STRUCTURE OF THE VERTEBRATES 191 



dermal plates, these muscles are very large. During the evolu- 

 tion of the primates the integumentary muscles of the body re- 

 gion become of increasingly less importance, until in man they 

 have all but disappeared. 



The panniculus carnosiis is the major integumentary muscle 

 of the body, and in most mammals is distributed over the entire 

 back and sides of the animal. The panniculus of the lemurs is 

 large, enveloping the body. In the lower monkeys it becomes re- 

 stricted to a band on either side of the body, centering in the 

 axillary and inguinal regions ; and in the anthropoids it is further 

 reduced to small slips which spread out from the axilla to the 

 skin. The appearance of the panniculus in man is only occasional, 

 and it is then limited to thin bands in the axilla or to elongated 

 slips along the sternum. 



The integumentary muscles of the neck and face, however, do 

 not undergo the same degeneration. The alligator and some other 

 reptiles have a well developed, transverse band of muscles in 

 the neck, the sphincter colli. In the mammals this muscle divides 

 to form two distinct groups: a neck muscle which retains the 

 original name; and a larger portion, the platysma, which spreads 

 over the face and moves the skin, scalp and ears. The mimetic 

 muscles of the higher primates are derived from the platysma. 

 These become progressively differentiated in the anthropoids and 

 man. In the latter the muscles attached to the ears and scalp 

 have degenerated, the ability to move either of these structures 

 appearing only as an anomaly. The derivatives of the platysma 

 which control the eyes, lips and facial expression, on the other 

 hand, are more developed in man than in other mammals. As 

 with other muscle functions, it is incorrect to say that frowning 

 or smiling is limited to any one genus. The chimpanzee and 

 gorilla have every facial expression known to man, although in 

 many cases the coordinations are less well developed. 



