X. THE COMPARATIVE ANATOMY OF THE COELOM, THE DIGESTIVE, 

 AND RESPIRATORY SYSTEMS 



A. 



THE ORIGIN AND PARTS OF THE COELOM AND THE MESENTERIES 



i. Origin.— The coelom or body cavity of vertebrates is t/he cavity of the hypomere. It is 

 never at any stage segmented. The outer wall of the hyrjomere comes in contact with the 

 inner surface of the layer of voluntary muscles and forms ( the lining of the body wall. This 

 lining is known as the parietal peritoneum. The inner walls of the hypomeres of the two sides 

 come in contact in the median plane folding around the intestine and inclosing the intestine 

 between their two walls. The inner walls of the hypomere thus become the covering layer of 

 the intestine and other viscera, and are then named the, visceral peritoneum or serosa. Above 



Fig. 44. — Diagrams to show the relations of certain viscera to the mesenteries. A, showing intes- 

 tine d, supported by the dorsal mesentery c, and the heart g, inclosed in the ventral mesentery e and h. 

 B, showing the liver n, inclosed in the ventral mesentery, part of which, the lesser omentum /, extends 

 between the intestine k and the liver, and part of which, the falciform ligament p, extends between the 

 liver and the ventral body wall. C, showing relation of the intestine q to the dorsal mesentery 0. 

 a, neural tube; b, notochord; c, dorsal mesentery of the digestive tract; d, esophagus; c, dorsal mesen- 

 tery of the heart or dorsal mesocardium; /, pericardial cavity; g, heart; h, ventral mesentery of the 

 heart or ventral mesocardium; i, dorsal aorta; _/', posterior cardinal vein; k, duodenum; /, lesser omen- 

 tum or hepatoduodenal ligament; m, serosa or visceral peritoneum; n, liver; 0, parietal peritoneum: 

 p, falciform ligament of the liver; q, small intestine; r, peritoneal cavity; s, bile duct. In .4, e and h. 

 and in B, I and p, form the ventral mesentery of the digestive tract which incloses the heart and liver: 

 in C the ventral mesentery is absent. (From Prentiss and Arey's Textbook of Embryology, courtes> 

 of the \V. B. Saunders Company.) 



and below the intestine the two walls of the hypomere are in contact and form a double-walled 

 membrane, which is designated as a mesentery. That portion of the mesentery between the 

 dorsal wall of the coelom and the intestine is called the dorsal mesentery, that between the 

 ventral wall and the intestine is the ventral mesentery. Different portions of these mesenteric; 

 receive special names, which will be given in the course of the dissection. The dorsal mesenterj 

 is intact for its entire length in most vertebrates, but the ventral mesentery very earh 

 disappears except in certain regions which will be noted later. These matters have already beei 



158 



