MUSCULAR SYSTEM. 135 



which lift lips, nose and lids and those which move the ears muscles 

 which as a whole have their highest development in man (fig. 142). 



THE DIAPHRAGM. 







The diaphragm is a transverse voluntary muscle which crosses the 

 body cavity of the mammals just behind the pericardium and lungs. 

 Its muscles are in part derived from those of the back, in part from the 

 rectus muscles of the lower surface. Various attempts have been made 

 to recognize similar muscles in the lower vertebrates, in some cases 

 with considerable success. Its development is outlined in the section 

 on the ccelom (p. 123). The diaphragm is dome-shaped and is attached 

 to the vertebral column and to the ribs. It is traversed by the 

 oesophagus and the large arterial and venous trunks. In some verte- 

 brates the muscular portion is confined to the margin, the centre being 

 membranous; in others the muscle fibres extend across it. Contrac- 

 tion of the muscles flatten it, thus enlarging the pleural cavities and 

 drawing air into the lungs, thus aiding in respiration. It is supplied by 

 the phrenic nerve. 



ELECTRICAL ORGANS. 



It is well known that the contraction of a muscle causes the dis- 

 charge of a minute amount of electrical energy, so it is not surprising 

 that in certain cases muscles are modified into electrical organs. The 

 known cases occur only in elasmobranchs and teleosts. The discharge 

 is weak in most species, but is strong in Torpedo and Gymnotus. In all 

 but Malapterurus the electrical organs are clearly modified muscles, situ- 

 ated in the head in Torpedo and Astroscopus, in the trunk of Gymnotus, 

 and in the tail of Raia, the nerve supply being correspondingly varied. 

 Thus in Torpedo the seventh, ninth and tenth cranial nerves are con- 

 cerned, while in Gymnotus and the skates the supply is from the spinal 

 nerves. Malapterurus is peculiar in that the organ is in the integu- 

 ment and has been supposed by some to arise from modified glands. 

 It is more probable that here as elsewhere it is derived from the muscles, 

 as the organ is under control of the will; the development has yet to be 

 studied. This diversity of origin clearly shows that the electrical 

 functions have been separately acquired in the different species. 



The organs are composed of a large number of electrical plates 



