ALIMENTARY CANAL. 623 



more advanced iu fig. 119, M. It is covered on either side by a layer 

 of flat cells, which form part of the general peritoneal epithelioid 

 lining, while its interior is composed of indifferent tissue. 



The primitive simplicity in the arrangement of the mesentery is 

 usually afterwards replaced by a more complicated disposition, owing 

 to the subsequent elongation and consequent convolution of tlie 

 intestine and stomach. 



The layer of peritoneal epithelium on the ventral side of the 

 stomach is continued over the liver, and after embracing the liver, 

 becomes attached to the ventral abdominal wall (fig. 880). Thus in 

 the region of the liver the body cavity is divided into two halves by 

 a membrane, the two sides of which are covered by the peritoneal 

 epithelium, and which encloses the stomach dorsally and the liver 

 ventrally. The part of the membrane between the stomach and 

 liver is narrow, and constitutes a kind of mesentery suspending the 

 liver from the stomach : it is known to human anatomists as the 

 lesser omentum. 



The part of the membrane connecting the liver with the anterior 

 abdominal wall constitutes the falciform or suspensory liga- 

 ment of the liver. It arises by a secondary fusion, and is not a 

 remnant of a primitive ventral mesentery (vide pp. 513 and 514). 



The mesentery of the stomach, or mesogastrium, enlarges in Mam- 

 malia to form a peculiar sack known as the greater omentum. 



The mesenteron exhibits very early a trifold division. An 

 anterior portion, extending as far as the stomach, becomes separated 

 off as the respiratory division. On the formation of the anal 

 invagination the portion of the mesenteron behind the anus becomes 

 marked off as the postanal division, and between the postanal 

 section and the respiratory division is a middle portion forming an 

 intestinal and cloacal division. 



The respiratory division of the mesenteron. 



This section of the alimentary canal is distinguished by the fact 

 that its walls send out a series of paired diverticula, which meet 

 the skin, and after a perforation has been effected at the regions of 

 contact, form the branchial or visceral clefts. 



In Amphioxus the respiratory region extends close up to the 

 opening of the hepatic diverticulum, and therefore to a position 

 corresponding with the commencement of the intestine in higher 

 types. In the craniate Vertebrata the number of visceral clefts 

 has become reduced, but from the extension of the visceral clefts in 

 Amphioxus, combined with the fact that in the higher Vertebrata the 

 vagus nerve, which is essentially the nerve of the branchial pouches, 

 supplies in addition the walls of the oesophagus and stomach, it may 

 reasonably be concluded, as has been pointed out by Gegenbaur, that 

 the true respiratory region primitively included the region which in 

 the higher types forms the oesophagus and stomach. 



