MUSCULAR SYSTEM. 



107 



the same. The diaphragm is therefore to be regarded as a 

 paired structure, extending from the lateral walls behind the 

 lungs to middle part of the septum transversum. This explains 

 why it is that the pericardium appears as if enclosed in the 

 pleural cavity, although it is morphologically outside of it. 



Fig. 115. Diagrams illustrating the relations of the pericardium to the rest 

 of the coelom : A in fishes, B in amphibia and sauropsida, C in mammalia, d^ dia- 

 phragm ; /i, heart ; /, liver; /, lungs; i, septum (false diaphragm) between peri- 

 cardium and the rest of the ccelom. In B the lungs project into the general ccelom 

 (pleuro-peritoneal cavity), in C, by the formation of the diaphragm, pleural and 

 peritoneal cavities are distinct, while the pericardial cavity, containing the heart, has 

 been shoved backwards between the two pleural cavities. 



The abdominal coelom is not completely closed off from the 

 outer world ; for the urogenital ducts, to be described later, form 

 a means of communication. Besides these there occur in cyclos- 

 tomes, many fishes, dipnoi, turtles, and crocodiles, from one to 

 two small openings (known as abdominal pores), beside or be- 

 hind the vent, by means of which the coelom is connected with 

 the outside world. Little is known as to their function. 



MUSCULAR SYSTEM. 



The history of the muscle plates or myotomes is next to be 

 taken up. After their separation from the other portions of the 

 primitive mesothelial tissue, these form a series of approximately 



