The peritoneum, as has been shown, is a serous membrane which lines the cavity of the abdomen from the diaphragm to the pelvic floor, and invests or covers to a varying extent the viscera which that cavity contains. Viewed in its very simplest condition, it may be regarded as a closed sac, the inner surface of which is smooth, while the outer surface is rough and is attached to the tissues which surround it.

In the male subject the peritoneum forms actually a closed sac; but in the female its wall exhibits two minute punctures, which correspond to the openings of the Fallopian tubes. That part which lines the walls of the abdomen is termed the parietal peritoneum; that which is reflected on to the viscera is the visceral peritoneum. The disposition of the peritoneum may first be studied by noting its arrangement as made evident in transverse sections of the abdomen at certain levels.

The first section to be described shows the peritoneum in its simplest condition. This is a transverse section through the body, at about the level of the upper sur- face of the fourth lumbar vertebra, and therefore about the site of the umbilicus.

Starting on the inner surface of the anterior abdominal wall, the peritoneum is seen to cover the transversalis fascia, and indirectly the anterior abdominal muscles; then, passing to the left, it lines the side of the abdomen, until it reaches the descending colon. This it covers, as a rule, in front and on the sides, though occasionally it forms a mesocolon. Then it passes over the bodies of the vertebrae with the large vessels upon them, and leaves the back of the abdomen to run forward and enclose the small intestine, returning again to the spine. The two layers thus form the mesentery, having between them a middle layer [lamina mesenterii propria] containing the terminal branches of the superior mesenteric vessels. It then passes over the right half of the posterior abdominal wall, covering the ascending colon in front and at the sides only (unless there be a mesocolon), and then passes on to the side and front of the abdomen to the point from which it was first traced.

In tracing the peritoneum in a section of the body opposite the stomach, on a level with the first lumbar vertebra, its course becomes more complicated and difficult to follow.

In the section already given the peritoneum as a simple closed sac can be readily conceived; but at the level now exposed the serous membrane has been so introverted that there appear to be two sacs, one leading from the other, and known respectively as the greater and the lesser sac of the peritoneum. They communicate through the epiploic foramen (of Winslow). The lesser sac [bursa omentalis] is situated behind the stomach, so that on first opening the abdomen no trace of it is to be seen. It extends downward [recessus inferior] between the layers of the great omentum (though this part of the lesser sac is largely obliterated by adhesion in the adult). It extends upward [recessus superior] behind the caudate lobe of the liver. The vestibule [vestibulum bursae omentalis] is the portion which lies just behind the lesser omentum, and communicates with the greater sac through the epiploic foramen. In general, the lesser sac is Umited anteriorly by the liver, stomach, and omenta; posteriorii by the posterior abdominal wall, and below, behind the great omentum, by the transverse mesocolon.

The epiploic foramen (foramen of Winslow) is situated just below the liver; it looks toward the right, and will readily admit one or two fingers. It is bounded superiorly by the caudate lobe of the liver; inferiorly, by the duodenum (pars superior); posteriorly, by the vena cava; and anteriorly by the right margin of the gastro-hepatic or lesser omentum, containing the structures passing to and from the liver. Starting at the epiploic foramen, the lesser sac will be found to turn to the left.

 If, now, the peritoneum be viewed in a transverse section of the body at the level named, viz., through the first lumbar vertebra, it will be found that the section has probably passed through the epiploic foramen. Starting at the front of the abdomen and- going to the right, the peritoneum is seen to line the anterior abdominal wall, to pass over the side of the abdomen, and to cover the front of the right kidney; it then extends on to the vena cava, when it becomes a part of the lesser sac; then along the back of the lesser sac, over the aorta and pancreas, which separate it from the vertebral column ; next it reaches the anterior of the two internal surfaces of the spleen internal to the hilus. Here it meets with another layer of peritoneum, and helps to form the gastro-splenic ligament [lig. gastrolienale]. Leaving the spleen, it changes its direction forward and to the right, and runs to the stomach, forming the posterior, layer of the gastro-splenic ligament; it covers the posterior surface of the stomach, and leaves its mesial border (lesser curvature) to form the posterior layer of the lesser omentum, and then passes upward and to the right to the liver. In this transverse section it is only seen passing on the right margin of the lesser omentum, where it forms the anterior boundary of the epiploic foramen. Here it bends sharply around the omental margin enclosing the hepatic vessels continuing to the left as the anterior layer of the lesser omentum; and then passing to the left reaches the stomach, which it covers in front. It then forms the anterior layer of the gastro-splenic ligament, and once more reaches the spleen. It passes right around the spleen to the back of the hilus, where it is reflected on to the left kidney as the lieno-renal ligament. Hence the peritoneum passes along the side and front of the abdomen to the point from which it started. In this section the liver is so divided as to appear separated from all connection with the other viscera and the abdominal wall, and to be surrounded by peritoneum.

The course of the peritoneum in a longitudinal section of the body will now be considered. Starting at the umbilicus and passing downward, the peritoneum is seen to line the anterior abdominal wall. Before reaching the pelvis, it covers also the urachus, the deep epigastric arteries, and obliterated hypogastric arteries, which form ridges beneath it. For some little way above the pubis bone the peritoneum is loosely connected with the abdominal wall, a circumstance which is made use of in supra-pubic cystotomy. Moreover, as the distended bladder rises from the pelvis it can detach the serous membrane to some extent from the anterior abdominal wall. In extreme distension of the bladder the peritoneum may be lifted up for some 5 cm. vertically above the symphysis. On reaching the OS pubis it is reflected on to the upper part of the bladder, covering it as far back as the base of the trigone; thence it is reflected on to the rectum, which it covers in front and at the sides on its upper part, rarely forming a distinct mesorectum. Between the bladder and rectum it forms in the male the recto-vesical pouch. The mouth of this pouch is bounded on either side by a crescentic fold, the plica semilunaris. In the female the peritoneum is reflected from the bladder on to the uterus, which it covers; it then extends so far down in the pelvis as to pass over the upper part of the vagina behind; thence it extends to the rectum. The peritoneum which invests the uterus is reflected laterally to form the broad ligaments. The fold between the vagina and rectum forms the recto-vaginal pouch, or pouch of Douglas. The membrane has now been traced back to the spine.

Following it upward, the sigmoid colon will be found to be completely covered by peritoneum, a mesocolon attaching the gut to the abdominal wall. A little higher up in the median line the peritoneum passes forward, to enclose the small intestine, and, returning to the spine, forms the mesentery. It now passes over the third part of the duodenum to the pancreas, from which point it again passes forward to form the lower layer of the transverse mesocolon. It invests the transverse colon below and partly in front, and then leaves it to pass downward to take part in the great omentum. Running down- ward some distance, it returns and forms the anterior layer of the omentum. On reaching the stomach it goes over the anterior surface, and at the upper border forms the anterior layer of the lesser or gastro-hepatic omentum, which extends between the stomach and the liver. It invests the inferior surface of the liver in front of the transverse fissure, and, turning over its anterior border, covers the upper surface. At the posterior limit of the upper surface it leaves the liver and goes to the diaphragm, forming the superior layer of the coronary ligament. It covers the anterior part of the dome of the diaphragm, and, once more reaching the anterior abdominal wall, can be followed to the umbilicus, where it was first described. This completes the boundary of the greater sac. On reference to the diagram the student might be led to suppose that the two sacs are quite separate. This, of course, is not the case; but in a longitudinal section of the body made anywhere to the left of the epiploic foramen (foramen of Winslow), it is impossible to show the direct connection between the two sacs.

The peritoneum has only been traced in this longitudinal section so far as it concerns the greater sac. It now remains to follow upon the same section such part of the membrane as forms the lesser sac. The peritoneum here will be seen to cover the posterior surface of the stomach; thence from the lesser curvature it runs upward to the liver, forming the posterior layer of the lesser or gastro-hepatic omentum. It reaches the liver behind the transverse fissure. It covers only a part of its posterior surface (caudate lobe), and is reflected on to the diaphragm, forming the lower layer of the coronary ligament. It now goes down- ward over the posterior part of the dome of the diaphragm to the spine, separated from the latter by the great vessels. On reaching the anterior border of the pancreas it passes forward, and forms the upper layer of the transverse mesocolon. It then covers the upper half of the transverse colon, and, descending, forms the innermost layer of the great omentum. (The inner layers of the great omentum are usually fused in the adult, however, thus obliterating this portion of the lesser sac.) It now ascends, and, arriving at the greater curvature of the stomach, passes on to its posterior wall. At this point its description was commenced. The general relations of the greater and the lesser sac are also evident showing the lines along which the parietal peritoneum is reflected from the posterior abdominal wall as the visceral peritoneum, forming the various mesenteries and covering the various abdominal organs.

The precise manner in which certain organs - such as the liver, the caecum, the duodenum, and the kidneys - are invested by peritoneum is described in the accounts of those viscera. To such accounts the reader is referred for a description of the many 'ligaments' (such as those of the bladder and liver) which are formed by the peritoneum.

The great omentum

As is evident from its development, the great omentum [omentum majus] is formed of four layers of peritoneum, though this is quite impossible to demonstrate in an adult, the individual layers having become adherent.

The great omentum acts as an apron, protecting the intestines and providing them with a heat-economising covering of fat. It is nearly quadrilateral in shape, and is variable in extent. The great omentum is shown to be connected with the greater curvature of the stomach, on the one hand, and the transverse colon, on the other. Originally it extended backward above the transverse colon and mesocolon to the posterior abdominal wall.

Mr. Lockwood has made some investigations on the lengths of the transverse mesocolon and great omentum in thirty-three cases. In twenty, under the age of forty-five, only one subject had a great omentum long enough to be drawn beyond the pubic spine; in five, the omentum reached as far as the pubes. In the cases beyond forty-five years it was the exception rather than the rule to find an omentum which could not be pulled beyond the lower limits of the abdomen.

The lesser omentum [omentum minus] consists of a double layer of peritoneum extending between the stomach and the liver. If the two anterior layers of the great omentum are traced upward, they are seen to enclose the stomach, and then join together again at the lesser curvature to form the lesser omentum. It is connected above with the portal (transverse) fissure and the fissure for the ductus venosus; below, with the lesser curvature of the stomach; the left extremity encloses the oesophagus; the right border contains the hepatic vessels and is free, forming the anterior boundary of the epiploic foramen.

The lesser omentum is divided into two parts. The portion connecting the portal fissure of the liver with the first part of the duodenum, and enclosing the root structures of the liver, is called the hepato-duodenal ligament [lig. hepatoduodenale]. The portion of the lesser omentum connecting the lesser curvature of the stomach with the fissure of the ductus venosus is the gastro-hepatic ligament [lig. hepatogastricum].

The gastro-splenic ligament [lig. gastrolienale] connects the left extremity of the stomach with the spleen, continuing the layers of peritoneum which enclose the stomach.

The gastro-phrenic and phreno-colic ligaments

As the peritoneum passes from the diaphragm to the stomach it forms a small fold just to the left of the oesophagus. This is the gastro-phrenic ligament. A strong fold of the membrane also extends from the diaphragm (opposite the tenth and eleventh ribs) to the splenic flexure of the colon, and is known as the phreno-colic (costo-colic) ligament [lig. phrenicolienale].

Minute anatomy

The peritoneum, like all serous membranes, consists of two layers; a lining layer composed of simple squamous epithelium (mesothelium), and an underlying layer of fibrous connective tissue. The latter is highly elastic, and denser in the parietal than in the visceral layer. It often contains fat. In mesenteries and similar structures, the connective tissue is usually very scanty, except surrounding the vessels and nerves. Ruptures often occur in the omenta, which thus become fenestrated in structure. The visceral peritoneum is usually closely attached to the organs for which it forms the outer serous tunic, but the parietal peritoneum is often loosely attached to the adjacent wall by a fatty subserous layer [tela subserosa]. Smooth muscle occurs frequently in the various peritoneal folds.

The peritoneal cavity contains normally a very slight amount of watery fluid, which serves to lubricate the smooth peritoneal surface and thus to eliminate friction between adjacent surfaces during the movements of the alimentary canal.

Vessels and nerves

The peritoneum is in general somewhat sparsely supplied with blood- vessels from various adjacent trunks. Lymph-vessels also occur, but they probably do not connect directly with the peritoneal cavity by stomata (as is found in the frog and as claimed by some to occur in man). They communicate with the lymphatics of neighboring regions. The nerves are also comparatively scarce. They are partly of sympathetic origin (vasomotor), and partly sensory nerves from the intercostal (7th to 12th), and lumbar nerves. The sensory nerves are more frequent in the parietal peritoneum and end in the connective tissue, either freely or in special end-organs (varying from simple end-bulbs to Pacinian corpuscles).

Development

The principal features in the development of the peritoneum have already been mentioned in the section on Morphogenesis and in the remarks on the general morphology of the intestinal canal. Further details will be included later under the development of the intestine, etc.

Variations

Variations in the form and relations of the peritoneum are exceedingly common, and are most commonly of developmental origin. Variations in the form and relations of the various abdominal organs necessarily involve corresponding modifications in the peritoneum. The diaphragm may be incomplete!}' formed, leaving the peritoneal cavity in communication with the pleural, or more rarely the pericardial cavity. The primitive dorsal mesentery of the intestine [mesenterium commune] may persist unmodified (in about 2 per cent, of adults), or the various secondary changes may be inhibited at any stage. Thus the stomach or the intestinal loop may fail, either wholly or partly, to undergo their characteristic rotations. The adhesions of the various mesenteries may be incomplete, or they may be more extensive than usual. For example, the sigmoid mesocolon may be more or less completely obliterated by adhesion, and numerous unusual peritoneal pockets or ligamentous bands may be formed in this way in various localities. Variations thus due to extensions of the normal developmental process are sometimes difficult to distinguish from pathological adhesions caused by peritonitis.

Comparative

As previously mentioned, the primitive body cavity in vertebrates extends throughout the trunk region. In the oyclostomata, this primitive relation persists, the pericardial cavity remaining in communication with the general body cavity. In all higher forms, however, the pericardial cavity becomes entirely separated. In amphibia the lungs he in the general (pleuroperitoneal) body cavity; in the reptiles and birds, they are partially separated; but a complete separation of the pleural cavities occurs only with the formation of the definite diaphragm in mammals.

The formation in the peritoneal cavity of a complete dorsal mesentery, and an incomplete ventral mesentery (in the hepatic region) is typical for all classes of vertebrates. Slight modifications in the form of the mesenteries depend chiefly upon the different degrees of complexity in the development of the various parts of the intestinal tract. The marked changes associated with extensive secondary adhesions of the primitive peritoneal structures are found only among the higher Mammalia, especially in man.