100 Basic Structure of Vertebrates 



therefore be regarded as highly specialized body-muscles. The first 

 myotome produces the four muscles which are supplied by the third 

 (oculomotor) cranial nerve. From the second and third myotomes 

 come, respectively, the muscle supplied by the fourth (trochlear) 

 and the one supplied by the sixth (abducent) nerve. 



SUBCUTANEOUS (iNTEGUMEINTAL) MUSCLES 



These are thin sheets or slips of muscle which split off from super- 

 ficial muscles of the body and acquire attachment to the skin. The skin 

 of a fish is quite firmly adherent to the body. The layer of highly elastic 

 subcutaneous connective tissue intervening between the dermal layer 

 of mammalian skin and the surface of the adjacent muscle makes pos- 

 sible some movement of the mammal's skin in relation to the body- 

 muscles. Subcutaneous muscles have their origin on external surfaces 

 of body-muscles, or on such skeletal parts as are near the external sur- 

 face, and have their insertion on the inner surface of the dermal layer 

 of the skin. Their contraction causes the skin to slide over the surface 

 of the body-muscle. 



Subcutaneous muscles are most highly developed in mammals, but 

 are represented in birds and, to a small extent, in reptiles. In many 

 mammals a very thin sheet of muscle, the panniculus carnosus, 

 spreads over a large part of the surface of the body. It seems to have 

 been derived by cleaving off from certain muscles which have extensive 

 superficial exposure, the latissimus dorsi above and the ventral pec- 

 toralis (Fig. 91C). The sphincter colli of reptiles (Fig. 91B), birds, 

 and mammals is a subcutaneous sheet investing more or less of the 

 neck, the fibers of the muscle extending transversely to the neck. The 

 platysma of mammals (Fig. 96), an extensive subcutaneous sheet 

 covering more or less of the lower part of the side of the face and neck, 

 is probably derived from the sphincter colli. Many of the thin super- 

 ficial facial muscles, best developed in man (Fig. 96), are essentially 

 subcutaneous in nature. These facial muscles, together with the plat- 

 ysma and sphincter colli, in their general location and in being inner- 

 vated by the seventh (facial) cranial nerve, correspond to superficial 

 parts of the branehiomeric group of fishes. 



The subcutaneous muscles are striated and "voluntary." They en- 

 able the animal to wrinkle and twitch the skin. Their action is well ex- 

 emplified by the horse as it twitches the skin of an area where a fly has 

 alighted. The extensive panniculus possessed by the hedgehog and 

 armadillo assists the animal in its defensive act of curling itself into a 

 ball. The human facial muscles serve for small movements of the lips 

 and nose and to close the eyelids and wrinkle the forehead. They are, 

 in general, "muscles of expression." 



