112 INITIATIVE IN EVOLUTION 



Professor Keith, a short account of that muscle in simiadae. It 

 is taken from an unpublished work of his on The Myology of the 

 Catarrhini — a Study in Evolution. The account may be only 

 interesting to the professed anatomist, but the conclusions in the 

 summary bear closely on the present problem. I give the exact 

 words from Chapter II., pp. 472, 479. The simian forms 

 examined are semnopithecus, gorilla, chimpanzee, orang, gibbon, 

 macacus, cercopitheci, cynocephali. " Summary .■ Every gradation 

 is found between the cynocephalic and human forms . The evolution 

 lies in the disappearance of the supra-trapezial origin and the 

 superficial labio-mental insertion. The opposite nuchal and 

 mental angles of a trapezoidal sheet are obliterated and a rhomboid al 

 figure is left. The change may be seen step by step through the 

 macaci, semnopitheci, hylobates, troglodytes and the orang to Man. 



" The maxillary insertion in man is more extensive than the 

 others, and the insertion is more distinctly demarcated from the 

 quadratus menti origin. But slips between the two muscles are 

 not uncommon. 



" The sub-mental interdigitation occurs frequently in man, 

 and although its extent varies in the other Catarrhini it is always 

 present. 



" The upper nuchal fibres, being cut loose in the higher members 

 of the orthorachitial group from their primary origin, became 

 aberrant in their behaviour. Auriculo -labial slips, slips of union 

 with the zygoma tici, or simulating a risorius, or a relapse to the 

 primitive medial dorsal origin and connection with the occipito- 

 auricular muscles may occur in man as in the others. 



" Fasciculation of the muscle may occur in man and the 

 troglodytes, 



"That the functions of this muscle are indefinite is shown 

 by the numerous individual and generic variations. But that 

 its presence is essential may be judged by its persistence. It may 

 depress the angle of the mouth or the lower jaw, or help to flex 

 the head upon the chest, or help to empty the laryngeal air-sac 

 if it be present. But as a matter of fact all these functions are 

 otherwise provided for. When tense it protects the deep part of 

 the neck somewhat, and it is usually active in temper. The 

 axillary part of the same sheet in the cynomorphce offers a similar 

 puzzle as regards its functions." 



We have it thus on the highest authority that the platysma 

 muscle is active and persistent in a large series of monkeys, apes 

 and man. But the whole work has for its sub-title, " A Study in 

 Evolution," and in the story of the platysma there is a picture of 



