140 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



mates the dermal muscles are restricted to the neck (platysma my- 

 oides) and the head, all parts of them being supplied by the facial 

 nerve belonging primitively to the hyoid region. The platysma ex- 

 tends forward from the neck and by growth and division gives rise to 

 the muscles of expression — the orbiculares which close the lips and 

 eyelids, the muscles which lift lips, nose and lids and those which 

 move the ears — muscles which as a whole have their highest devel- 

 opment in man (fig. 149). 



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 anlagen in the neck, in part from 

 the rectus and transverse 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 develop- 

 ment is outlined in the section on the coelom (p. 18); it being only 

 necessary to add here that parts are also contributed by the pleuro- 

 peritoneal membrane and by the dorsal mesentery. 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 mammals the muscular portion is confined 

 to the margin, the centre being membranous; in others the muscle 

 fibres extend across it. Contraction of the muscles flatten it, thus 

 enlarging -the^xleural caviti es a adjdrawing air into the lungs, and so 

 ai3ing in respiration. It is supplied by the phrenic nerve which, like 

 the muscles, arises in the cervical region, a fact of interest when it is 

 recalled that lungs and heart (anterior to the diaphragm) arise in the 

 primitive head region, and later migrate into the trunk. 



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 dis- 

 charge is weak in most species, but is strong in Torpedo and Gymnoius. 

 In all but Malapterurus the electrical organs are clearly modified 

 muscles, situated in the head in Torpedo and Astroscopus, in the trunk 

 of Gymnotus, and in the tail of Rata, the nerve supply being corre- 

 spondingly varied. Thus in Torpedo the seventh, ninth and tenth 



