1234 THE DIGESTIVE SYSTEM. 



bladder, or the ureters, when usually no anus is evident ; or finally the cloaca may persist. 

 Other forms are also described, but the foregoing are those most commonly found. 



For the development of the rectum and anus, see pp. 39 and 42. 



PERITONEUM. 



An introductory sketch of the peritoneum was given on p. 1160 ; subsequently, 

 when describing the abdominal viscera, an account of its detailed relations to 

 each of them was included. We shall here consider the membrane and its folds as 

 parts of one continuous whole ; and we shall also describe its arrangement as seen 

 on horizontal and vertical sections of the abdomen. 



As already explained, 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 ; that which clothes the viscera 

 is called the visceral peritoneum. The membrane is connected to both walls and 

 viscera by a layer of areolar tissue tela subserosa, the extra or subperitoneal con- 

 nective 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. 475.) 



The peritoneal cavity is described as consisting of two portions the general 

 peritoneal or great sac and the bursa omentalis. The great sac is opened when the 

 anterior abdominal wall is removed or incised, and the peritoneum which encloses 

 it lines the greater portion of the wall of the abdominal cavity, and invests most 

 of the abdominal viscera ; the omental bursa lies chiefly on the posterior aspect of 

 the stomach, and is much smaller. It must be clearly understood that these two 

 sacs are not two separate cavities, but simply subdivisions of one great cavity ; for 

 the omental bursa is merely a recess of the greater sac, from which it has become 

 partly shut off largely by changes that take place in the position of the adjacent 

 viscera during their development. If the general peritoneal cavity is compared to 

 a bag, the bursa omentalis might be represented as a pocket lying behind, and 

 opening into it by a narrow orifice the foramen epiploicum [Winslowi]. 



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 caudate lobe of the liver, the left supra-renal gland, the upper surface of 

 the pancreas, and also parts of the spleen, left kidney, and transverse colon ; all of these, 

 as well as the parietes behind the caudate lobe, are clothed by the bursa omentalis. 



The general peritoneal sac is placed between the parietes anteriorly and 

 the abdominal viscera posteriorly. It is composed of two layers : an anterior, which 

 lines the anterior abdominal wall ; and a posterior, which mainly 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 peritoneum covers the anterior abdominal wall com- 

 pletely, from the diaphragm above to the pelvis below. Over the greater part of 

 its extent the connexion of the serous membrane 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 connexion between the two becomes much looser. 

 This is to allow of the movement of the peritoneum which takes place there during 

 distension of the bladder. As the bladder enlarges it passes up in the extra- 

 peritoneal tissue of the lower part of the anterior abdominal wall, off which it 

 raises the peritoneum, so that, in the fully distended condition, the anterior 

 surface of the bladder is 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. 964). 



Running up in the fatty subserous tissue are found five cord-like structures, 

 one placed in the median plane, and two at each side. These are (a) the lig. umbili- 

 cale medium (O.T. urachus) the remains of the allantois of the foetus which in 

 the adult is a slender fibrous band connected to the umbilicus above, and to the 



