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. (Figs. , , , .)

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. 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.

The general arrangement of the great omentum has already been indicated. We may now trace its line of attachment, which would correspond to the mouth of the sac. Beginning at the ventral part of the great curvature of the stomach, the line passes to the ventral face of the pylorus, then crosses obliquely the first part of the duodenum to the point where the pancreas is adherent to it. Here it passes to the anterior face of the terminal transverse part of the great colon, runs along this transversely (from right to left), and continues for some ten or twelve inches (ca. 25 to 30 cm.) on the small colon. It then forms an acute angle, passes inward and forward along the small colon to the dorsal part of the hilus of the spleen, where it blends with the suspensory ligament of the latter, and forms a recess (Recessus lienalis) behind the saccus cæcus of the stomach. It now passes along the hilus of the spleen, and is continued to the great curvature of the stomach by the gastro-splenic omentum. It is convenient to regard the spleen as being intercalated in the left part of the great omentum; on this basis the gastro-splenic omentum would be that part of the great omentum which connects the hilus of the spleen with the great curvature of the stomach. The great omentum is relatively small in the horse, and is usually not visible when the abdomen is opened. It is generally folded up in the space between the visceral surface of the stomach and the intestine.

The lesser sac furnishes the peritoneal covering for: (1) the visceral surface of the stomach and a small area of the first curve of the duodenum; (2) a large part of the dorsal surface of the pancreas and portal vein; (3) a small part of the visceral surface of the liver above the attachment of the lesser omentum and the portal fossa; (4) the posterior vena cava, from the level of the epiploic foramen [of Winslow] to its passage through the diaphragm (in so far as it is not embedded); (5) the part of the parietal surface of the liver between the right and middle divisions of the coronary ligament; (6) the corresponding part of the diaphragm, and the right part of the right crus of the same; (7) part of the anterior surface of the terminal part of the great colon, and the origin of the small colon; (8) the left extremity of the pancreas (inconstant); (9) the spleen.

We may now trace the peritoneum in a longitudinal direction, beginning in front. It is reflected from the ventral abdominal wall and the diaphragm upon the liver, forming the ligaments and serous coat of the gland. It leaves the visceral surface of the liver as lesser omentum, and the crura of the diaphragm as the gastro-phrenic ligament, reaches the saccus cæcus and lesser curvature of the stomach and the first curve of the duodenum, covers these organs, and is continued by the great omentum.

On the left it passes from the left crus of the diaphragm and the left kidney to form the suspensory ligament of the spleen, clothes that organ, and leaves it to be continued by the great omentum.

On the right it passes from the right crus of the diaphragm and the dorsal border of the liver to the concave border of the duodenum, forming the gastro-pancreatic fold (second part of the mesoduodenum), and covering part of the dorsal surface of the pancreas. From the margin of the pancreas, the right kidney, and a small area of the sublumbar region behind the latter, it passes on to the base of the cæcum and the terminal part of the great colon. From these it passes on the right to the duodenum, forming the third part of the mesoduodenum. On the left it clothes part of the ventral surface and the outer border of the left kidney, from which it passes to the base of the spleen, forming the ventral layer of the suspensory ligament of the latter. Behind the terminal part of the great colon it is reflected around the great mesenteric artery to form the great mesentery. Behind this it is reflected almost transversely from the roof of the cavity and from the origin of the small colon on to the duodenum, forming the terminal part of the mesoduodenum. The line of origin of the colic mesentery begins on the inner part of the ventral surface of the left kidney, and extends to the sacral promontory, where the mesorectum begins. At the termination of the latter the peritoneum is reflected from the rectum on the dorsal and lateral walls of the pelvic cavity. Below the rectum it forms the urogenital fold, and passes on to the dorsal surface of the bladder, covers its anterior part, and is reflected on to the body-wall laterally and ventrally, forming the lateral and middle ligaments of the bladder. In the female the broad ligaments of the uterus replace the urogenital fold, with which they are homologous.

In the new-born foal certain folds are specially large. The falciform ligament of the liver extends to the umbilical opening, and contains in its free edge the large umbilical vein. The bladder—at this time an abdominal organ—has a ventral median fold, which connects it and the urachus with the abdominal floor. This is flanked on either side by a fold which also extends to the umbilicus, and contains the large umbilical artery.