270 



THE SALAMANDER 



5. The Diaphragm. 



Although there is no thorax in Urodela and the lungs retain their 

 primitive position in the abdomen, nevertheless the coelomic cavity 

 is bounded anteriorly by precisely the same structures as it is in the 

 Mammalia. The comparison between the Amphibia and Mammalia, 

 and also with Birds and Reptiles, has been discussed in detail by 

 Keith ( 1 905), who regards the pleural coelom of Mammals as having 

 been cut off from the abdominal cavity by the evagination of the 

 lungs through the diaphragm, the points at which this occurred 

 being marked by the pleuro-peritoneal openings of the embryo. 

 Keith agrees with Rabl in regarding the pleuro-peritoneal membrane 

 as 'merely a cresentic fold of peritoneum forming part of the 

 genital mesentery'. 



The three structures entering into the composition of the dia- 

 phragm in Salamandra are : (i) ventrally, a portion of the superficial 

 stratum of the M. rectus abdominis which is closely attached to the 

 ventral wall of the pericardium, while a few fibres are actually in- 

 serted into it; (ii) a central portion formed by the septum trans- 

 versum^ i.e. by the membrane dividing the pericardial coelom from 

 the main body cavity; and (iii) a dorsal part formed by those fibres 

 of the transversalis muscle-sheet which are inserted into the dorsal 

 pericardial wall and around the tracheal chamber (cf. Fig. 77; m.t.). 



Regarding the function of the diaphragm, Keith declares that it 

 is primarily concerned with assisting the venous circulation of the 

 abdominal cavity and filling the heart with blood by compressing 

 the viscera, and that its respiratory functions are secondary. The 

 latter function is expiratory in character in Salamandra, but becomes 

 inspiratory in Mammalia, where the lungs are anterior to the dia- 

 phragm. 



BIBLIOGRAPHY OF SECTION VII 



435 44, M, 1% 108, 121, 189, 218, 234, 255, 298, 309, 315, 350, 356, 368, 

 375, 3795 390j 418, 43O5 44^, 484, 505, 507, 51O5 606, 615, 627, 654, 694, 

 706, 786. 



