MUSCULAR SYSTEM 141 



for the sternum is always interposed between them, as it is in 

 the Sauropsida. 



In Monotremes and Marsupials, a strong 7;2/ra??it(f(;6/ts muscle 

 lies on the ventral side of the rectus abdominis. It arises from 

 the inner border of the "marsupial bones" (epipubes, p. 121) and 

 may extend forwards as far as the sternum. In the higher 

 Mammals, where the epipubes are absent, the pyramidalis usually 

 becomes greatly reduced or entirely lost. Traces of it are, however, 

 commonly to be met with even in the Primates, and always arise 

 from the anterior border of the pubis, right and left of the middle 

 line. 



The external and internal oblique muscles are represented in 

 the thoracic region in Mammals, as in the Sauropsida, in the form 

 of external and internal intercostals. 



What has been said above as to the differentiation of the dorsal 

 portion of the trunk-muscles in Reptiles applies also essentially 

 to Mammals. 



The greater number of the muscles in connection with the 

 external genital organs become differeiitiated from the primitive 

 sphincter cloacce : the origin of the others is not known. 



B. Muscles of the Diaphragm. 



A complete diaphragm dividing the coelome into thoracic and 

 abdominal cavities occurs only in the Mammalia. It is dome- 

 shaped and muscular, its muscles arising from the vertebral column, 

 ribs, and sternum. The diaphragm is of great importance in 

 respiration, as it allows of a lengthening of the thoracic cavity in 

 a longitudinal direction. It is supplied by a phrenic nerve, arising 

 from one or more (3rd to 6th) of the cervical nerves ; and usually 

 consists of a central tendon, perforated by the oesophagus and post- 

 caval vein, and of muscular fibres radiating from this to the 

 periphery and forming dorsally two strong " pillas'S of the dia- 

 phragm." In some cases {e.g., Echidna, Phocasna) the diaphragm 

 is entirely muscular. 



Amongst the Sauropsida, a partition is present between the 

 pleural and peritoneal cavities in Chelonians, and is still more 

 marked in Crocodiles and Birds ^ : this is connected with the 

 ribs by muscular fibres. It, however, does not enclose the peri- 

 cardium, which, as in the Anamnia, lies in the general peritoneal 

 cavity. 



The evolution of the mammalian diaphragm is not yet tho- 

 roughly understood. 



' In Birds, two entirely different structures liave been described as a 

 diaphragm. {See under Air-sacs.) 



