458 ELEMENTARY ANATOMY. [LESS. 



ultimately disappears with the exception of the serous mem- 

 brane coating it, and thus, the two serous coats of its two 

 sides meeting together, the alimentary canal comes to be 

 slung by means of them from the skeletal axis in a peritoneal 

 fold, or, as it is called, mesentery. In man and in allied 

 animals, however, the alimentary canal is very tortuous and 

 convoluted, and has annexed to it many complex accessory 

 structures. Thus the lining peritoneal membrane, passing 

 from one digestive organ to another and wrapping them 

 round in various degrees and with various adhesions, comes 

 to form a wonderfully complex structure. 



In man, beginning from the under surface of the dia- 

 phragm, we find a fold of peritoneum constituting the falci- 

 form ligament, before described, attaching the liver to the 

 diaphragm. The peritoneum then descends, coating the 

 abdominal aorta and inferior vena cava. It also descends, 

 and coats the liver. From the under surface of the liver it 

 again descends to the stomach, in a double fold of membrane 

 called the gastro-hepatic amentum. It next encloses the 

 stomach and also the spleen. It also invests the intestines 

 except the greater part of the rectum, but it coats the anterior 

 surface of that gut as well as the upper and inner surface of 

 the bladder. From between the stomach and the transverse 

 colon, a great, free, apron-like flap of peritoneum, called the 

 great amentum, hangs down loosely in front of the bowels. 



This great complex bag, the peritoneum, exists in all Ver- 

 tebrates, but in lower forms is greatly simplified in conformity 

 with the simple arrangement of the viscera (Fig. 390). In 

 Fishes its cavity sometimes communicates with the exterior 

 by two apertures placed near the external termination of the 

 alimentary canal, as e.g. in the Lamprey, Eel, Salmon, and 

 others. It may communicate by a single opening placed 

 further forwards, as in the Lancelet. It may communicate 

 with the pericardium, e.g. in Myxine, the Sturgeon, and 

 Sharks, as was indicated in describing the diaphragm. 



It may, as in most Reptiles, and in Batrachians and Birds, 

 line uninterruptedly the thoracic and abdominal cavity, 

 forming one pleuro-peritoneal sac ; thus differing from man 

 (and Mammals), in whom, as we shall see in the next Lesson, 

 the thorax has its own proper serous sac, that muscular par- 

 tition, the diaphragm, dividing the two cavities in Mammals. 

 The peritoneum may be delicate and transparent, as in man. 

 It is so in his class generally, and in Birds. It may, how- 

 ever, be black, as is sometimes the case in Reptiles and 



