The peritoneum is the thin serous membrane which lines the abdominal and
(in part) the pelvic cavity, and covers to a greater or less extent the
viscera contained therein. In the male it is a completely closed sac,
but in the female there are two small openings in it; these are the
abdominal orifices of the Fallopian tubes, which at their other ends
communicate with the uterus, and so indirectly with the exterior. The
peritoneal cavity is only a potential one, since its opposing walls are
normally separated only by the thin film of serous fluid (secreted by
the membrane) which acts as a lubricant.
The free surface of the membrane has a glistening appearance and is very
smooth. This is due to the fact that this surface is formed by a layer
of flat endothelial cells, and is moistened by the peritoneal fluid.
Friction is thus reduced to a minimum during the movements of the
viscera. The outer surface of the peritoneum is related to the
subperitoneal tissue, which attaches it to the abdominal wall or the
viscera.
In order to understand the general disposition of the peritoneum, we may
imagine the abdominal cavity to be empty and lined by a simple layer of
peritoneum, termed the =parietal layer= (Lamina parietalis). We may
regard the organs as beginning to develop in the subperitoneal tissue,
enlarging, and migrating into the cavity to a varying extent. In doing
so they carry the peritoneum before them, producing introversion of the
simple sac, and forming folds which connect them with the wall or with
each other. The viscera thus receive a more or less complete covering of
peritoneum, termed the =visceral layer= (Lamina visceralis). The
connecting folds are termed =omenta=, =mesenteries=, =ligaments=, etc.
They contain a varying quantity of connective tissue, fat and lymph
glands, and furnish a path for the vessels and nerves of the viscera.
Some contain unstriped muscular tissue. An omentum is a fold which
passes from the stomach to other viscera. There are three of these,
namely: (1) the =small= or =gastro-hepatic omentum= (Omentum minus),
which passes from the lesser curvature of the stomach to the liver; (2)
the =gastro-splenic omentum= (Ligamentum gastrolienale), which extends
from the greater curvature of the stomach to the spleen; (3) the =great
omentum= (Omentum majus), which passes from the greater curvature of the
stomach and from the spleen to the terminal part of the great colon and
the origin of the small colon. It does not pass directly from one organ
to the other, but forms an extensive loose sac (Figs. 278, 279). A
=mesentery= (Mesenterium) is a fold which attaches the intestine to the
dorsal wall of the abdomen. There are two mesenteries, namely: (1) the
=great mesentery= which connects the greater part of the small intestine
with the dorsal abdominal wall; (2) the =colic mesentery=, which
attaches the small colon to the dorsal abdominal wall. =Ligaments= are
folds which pass between viscera other than parts of the digestive tube,
or connect them with the abdominal wall. The term is also applied to
folds which attach parts of the digestive tract to the abdominal wall,
but do not contain their blood-vessels and nerves. In some cases (_e.
g._, the lateral and coronary ligaments of the liver) they are
strengthened by fibrous tissue; in other cases (_e. g._, the broad
ligaments of the uterus) they contain also unstriped muscular tissue.