THE MUSCULAR SYSTEM 



203 



myotomes and the consequent loss of metamerism leads towards the con- 

 ditions in mammals. An extreme degree of delamination affects the 

 lateral trunk muscles, some reptiles having as many as eight layers in the 

 body-wall. 



FRONTAL- 

 M. ORBICULARIS OCULI 



NASAL- 

 ZYGOMATIC 

 AUR I CULO- LABIAL SUR 

 AURICULO-LABIAL INF. 



TRIANGULAR 



A. ATELES. 



SUR AURICUI-AR 

 OCCIPITAL 

 POST. AURICULAR 



1-. / 



M. ANTITRAGICUS 



ANT. AURICULAR 



M. PLATYSMA- 



ANT. AURICULAR 



M. ORBICULARIS OCULI 



M. OUADRATUS LAB 1 1 SUR 

 CANINE- 



M. ORBICULARIS ORIS 



M. RISORIUS 



M. QUADRATUS LABI! INF 



M. MENTAL IS 



SUR AURICULAR 



POST. AURICULAR 



TRIANGULAR' 

 B. HOMO. 



Fig. 193. — Mimetic muscles in monkey (Ateles) and man. A, Ateles (redrawn from 

 Wilder after Ruge) and B, Homo. The similarity of these muscles both in function 

 and relations attests their similar genetic derivation. 



With the great enlargement of the appendages of mammals, there 

 appears a corresponding increase in the appendicular musculature and 

 trunk muscles become relatively reduced. Subdivision and migration of 

 muscles increase. Caudal muscles dwindle with the reduction of the tail. 

 In the trunk region, metamerism is preserved only in the intercostals, 

 the rectus abdominis, and the intervertebral muscles. 



