MUSCULAR SYSTEM 185 



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 paired phrenic nerves 

 arising from one or more of the cervical nerves (usually the 4th or 

 4th and 5th, but varying from the 3rd to the 8th) ; and is per- 

 forated by the oesophagus, aorta, postcaval and azygos veins, 

 thoracic duct, &c. In most cases it consists of a central tendon 

 from which muscular fibres radiate to the periphery and form 

 dorsally two strong " pillars " of the diaphragm. In some Mammals 

 (e.g. Echidna, PhocEena) the diaphragm is entirely muscular : in 

 the higher Primates the central tendon unites secondarily with 

 the pericardium. 



The nerve-supply of the diaphragm indicates a polymeric 

 origin from the ventral portions of several myomeres. In the 

 course of development, it, like the pericardium, becomes shifted 

 backwards. The first rudiment of the diaphragm (" septum 

 transversum ") is composed of connective tissue into which the 

 musculature extends secondarily, and is situated ventrally on 

 either side of the median line: eventually it becomes closed in 

 laterally and posteriorly. 1 It is important to note that in the 

 innervation, as well as the grouping of the muscles, a costo- 

 sternal and a lumbar portion can be recognised in the mammalian 

 diaphragm. 



Although in many respects the mode of evolution of the 

 mammalian diaphragm still requires elucidation, it is at any rate 

 certain that a close connection exists between its development and 

 that of the thorax and the changed respiratory conditions. The 

 diaphragm acts as an important respiratory muscle, and also aids 

 the abdominal muscles in the compression of the abdomen. 



c. Muscles of the Appendages. 



All the muscles of the appendages of Vertebrates are primarily 

 to be looked upon as derivatives of the ventral muscles of the 

 trunk, i.e., of the myomeres. This is indicated, apart from the 

 nerve-supply, by their mode of development in numerous Anamnia, 

 although in the Amniota the primitive mode of formation is not 

 clearly recognisable owing to an abbreviation of development. 



Two principal groups of appendicular muscles may always be 

 distinguished : one lying in the region of the pectoral and pelvic 

 arches, dorsally and ventrally, the other in the free extremity. In 



1 ri- 



This mode of formation can be recognised not infrequently in those terato- 

 logical cases in which the costal and lumbar portions of the diaphragm do not 

 become united. 



