Basic Structure of Vertebrates 



Fig. 17. Diagrams showing the relations of the coelomic cavities (black) in 

 (A) fishes, (B) amphibians, reptiles, and birds, and (C) mammals. (L) Liver; (P) 

 lungs; (S) septum transversum; (D) diaphragm. In B the lungs lie in the peritoneal 

 (or pleuroperitoneal) cavity; in C they occupy special pleural subdivisions of the 

 coelom. (Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," Phila- 

 delphia, The Blakiston Company.) 



septum is the median partition dorsal to the heart and between the 

 two pleural cavities. The thin transverse partition between the ab- 

 dominal cavity and the three cavities anterior to it is the diaphragm 

 which, unlike the septum transversum, is muscular and acts as part of 

 the mechanism of breathing. The space occupied by the three cavities 

 anterior to the diaphragm is called the thoracic cavity. 



Peritoneum 



The abdominal coelomic spaces are lined by a thin membrane, the 

 peritoneum (Fig. 16). Along the middorsal line of the abdominal 

 coelom, the peritoneal layers of the right and left sides are deflected 

 from the surface of the body-wall and join to form a single two-layered 

 sheet (mesentery) which crosses the coelomic space and attaches to 

 the digestive tube. At the surface of this organ the two layers separate 

 and cover opposite sides of the tube. They may be deflected from the 

 tube and joined to form another mesentery connecting it with some 

 other organ (e.g., the liver) which is similarly invested by the right and 

 left peritoneal sheets. In the case of the liver, the peritoneal layers form 

 a ventral mesentery (suspensory ligament) which extends between 

 the liver and the ventral body wall (Fig. 16). 



That part of the peritoneum covering the body-wall is called pa- 

 rietal or somatic; the part covering coelomic surfaces of organs is 

 called visceral or splanchnic. The peritoneum-like lining of the peri- 

 cardial cavity is called the pericardium and that of a pleural cavity 

 is the pleura. The mediastinal septum is a double membrane con- 

 sisting of right and left pleurae. 



It is a noteworthy principle of animal structure that muscles, 



