MUSCULAR SYSTEM 1 39 



amphibians these various muscles undergo considerable differentia- 

 tion, while in the amniotes this is, in part, carried farther, in part they 

 are reduced on account of the loss of branchial respiration and the 

 degeneration of the parts connected with it. Hence the most notice- 

 able of the visceral muscles in the higher groups are those connected 

 with opening and closing the mouth. 



THE DERMAL MUSCLES 



The muscles already mentioned are connected with the skeleton, 

 but in the higher vertebrates a dermal musculature appears in which 

 the muscles are inserted in the skin, although they are derived from 



FiG. 149. — Principal dermal muscles of head of man. aa, as, auriculares anterior 

 and superior;/, frontalis; m, masseter; oc, occipitalis; 00, orbicularis oris; op, orbicularis 

 palpebrarum; pm, platysraa myoides; s, sternocleidomastoid; /, trapezius. 



the skeletal muscles. This system is poorly developed in the am- 

 phibia, and increases in the reptiles and birds, where it serves to move 

 the scales, scutes and feathers. It is especially noticeable in the 

 ''nakes, where it is largely concerned in the movement of the scutes 

 in creeping. 



The system acquires its greatest development in mammals. In 

 the marsupials, for instance, there is an extensive dermal musculature, 

 the panniciilus camosus, covering a large part of the body and the 

 appendages. It is by means of this that various mammals twitch the 

 kin to dislodge insects, etc., while armadillos and hedgehogs roll 

 themselves into a ball by means of a part of the layer. In the pri- 



