178 



COMPARATIVE ANATOMY 



ment in Mammals, and exhibits numerous modifications in passing 

 from Monotremes to Man. In lower forms (Monotremes, Fig. 138, 

 as well as, e.g. Dasypus, Centetes, Erinaceus, Pinnipedia, &c.), 

 it extends over the trunk and limbs (panniculus carnosus), while 

 in Primates it becomes reduced, and confined essentially to the 

 neck (jplatysma myoidcs) and head (mimetic muscles) : these 

 muscles are closely related genetically, and are all supplied by the 

 facial nerve. Two layers can be distinguished in the platysma 

 (Figs. 138 and 139), the more superficial of which has an oblique 

 or longitudinal direction, while the deeper layer (sphincter colli] is 

 circular : the two layers together correspond to the sphincter colli 



MJevatwlqbii 



M.orMo auric. M. helms 



M.oiit.ocuH \ M.auric.sup / 



\. M.maiicli- 

 bulo-miricnl 



Via. 139. SUPERFICIAL FACIAL MUSCLES OF Lepilcmin- mnxtefiint.t. The deep 

 layer is recognisable on the neck. (After Huge.) 



of the Sauropsida. They are continued on to the head, and there 

 give rise to a number of new muscles which are mainly grouped 

 around the eye, mouth, nose, and ear (Fig. 139). These mimetic 

 muscles are most highly differentiated in Man, but at the same 

 time reduction or tendinous transformation of certain of them 

 takes place, and some disappear entirely. 



The action of the integumentary muscles is very varied in 

 different Vertebrates. It may serve to roll up the body into a ball 

 (e.g. Hedgehog, Armadillo), or aid in the movements of the limbs 

 and tail in swimming (e.g. Ornithorhynchus), or serve to erect the 



