THE MUSCULAR SYSTEM 255 



the side of the neck, and, reaching- the ear, divides into two 

 sheets, dorsal and ventral. The dorsal sheet, auriculo-occipit- 

 clis, subdivides once more and furnishes the auricularis pos- 

 terior \_retrahens auris~\ and the occipitalis (i.e., the occipital 

 portion of the " occipito-frontalis " of human anatomy), and 

 from the latter are derived the intrinsic muscles of the dorsal 

 surface of the ear-flap, rudimentary in man. The ventral or 

 facial portion gives off along the sides of the mandible the two 

 slips, levator menti and quadratus labii inferioris [depressor 

 labii inferioris'] , which latter becomes attached to the bone, and 

 is continued over the face as M. sub-cutaneus faciei. The ulti- 

 mate differentiations of this latter portion are quite complex 

 and concern three portions into which the sheet divides itself. 

 Of these an auriculo-labialis inferior furnishes the intrinsic 

 muscles upon the ventral or forward surface of the ear-flap 

 and an auriculo-labialis superior differentiates into the zygo- 

 matlcus [major], the orbicularis oculi [palpebrarum~\ and the 

 levators of the lip and side of the nose. Finally a third element, 

 the orbit o-auricularis, furnishes two of the extrinsic ear mus- 

 cles, auricularis anterior and superior [attrahens and attol- 

 leus flttfw], and the frontalis, which in the apes comes nearly 

 in contact with the occipitalis previously mentioned, the two 

 becoming connected by a fascia. The gradual lifting of the 

 cranial dome and the formation of a forehead, culminating 

 in man, spreads apart the two muscular elements of this 

 occipitalis-frontalis sheet and extends the intervening fascia 

 to become the galea aponeurotica, so extensive in Man. That 

 portion of the platysma which covers the sides of the neck 

 in Man remains in its original undifferentiated condition, and, 

 although quite variable in its occurrence and in the* control 

 over it, is yet often capable of throwing the skin into longi- 

 tudinal folds, its original function. 



The deeper layer, the sphincter colli proper, extends also 

 to the face, but is mainly confined to the region about the 

 mouth, where it gives rise to orbicularis oris, caninus [levator 

 anguli oris'], buccinator and the intrinsic muscles of the nose. 



The original sphincter colli, as found in reptiles and mono- 

 tremes, lies within the province of the seventh cranial nerve 



