ALIMENTARY CANAL. 757 



Coincidcntly with their differentiation the connective tissue 

 stratum of the peritoneum becomes established. 



The Mesentery. Prior to the splanchnic mesoblast growing 

 round the alimentary tube above, the attachment of the latter 

 structure to the dorsal wall of the body is very wide. On the 

 completion of this investment the layer of mesoblast suspending 

 the alimentary tract becomes thinner, and at the same time the 

 alimentary canal appears to be drawn downwards and away from 

 the vertebral column. 



In what may be regarded as the thoracic division of the general 

 pleuroperitoneal space, along that part of the alimentary canal 

 which will form the oesophagus, this withdrawal is very slight, but 

 it is very marked in the abdominal region. In the latter the at 

 first straight digestive canal comes to be suspended from the body 

 above by a narrow flattened band of mesoblastic tissue. This 

 flattened band is the mesentery, shewn commencing in fig. 117, 

 and much more advanced in fig. 1 19, 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 the 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. 380). 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 suspen- 

 sory ligament of the liver. It arises by a secondary fusion, and 

 is not a remnant of a primitive ventral mesentery (vide pp. 624 

 and 625). 



