The peritoneal cavity is the space enclosed by the peritoneum. In contrast with the abdominal cavity, the peritoneal cavity is empty except for the film of lymph that keeps the surfaces moist; for the viscera are so closely packed together that at all points the space is reduced to the mere capillary interval that contains the lymph. But it can become capacious in abnormal circumstances: (1) when air is let into it through a cut in the abdominal wall; (2) when a large amount of lymph exudes into it through the peritoneum in the condition called ascites; or (3) when the contents of the gastrointestinal tract escape into it through a ruptured stomach or intestine.
The distinction between the abdominal cavity and the peritoneal cavity may be easily understood if a familiar object is taken as an example – a leather bag with a cotton lining. The leather represents the abdominal walls including the fascia; the cotton is the peritoneum. If a hole is made in the leather and articles are pushed through it into the bag, between the leather and the cotton lining, till it can hold no more, the bag is then full, but, if its mouth is opened, its "peritoneal cavity" is seen to be empty.
In the male, the peritoneum is unbroken, and the peritoneal cavity is entirely closed. Not so in the female. On each side, the Fallopian tube, which conveys the ova from the ovary to the uterus, opens into the peritoneal cavity in the pelvis. Thus, in contrast to the condition in the male, the peritoneal cavity in the female is placed indirectly in communication with the exterior.
Divisions of the peritoneal cavity
Owing to the disappearance, fusion, shifting, shortening, or the redundant growth of different folds of peritoneum during development, the peritoneal cavity has become divided into two distinct parts: the cavities of the greater and lesser sacs.
The greater sac is the part opened when, in dissection, the anterior wall of the abdomen is cut or removed. The lesser sac is situated behind a part of the liver called the caudate lobe, behind the lesser omentum, behind the stomach, and in the greater omentum. It is almost entirely shut off from the greater sac; their cavities communicate with each other only through one short canal, called the opening into the lesser sac, which is barely wide enough to admit two fingers.