudoventral view, semischematic. 1, Fundus; 2, diverticulum; 3, esophagus; 4, nonglandular mucosa; 5, lesser curvature; 6, cardiac gland region; 7, region of proper gastric glands; 8, approximate position of median plane; 9, pyloric gland region; 10, torus pyloricus; 11, duodenum.

THE STOMACH

The stomach is of the simple type, presenting fundus, corpus, and a pyloric part (Figure 34–4/2). The first two are generally confined to the left side of the abdomen but may extend across the median plane when the stomach is grossly distended. They are cranially related to the liver and diaphragm. The pyloric part extends to the right and is also in contact with the liver. All parts are related caudally to various parts of the intestinal mass; the principal relation is to the ascending colic spiral. It is only when grossly distended that the stomach makes contact with the abdominal floor and, on the left, extends beyond the protection of the rib cage. A feature unique to the pig among domestic species is the presence of a conical diverticulum (Figure 34–5/2) projecting caudally from the fundus.

The interior displays a narrow nonglandular strip of mucosa that extends into the diverticulum and follows the lesser curvature for some distance below the cardia (Figure 34–5/1). The remainder of the mucosa is divided into the usual three glandular regions, which are more clearly distinguished by color than in most species, although their borders are not always sharply defined (Figure 34–5/2a, 2b, 2c). A second feature of distinction is the very prominent torus narrowing the pyloric canal at the exit into the duodenum (Figure 34–4/10).

Figure 34–5 The stomach laid open (cardia to the right). 1, Nonglandular region; 2a, region with cardiac glands; 2b, region with proper gastric glands; 2c, region with pyloric glands.

Although the omenta are arranged much as in the dog, the greater one is less extravagantly developed, does not intervene between the intestines and the abdominal floor, and is therefore not encountered when the abdomen is first opened (Figure 33–4/5).

THE SMALL INTESTINE

The duodenum is also arranged rather like that of the dog, descending toward the pelvis before turning to run forward to the left of the root of the mesentery before dipping ventrally to be continued by the jejunum (Figure 34–7/1,2,3). It is entered by the bile duct about 3 cm beyond the pylorus and by the single (accessory) pancreatic duct about 10 cm farther on. Both openings are raised on papillae.

Figure 34–7 The development of the ascending colon, left lateral view. 1, Descending duodenum; 2, caudal flexure of duodenum; 3, jejunum; 4, ileum; 5, cecum; 6, ascending colon; 7, transverse colon; 8, descending colon; 9, descending mesocolon; 10, mesoduodenum; 11, mesentery.

The jejunum is arranged in many small loops (Figure 34–9/4) suspended by a mesentery that gives them much freedom of position (Figure 34–7/11). The greater part lies in the right half of the abdomen, ventrally and toward the pelvis, but some part may be in contact with the left flank behind the colic spiral. Like many other abdominal organs, the jejunum must accommodate its position to the condition of the stomach and, in sows, to that of the uterus.

Figure 34–9 Abdominal and thoracic viscera, right lateral view. 1, Wing of ilium; 2, uterine horns; 3, bladder; 4, jejunum; 5, right kidney; 6, last rib; 7, 8, right lateral and medial lobes of liver; 9, heart in pericardium; 10, diaphragm, cut; 11-13, caudal, middle, and cranial lobes of right lung.

THE LARGE INTESTINE

The large intestine is capacious and, like that of the horse, is much sacculated, being drawn into a series of pouches by two (on the colon) or three (on the cecum) teniae that run along its length. The peculiar disposition presented by the cecum and ascending colon in this animal, unique among domestic species, results from the greater than 360° rotation performed by the loop of bowel that it is herniated into the umbilical cord early in development (Figures 3–64 and 3–65). This carries the caudal limb of the loop, including the cecocolic junction, to the left of the mesenteric axis, where it remains throughout later development and into adult life. The ascending colon thus commences on the left side and only gains its usual continuation into the transverse colon on the right side of the abdomen in consequence of the reversal of course described below.

The cecum and colon must be considered together as they combine in a conical, ventrally tapering mass suspended from the roof of the abdomen (Figure 34–6). The cecum, which has a capacity of about 2 L, has its origin below the left kidney and extends ventrally or caudoventrally against the left flank to its rounded, blind apex. The ascending colon is arranged around its mesentery in a cone that points ventrally to reach the abdominal floor (with some deviation possible in any direction) (Figure 34–7). The outer part of the cone is provided by the wide, sacculated portion continuing from the cecum; when viewed from above, it spirals ventrally, clockwise and centripetally, before reversing course at the apex of the cone to ascend in narrower, smoother, and tighter centrifugal coils concealed within the center of the cone. These carry it dorsally to emerge from the base of the cone, pass to the right of the root of the mesentery, and continue as the transverse colon. The cecocolic mass mainly occupies the middle third of the left side of the abdomen, leaving the caudal and right regions available to the jejunum. However, variation is common and, especially where the jejunum is concerned, can be considerable. There is little notable about the remainder of the large intestine beyond the existence of a rectal ampulla.

Figure 34–6 The large intestine, schematic view from the right side. 1, Aorta; 2, caudal mesenteric artery; 3, cranial mesenteric artery; 4, celiac artery; 5, ileum; 6, cecum; 7, ascending colon; 8, transverse colon; 9, descending colon; 10, rectum.

THE LIVER

The liver resembles that of the dog in position and lobation. It is divided by deep fissures into left lateral and medial lobes and right medial and lateral major lobes, supplemented by a smaller quadrate lobe and caudate process (Figure 34–8).

Figure 34–8 Visceral surface of the liver. 1, Left lateral lobe; 2, left medial lobe; 3, right lateral lobe; 4, right medial lobe; 5, quadrate lobe; 6, caudate process; 7, porta; 8, gallbladder; 9, approximate position of median plane; 10, caudal vena cava.

The gallbladder is situated between the quadrate and right medial lobes. Apart from its ventral margin, the liver lies under the protection of the ribs (Figure 33–4/3); the somewhat larger part is situated to the right of the median plane (Figure 34–8; see also Figure 3–53, B). The cranial surface is shaped to the diaphragm, and the caudal surface is indented by the stomach and duodenum; other contacts with the pancreas, jejunum, and colon leave less distinct or no impressions.

The two most notable features of the liver of this species are the lack of contact with (and molding by) the right kidney and the very well-developed fibrous tissue framework that prominently outlines the hepatic lobules on the surface and in section (Figure 3–52, A-B). The latter feature is relevant to the clinician because surgery is required if a biopsy is indicated (aspiration is impossible with so fibrous a tissue) and is also relevant to the producer because it limits the price the consumer can be charged for a not very palatable foodstuff.

THE PANCREAS

The pancreas is related to the abdominal roof, largely on the left side. It is related ventrally to the gastric fundus, the spleen, and the left kidney (through fat) and, on the right, follows the duodenum. Other contacts are with the liver and right kidney. As happens in most mammals, it is penetrated by the portal vein traveling to the liver.

THE KIDNEYS

The shape of the pig’s kidneys is very distinctive. They are flattened (see Figure 5–21, C) against the abdominal roof (within a fatty capsule), extending between the level of the last rib to that of the fourth lumbar vertebra (Figure 34–9/5). This symmetry of position is most unusual and deprives the right kidney of the expected contact with the liver. The left kidney is related ventrally to the colic spiral, the cecum, and the pancreas; the right one is related to the descending duodenum and also possibly to the pancreas.

The internal structure resembles that of the human kidney (Figure 34–10). A central cavity with two recesses (major calices) directed toward the poles comprise the pelvis, which extends about a dozen minor calices, each embracing a renal papilla through which the papillary ducts discharge urine. The papillae correspond to renal pyramids, and because the number of these is reduced by fusions in the course of development, there is some inequality in the size of the units presented by the mature organ.

Figure 34–10 Kidney sectioned through poles and hilus. 1, Cortex; 2, medulla; 3, papilla; 4, pelvis; 5, ureter; 6, renal artery; 7, renal vein.