106 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



mesenterial regions, the details of which must be sought in 

 special works. 



Those mesenterial folds which bind the various regions of 

 the alimentary canal to each other have received the special 

 name of omenta. The small omentum has just been men- 

 tioned ; the gastro-splenic omentum connects the spleen with 

 the stomach ; while in the higher vertebrates the great omen- 

 tum is a large double fold formed from the mesogaster and 

 the mesocolon, which connects the stomach to the transverse 

 colon. 



In the region of the heart and of the lungs (when these 

 organs are present) the splanchnic layer of the ccelomic wall 

 becomes similarly related to these structures, and in a similar 

 way similar supporting folds (mediastinum for the lungs, meso- 

 cardium for the heart) are formed. In the abdominal region 

 similar mesothelial folds (mesorchium in the male, mesovarium or 

 mesoarium in the female) support the reproductive organs 

 (gonads). 



Divisions of Splanchnocoele. So far, that part of the ccelom 

 enclosed between the lateral plates has been considered as a 

 single space on either side, as it is in the early development. 

 Soon, however, the anterior portion becomes cut off from the 

 rest and forms a sac, the pericardium, enclosing the heart, the 

 relations of which are described in connection with the circu- 

 latory organs. In the lower vertebrates the posterior wall of 

 this pericardium is known as the false diaphragm or septum 

 transversum, and in many is perforated by one or more small 

 pericardio-peritoneal canals, connecting the pericardium with the 

 abdominal cavity, a result of incomplete closure. 



In the mammals the true diaphragm appears, although rudi- 

 ments, sometimes even muscular, appear in some sauropsida. 

 This diaphragm is a transverse muscle, usually described as 

 crossing the abdominal cavity from side to side, completely 

 dividing it into two cavities, an anterior or pleural, in which 

 the lungs are placed, and a posterior or peritoneal cavity con- 

 taining the remaining viscera. This statement is not exactly 

 correct. In the lower forms the liver abuts directly against 

 the septum transversum. In the mammals these relations are 



