THE PERITONEUM. 1047 
on horizontal and vertical sections of the abdomen—a favourite method of studying 
the peritoneum. 
As already alates the peritoneum is the serous membrane which, on the 
one hand, lines the abdominal cavity, and on the other forms a more or less 
complete covering for the contained viscera. The portion which lines the 
walls of the cavity is known as the parietal peritoneum (peritoneum parietale), 
that which clothes the viscera as the visceral peritoneum (peritoneum viscerale). 
The membrane is connected to both walls and viscera by a layer of areolar tissue— 
the extra or subperitoneal connective tissue—which is considerable in amount in 
certain regions, whilst it is reduced to a mere trace in others, particularly on the 
viscera. (The subperitoneal tissue is described at p. 995.) 
The peritoneum is described as consisting of two sacs—a greater and a. lesser. 
The former lines the greater portion of the abdominal cavity, “and invests most. of 
the abdominal viscera ; the latter hes chiefly behind the stomach, and is much more 
restricted in its distribution. It must be clearly understood that these two sacs 
are not two separate cavities, but simply subdivisions of one great cavity; for the 
lesser is merely a recess of the greater sac, from which it has become partly shut 
off by changes that take place in the position of the adjacent viscera during their 
dev elopment. If the great sac be compared to a bag, the lesser sac might be repre- 
sented as a pocket lying behind, and opening into it by a narrow orifice—the foramen 
of Winslow——on its posterior wall. 
Speaking generally, the great sac lines the walls of the abdominal cavity, and it also 
covers the various organs which receive a peritoneal investment, except the back of 
the stomach, the Spigelian and caudate lobes of the liver, the left suprarenal capsule, 
the upper surface of the pancreas, and also parts of the spleen, left kidney, and transverse 
colon ; all of which, as well as the parietes behind the Spigelian lobe, are clothed by the 
small sac. 
The great sac of the peritoneum is placed between the parietes in front and 
the abdominal viscera behind. It is composed of two layers, an anterior which 
lines the anterior abdominal wall, and a posterior which covers the viscera; but 
this posterior layer is carried forwards by the viscera, so that the two layers come 
in contact, and the cavity of the sac is practically obliterated. 
The anterior layer of the great sac covers the anterior abdominal wall com- 
pletely, from the diaphragm above to the pelvis below. Over the greater part of 
its extent the connection of the peritoneum to the wall is by a small amount of 
fatty extra-peritoneal connective tissue; but below, near the pubic region, the 
fat is more abundant, and the connection between the two becomes much looser. 
This is to allow of the peeling off of the peritoneum, which takes place here during 
distension of the bladder. As the bladder enlarges it passes up along the anterior 
abdominal wall, off which it strips the peritoneum, so that, in the fully distended 
condition, it may be in contact with this wall, without the interposition of peri- 
toneum, for a distance of two inches (5-0 cm., or occasionally more) above the pubes 
(Fig. 738). 
Running up in this fatty tissue are found five cord-like structures, one placed 
in the middle line, and two at each side. These are (a) the urachus—the remains of 
the allantois of the feetus—which in the adult is a slender fibrous band connected 
to the umbilicus above, and to the apex of the bladder below, where it usually 
becomes much stouter. External to the urachus, and some distance from it (Fig. 
703), will be found, in the same fatty tissue, (b) two stouter fibrous cords, the 
obliterated hypogastric arteries (arterice umbilicales). Traced upwards, these also 
become more slender, and approach the urachus, along with which they are con- 
nected to the umbilicus. Below, they grow thicker, and can be followed backwards 
along the side wall of the pelvis to the internal iliac arteries, which they join. 
(c) Further out still, the deep epigastric arteries are seen running upwards and 
inwards from the external ilac trunk on each side. 
When the anterior abdominal wall is examined from behind, it will be seen 
that these five structures which he on the front of the peritoneum carry that 
membrane inwards towards the abdominal cavity in the form of five more or less 
a 
