134 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
THE DERMAL MUSCLES. 
The muscles already mentioned are connected with the skeleton, 
but in the higher vertebrates a dermal musculature appears in which 
the muscles are inserted in the skin, although they are derived from the 
skeletal muscles. This system is poorly developed in the amphibia, 
and increases in the reptiles and birds, where it serves to move the 
scales, scutes and feathers. It is especially noticeable in the snakes, 
where it is largely concerned in the movement of the scutes in 
creeping. 
The system acquires its greatest development in mammals. In the 
marsupials, for instance, there is an extensive dermal musculature, the 
panniculus carnosus, covering a large part of the body and the ap- 
pendages. It is by means of this that various mammals twitch the 
Fic. 142.—Principal dermal muscles of head of man. a, as, auriculares anterior and 
superior; 7, frontalis; m, masseter; oc, occipitalis; 00, orbicularis oris; of, orbicularis pal- 
pebrarum; pm, platysma myoides; s, sternocleidomastoid; ¢, trapezius. 
skin to dislodge insects, etc., while armadillos and hedgehogs roll them- 
selves into a ball by means of a part of the layer. In the primates the 
dermal muscles are restricted to the neck (platysma myoides) and the 
head, all parts of them being supplied by the facial nerve belonging 
primitively to the hyoid region. The platysma extends forward from 
the neck and by growth and division gives rise to the muscles of ex- 
pression—the orbiculares which close the lips and eyelids, the muscles 
