The peritoneum is the extensive serous membrane that, in general, lines all borders of the abdominopelvic cavity and reflects from the body wall onto the organs that are contained within it. A general concept that one might have of the pleura or the serous pericardium can be carried over to the peritoneum. In all of these situations, the serous membrane lining the body wall is continuous with that on the surfaces of the viscera contained within the portions of the body cavity involved, and although one refers separately to parietal and visceral portions of the serous membrane, they are continuous. Also, under normal circumstances, the organs fill the respective portion of the body cavity so completely that the visceral and parietal portions of adjacent structures are separated from each other by only a thin film of fluid. The peritoneal cavity of the female is the only place where an organ’s lumen is in direct contact with the peritoneal space, as the opening of each uterine tube is open to the peritoneal cavity.
The peritoneum is much more complicated in its arrangement than either the visceral pleura or the serous pericardium. This is essentially due to the fact that parts of several viscera deform the peritoneal serous membrane to various degrees in the course of fetal development. The rotations of the gut, combined with the propensity of one free peritoneal surface to fuse with another free surface, result in complex changes of the arrangement that lead to the mature appearance of the abdominal organs. The stomach (to cite only one example of the manifold rearrangements in the visceroperitoneal relations) in its primary vertical position was attached by one double layer of peritoneum, the ventral mesogastrium, to the ventral body wall, and by another double layer, the dorsal mesogastrium, to the posterior wall. When the stomach rotated, its original left side became the anterosuperior surface and the original right side the posteroinferior surface; the dorsal mesogastrium was swept toward the left to form an outpouching of the peritoneal cavity, which presents itself at an early developmental stage (6 weeks) as the omental bursa (lesser sac), communicating with the rest of the peritoneal cavity (greater sac) by only a small opening, the omental foramen (of Winslow), located a little to the right of the midline posteroinferior to the liver.
The best way to obtain a general concept of the arrangement of the peritoneum is to trace it in three planes, a midsagittal plane and two horizontal planes, one at the level of the pylorus and the other at the level of the umbilicus, in a preferably fresh specimen at the autopsy table. Lacking this opportunity, the use of these three planes still remains methodically the most informative approach for studying the peritoneal continuity and its relationship to the abdominopelvic viscera.
In the midsagittal plane, the greater and lesser peritoneal sacs must be pursued separately, because they are not continuous anywhere in this plane. In following the cut edge of the greater sac, one can start with the parietal peritoneum on the inner surface of the anterior wall at the level of the umbilicus. Progressing superiorly, the peritoneum continues onto the inferior surface of the diaphragm and along it until it is reflected to the liver as the superior (anterior) layer of the left triangular ligament. From here it extends along the anterosuperior surface of the liver, around the free margin of the liver, and onto its visceral surface, until it is reflected toward the lesser curvature of the stomach as the anterior layer of the lesser omentum, which then advances onto the anterior surface of the stomach, leaving the latter as the anterior surface of the greater omentum. At the free margin of the greater omentum, this layer turns superiorly to become the posterior surface of the greater omentum, which proceeds superiorly to the transverse colon, where it appears to continue onto the posterior surface of the transverse colon and then as the posterior layer of the transverse mesocolon. From the posterior layer of the transverse mesocolon, the peritoneum turns interiorly from the lower border of the pancreas across the anterior surface of the third portion of the duodenum and becomes the right (superior) layer of the intestinal mesentery. At its free margin the mesentery entirely (except for the area of mesenteric attachment) surrounds the small intestine and continues to the posterior body wall as the left (inferior) layer of the mesentery. On reaching the body wall, it runs as the parietal peritoneum of the posterior wall interiorly on the anterior surface of the aorta and then on the vertebral column to about the second sacral level, where it comes to lie on the anterior surface of the rectum, from which, in the male, it is reflected onto the posterosuperior surface of the bladder, bounding the rectovesical pouch. In the female, the peritoneum passes from the anterior surface of the rectum to the posterior vaginal fornix, bounding the rectouterine pouch (of Douglas). It passes up the posterosuperior aspect of the uterus, over the fundus of the uterus, and down on its anteroinferior aspect to about the junction of the body and cervix, from whence it reflects onto the posterosuperior aspect of the bladder, bounding the vesicouterine pouch. In both the male and the female, the peritoneum passes from the superior surface of the bladder to the inner surface of the anterior body wall, a variable distance above the symphysis pubis, depending on the degree of distention of the bladder. From here it continues superiorly to the point at which this tracing of the peritoneum was started.
In following the cut edge of the omental bursa’s peritoneum in a midsagittal plane, a start can be made on the anterior surface of the pancreas, and the peritoneum can be traced superiorly from here onto the surface of the diaphragm until it reflects from the diaphragm to the liver as the inferior (posterior) layer of the left tri- angular ligament. From here it can be traced along the posterior and then inferior surfaces of the liver to the point at which it leaves the liver to go to the lesser curvature of the stomach as the posterior layer of the lesser omentum, which continues onto the posteroinferior surface of the stomach and to the greater curvature, where it leaves the stomach to extend for a variable distance into the greater omentum. This distance depends on the degree of fusion of the peritoneum which has taken place, typically not reaching beyond the transverse colon. The peritoneum turns superiorly on the anterior surface of the transverse colon, and then, in the adult, it usually forms the anterior layer of the transverse mesocolon if the fusion of the primitive dorsal mesogastrium with the primitive mesentery of the transverse colon has been complete. The transverse mesocolon comes to the posterior body wall just inferior to the point at which the tracing of the lesser sac peritoneum was started.
In tracing the peritoneum in a horizontal section at the level of the omental foramen, a start can be made with the greater sac peritoneum on the inner surface of the anterior abdominal wall in the midline. Following the cut edge of the parietal peritoneum to the left along the inner surface of the anterolateral wall to the region of the posterior wall, it will pass onto the anterolateral surface of the left kidney, from where it reflects to the hilar area of the spleen, forming the external layer of the splenorenal ligament, and then completely surrounds the spleen except at the hilar area. From the anterior margin of the hilum of the spleen, the peritoneum passes to the stomach as the external layer of the gastrosplenic ligament. The peritoneum can then be followed along the anterosuperior surface of the stomach to the lesser curvature, where it leaves the stomach as the anterior layer of the lesser omentum, which can be followed to the right until the free margin is reached a short distance to the right of the midline. Here the peritoneum passes around the free margin of the lesser omentum (anterior boundary of the omental foramen) to become the peritoneum of the omental bursa, which continues to the left, as the posterior layer of the lesser omentum, to the lesser curvature of the stomach, where it continues onto the posteroinferior surface of the stomach, which it follows until it leaves the stomach to form the internal (lesser sac) layer of the gastrosplenic ligament. From the spleen the peritoneum forms the internal layer of the splenorenal ligament and then travels to the right anterior to the aorta and the inferior vena cava. At the right margin of the inferior vena cava, the peritoneum again becomes continuous with the greater sac and continues to the right onto the anterior aspect of the right kidney. From here the tracing of the peritoneum could differ, depending on whether the bare area of the liver were to extend down just far enough to be encountered in the plane of the tracing or whether the plane of tracing passes just inferior to the bare area of the liver. In the former case, the peritoneum would pass from the kidney as the inferior layer of the coronary ligament to the liver, and would follow around the liver to its anterosuperior surface, where it would leave the liver as the left layer of the falciform ligament, to go to the inner surface of the anterior body wall and to the left to the point from which the tracing started. To complete the tracing in this plane, one must follow the peritoneum from the right layer of the falciform ligament onto the anterosuperior surface of the liver, and to the right along this surface to the superior layer of the coronary ligament, along this to the diaphragm, and then anteriorly to the right layer of the falciform ligament. If the plane of section passes just inferior to the bare area of the liver as the peritoneum leaves the anterior surface of the inferior vena cava (the posterior boundary of the omental foramen), it passes across the anterior surface of the right kidney, then to the diaphragm, and forward on the inner surface of the body wall to the falciform ligament.
In tracing peritoneum in a horizontal section at about the level of the umbilicus, one can start at the midline of the inner surface of the anterior abdominal wall and follow from this point the parietal peritoneum to the left along the inner surface of the wall to the posterior wall, where it reflects onto the left side of the descending colon to cover also the anterior surface and right side of this structure, from which it passes to the posterior body wall. In early development the descending colon was suspended by the primitive dorsal mesentery, but peritoneal fusion during embryologic development brings it into the adult relationship to the peritoneum just described. The peritoneum continues to the right on the posterior body wall to about the midline, where it reflects forward to form the left (inferior) layer of the intestinal mesentery. The small intestine is completely surrounded (except at its mesenteric attachment) in the free margin of the mesentery; from here the peritoneum is traced posteriorly to the posterior body wall as the right (superior) layer of the mesentery. Thereafter, the peritoneum can be followed to the right onto the posterior body wall, until it reflects from here to cover the left, anterior, and right surfaces of the ascending colon. This structure was also suspended originally by the primitive dorsal mesentery. From the right side of the ascending colon, the peritoneum passes to the posterior body wall and then forward on the inner surface of the anterolateral abdominal wall until it reaches the midline, from where the tracing was started. Also in a section at about the level of the umbilicus, one would expect to find the greater omentum cut, which is present as an island of peritoneum not connected in this section to the rest of the peritoneum. If the transverse colon is hanging low enough, it too would be cut as an island with its peritoneum continuous with that of the greater omentum.
Worthwhile additions to the general concept of the distribution of the peritoneum, gained by tracing it in several planes as done above, can be obtained by careful study of a view of the posterior half of the abdominopelvic cavity, in which all of the viscera (except the bladder and rectum) that invaginate the peritoneum to any degree have been removed, cutting the peritoneum along its lines of reflection from the posterior body wall or the anterior surfaces of the viscera and vessels that do not project into the peritoneum. The right and left kidneys, the pancreas (except for the tip of its tail), the second, third, and most of the fourth parts of the duodenum, and the aorta and inferior vena cava do not project into the peritoneal cavity to a significant degree. The peritoneum covers the inner surface of the abdominopelvic body walls as parietal peritoneum, except where it is lifted away from them by the structures just listed (the bare area of the liver against the diaphragm, the ascend- ing and descending colon, the roots of the mesentery, the transverse mesocolon and sigmoid mesocolon, the ureters and inferior mesenteric vessels, and the rectum and bladder and, in the female, the uterus and broad ligaments, other folds in the pelvis, and folds on the inner surface of the anterior abdominal wall). The folds on the inner surface of the anterior abdominal wall are the falciform ligament of the liver (a remnant of the ventral mesentery, ventral to where the liver grew into it), running superiorly and a little to the right from the umbilicus, with the ligamentum teres (obliterated umbilical vein) of the liver in its free margin; the median umbilical fold, projecting from the superior aspect of the urinary bladder, running superiorly up the midline to the umbilicus; the medial umbilical folds, also running to the umbilicus and containing the obliterated right and left umbilical veins; and the right and left lateral umbilical folds, containing the inferior epigastric artery and vein on each side (which may produce a slight elevation remindful of a fold by pulling the peritoneum a little away from the body wall). The depression between the median and medial umbilical folds is called the supravesical fossa, whereas the one between each medial and lateral umbilical fold is the epigastric fold. Lateral of the lateral umbilical fold is the lateral inguinal fossa. Parietal peritoneum is thus seen to be applied to practically the entire extent of the inner surface of the anterolateral abdominal wall, and virtually any incision through this wall will open into the peritoneal cavity.
Much of the diaphragm has parietal peritoneum on its abdominal surface, but much less of the muscular portion of the posterior abdominal wall is directly lined by peritoneum on its inner surface. This is because several viscera, major vessels, and a significant amount of adipose tissue lie behind the peritoneum and most of the abdominal viscera project from the posterior wall into the peritoneal cavity.
From the preceding description it is obvious that the degree to which the various abdominal viscera are covered by peritoneum (visceral peritoneum) varies along a spectrum in which peritoneum may cover just part of one surface of the viscus in question to the other extreme in which peritoneum covers the viscus entirely, except for the area of attachment of a suspending double-layered fold of peritoneum. “Retroperitoneal” is a very commonly used descriptive term having the general meaning of “behind the peritoneum,” which is well agreed upon, but some authors refer to certain organs as retroperitoneal that other authors would not designate in this fashion. Generally, “primarily retro- peritoneal structures” (e.g., ureter, kidney) are those that develop posterior to the peritoneal lining and never develop a mesentery. “Secondarily retroperitoneal structures” once had a mesentery but lost it when the organ was laid back along the body wall and the mesentery fused to a degree with the parietal peritoneum (e.g., ascending colon, second portion of the duodenum). “Intraperitoneal structures” are those that are suspended from the posterior body wall by a mesentery containing blood vessels and nerves associated with the organ (e.g., stomach, ileum). Additional details will be given in the sections dealing with each organ or region.
The mesentery is commonly taken to mean the mesentery of the small intestine (i.e., the jejunoileal portion of the small intestine which is the portion having a mesentery or a double-layered fold of peritoneum suspending it from the posterior abdominal wall). The root of the mesentery is about 15 cm in length, and its line of attachment varies a bit with the shape of the duodenum, but, in general, it courses from a little to the left of the second lumbar vertebra inferiorly and to the right, crossing the third part of the duodenum, the aorta, the inferior vena cava, the right ureter, and the right psoas major muscle to reach a point near the right sacroiliac joint. The free or unattached border, which contains the loops of the small intestine, is frilled out to such an enormous degree that it may attain a length varying from 3 m to more than 6 m. The distance from the attached border to the free border measures 15 to 22 cm; it may definitely increase with age, probably owing to stretching of the mesentery due to laxity of the anterior abdominal wall. Between the two layers of peritoneum on the two surfaces of the mesentery are the superior mesenteric artery and its branches, the accompanying veins, lymphatics, approximately 100 to 200 lymph nodes, autonomic nerve plexuses, connective tissue, and varying amounts of adipose tissue, which is present in greater amounts near the root. The mesentery divides the area below the transverse mesocolon into two compartments, which are important in determining collections of fluid and the localization of infection.
The transverse mesocolon is the broad peritoneal fold suspending the transverse colon from the posterior body wall. The root of the transverse mesocolon crosses the anterior surface of the right kidney, the second portion of the duodenum, and the head of the pancreas, and then passes along the lower border of the body and tail of the pancreas superior to the duodenojejunal flexure, to end on the anterior surface of the left kidney. It contains the middle colic artery, branches of the right and left colic arteries, accompanying veins, lymphatic structures, autonomic nerve plexuses, as well as a considerable thickness of connective tissue.
The sigmoid mesocolon is the mesentery of the sigmoid colon. When the peritoneum begins to surround the large intestine near the crest of the ilium, the attachment of the sigmoid mesocolon follows a fairly straight line from the posterior part of the left iliac fossa inferiorly and medially to reach the third sacral segment.
If, as is the case in the other extreme of the range of variation, the colon is closely bound down in the iliac fossa, the line of attachment of the sigmoid mesocolon goes posteriorly along the pelvic brim until it crosses the anterior side of the sacroiliac joint, and then descends along the anterior aspect of the sacrum to the level of its second to third segment. The sigmoid colon is enwrapped by the free margin of the sigmoid mesocolon, which has its greatest width (distance from attached to free border) at its attachment to the first sacral segment. This width varies from about 5 to 18 cm, although it occasionally may be as much as 25 cm between the layers of the sigmoid mesocolon through which run the sigmoidal and superior rectal arteries, accompanying veins, lymphatics and autonomic nerve plexus, and connective tissue, which, of course, includes varying amounts of adipose tissue.
The greater omentum is the largest peritoneal fold; it may hang down like a large apron from the greater curvature of the stomach in front of the other viscera as far as the brim of the pelvis or even into the pelvis. It may even extend into an inguinal hernia, most commonly on the left side. It also may be much shorter than this, appearing as just a fringe on the greater curvature of the stomach, or it may be of some length and found folded in between coils of the small intestine, tucked into the left hypochondriac area or turned superiorly just anterior to the stomach. The superior end of the left border is continuous with the gastrosplenic ligament, and the superior end of the right border extends as far as the beginning of the duodenum. The greater omentum is usually thin, with a delicate layer of fibro- elastic tissue as its framework, and somewhat cribriform in appearance, although it usually contains some adipose tissue and may accumulate a large amount of fat in an obese individual. In the make-up of the greater omentum, the peritoneum of the omental bursa on the posteroinferior surface of the stomach and the greater sac peritoneum on the anterosuperior surface of the stomach meet at the greater curvature of the stomach and course inferiorly to the free border of the greater omentum, where they turn superiorly to the transverse colon. Early in development, these two layers of elongated dorsal mesogastrium course superiorly in front of the transverse colon and transverse mesocolon to the anterior surface of the pancreas. Owing to fusions of these two layers of peritoneum to each other and to the peritoneum on the transverse colon, and the anterior surface of the primitive transverse mesocolon, it appears, in the fully developed state, as though the “two layers” of peritoneum, running superiorly as the posterior layer of the greater omentum, separate from each other to surround the transverse colon and continue as the two layers of the transverse mesocolon. Frequently, there is enough fusion in the four-layered primitive greater omentum inferior to the transverse mesocolon that no extension of the omental bursa is present between layers. Close to the greater curvature of the stomach, the right and left gastroomental vessels course, anastomosing with each other in the greater omentum. The greater omentum, if of any length, has a great deal of mobility and can shift around to fill what would otherwise be temporary gaps between viscera or to build up a barrier against bacterial invasion of the peritoneal cavity by becoming adherent at a potential danger spot.
The lesser omentum, which can be subdivided into hepatogastric and hepatoduodenal ligaments, extends from the posteroinferior surface of the liver to the lesser curvature of the stomach and the beginning of the duodenum. It is extremely thin, particularly the part to the left, which is sometimes fenestrated. The right side is thicker and ends in a free, rounded margin, which contains the common bile duct to the right, the hepatic artery to the left, and the portal vein posterior to these two, and forms the anterior border of the omental foramen. In addition to the structures just listed, the lesser omentum contains the right and left gastric arteries (close to the lesser curvature of the stomach) and the accompanying veins, lymphatics, and autonomic nerve plexuses. The lesser omentum reaches the liver at the porta hepatis, and to the left of the porta hepatis it extends to the bottom of the fossa for the ligamentum venosum, the obliterated ductus venosus, which carried oxygenated blood from the umbilical vein to the inferior vena cava.
The omental bursa (lesser sac of the peritoneum) is a large fossa, or outpouching, from the general peritoneal cavity. It is bounded in front, from superior to inferior, by the caudate lobe of the liver, lesser omentum, posteroinferior surface of the stomach, and anterior layer of the greater omentum (at least in part). Posteriorly, from inferior to superior, are the posterior layer of the greater omentum (the amount of this depends on the variable inferior extent of the bursa), transverse colon, anterior layer of the transverse mesocolon, anterior surface of the pancreas, left suprarenal gland, superior end of the left kidney, and, to the right of the esophageal opening into the stomach, that part of the diaphragm supporting the caudate lobe of the liver. The horizontal extent of the bursa stretches from the omental foramen at the right to the hilum of the spleen at the left, where it is limited by the splenorenal and gastrosplenic ligaments. Inferiorly, the bursa may extend about as far as the transverse colon, its cavity having originally reached as far down as the free margin of the greater omentum before becoming obliterated by fusion of its layers. The portion of the bursa between the caudate lobe of the liver and the diaphragm is called the superior recess, and the narrow portion from the omental foramen across the head of the pancreas to the gastropancreatic fold is called the vestibule of the bursa.
The omental foramen (of Winslow) is the opening by which the omental bursa communicates with the general peritoneal cavity (greater sac). It is somewhat circular and is usually large enough to admit one or two fingers. Anterior to the foramen is the free margin of the lesser omentum, containing the common bile duct, hepatic artery, and portal vein. Its posterior border is the peritoneum covering the inferior vena cava; superior to it is the peritoneum on the caudate process of the liver, and its inferior boundary is the peritoneum that covers the beginning of the duodenum and the hepatic artery. Many extremely variable and inconstant fossae or recesses have been described which are of interest to the surgeon because of the possibility of herniation of a loop of intestine into any one of them. The more common ones are located either in the region of the fourth portion of the duodenum or in the region of the cecum and ileocecal junction. A relatively common “intersigmoid recess” is found on the left side of the line of attachment of the sigmoid mesocolon at the angle that is present in this line when the colon is tightly bound down in the iliac fossa.
A characteristic of the peritoneum covering the surfaces of the various parts of the colon is the presence of little outpouchings of peritoneum containing adipose tissue, which are called omental appendages (appendices epiploicae).
The parietal peritoneum is supplied by the nerves to the adjacent body wall and is thus pain sensitive. The visceral peritoneum is insensitive to ordinary pain stimuli but does respond to ischemia, distention, and inflammation. When moist surfaces of peritoneum which are in contact become irritated, adhesions tend to form that often become permanent.