THE DIGESTIVE APPARATUS IN MAMMALIA. 



Fig. 256. 



is of an oval form. Posteriorly, it is narrower, and is traversed by the rectum 

 and the e^enito-urinary organs, which open externally. 



The Peritoneum. — The abdominal cavity is lined, internally, by a serous 

 membrane, the peritoneum, which will now be briefly described. 



Like all the splanchnic serous membranes, the peritoneum is composed of a 

 parietal and a visceral layer, which together form a closed sac so arranged that the 

 organs contained in the abdomen are situated external to this sac. The adjoining 

 theoretical figure (256), representing a transverse section of the abdominal cavity, 

 will show at a glance this arrangement. Let a represent the section of the small 

 intestines floating at liberty in the interior of the cavity ; b, that of the aorta, at 

 the level of the great mesenteric artery : the 

 peritoneal membrane, c c, covers the walls of the 

 abdomen, and at the points D, D is folded around 

 the great mesenteric artery, in such a manner 

 as to form two layers, f, f, which come in con- 

 tact by their adherent faces, reach the intestine, 

 and then separate to envelop it. We then see 

 in this figure the parietal layer of the peritoneum 

 C C, the visceral layer, G g, and the two layers, 

 F, F, which establish the continuity of the 

 parietal and visceral portions ; the complete sac 

 formed by these is apparent, and it could be 

 rendered more so by the further separation and 

 development of the layers f, f ; so that there is 

 no difficulty in understanding how the small 

 intestine may be at the same time in the inferior of 

 the abdominal cavity, and outside the sac formed 

 by the serous membrane which lines that cavity. 

 This arrangemer.t is common to all the organs freely suspended in the abdo- 

 men. The serous folds which suspend them, in joining their peritoneal layer to 

 that which covers the parietes of the cavity, will be studied under the names of 

 ligaments, mesenteries, and omenta. Some organs — the kidneys, for example — 

 have no proper visceral layer, being contained between the abdominal wall and 

 the external face of the parietal peritoneum, and are invested with none of the 

 duplicatures just mentioned. 



We will briefly study the duplicatures, ligaments, mesenteries, and omenta that 

 the peritoneum forms, starting from the umbilicus and passing forwards and 

 backwards (Fig. 2,')7). 



On reaching t!ie supra-sternal region, the peritoneum forms a falciform dupli- 

 cature, extending from the umbilicus to the middle lobe of the liver, and which 

 is even prolonged between that lobe and the posterior face of the diaphragm ; at 

 the free border of this fold is a thickening, which is regarded as the remains of 

 the obliterated umbilical vein. In becoming doubled over the neighbouring 

 organs, the diaphragmatic portion constitutes : 1. The ligaments of the right and 

 left lobes of the liver. 2. The common ligament of that gland, which surrounds 

 the posterior vena cava. 3. The cardiac ligament that envelops the termination 

 of the oesophagus. Behind the liver is found the hepato-gastric ligament, which 

 fixes the stomach in the posterior fissure of the liver, and is, to the right and 

 backwards, attached to the duodenum at the lower face of the right kidney ; it 

 is then directed from right to left, and becomes continuous with the parietal 

 peritoneum of the sublumbar region and the mesentery proper. 



THEORETICAL TRANSVERSE SECTION 

 OF THE ABDOMINAL CAVITY, TO 

 SHOW THE DISPOSITION OF THE 

 PERITONEUM. 



