Page:Popular Science Monthly Volume 25.djvu/747

Rh —The muscular system of man is liable to many variations, nearly all of which are interesting from a morphological point of view.



It is not uncommon to find in man useless rudiments of muscles which exist in a well-developed state in some of our more humble fellow-creatures, and in them serve a definite purpose.

In man the "skin-muscles" are very feebly developed compared with those seen in many of the lower animals. The only remnants of these in man are, the muscle which wrinkles the forehead (occipito-frontalis), the muscle immediately under the skin covering the side of the neck (platysma myoides), and the palmaris brevis, a little bundle of muscular fibers in the palm of the hand; not unfrequently remnants appear abnormally in other situations, as, over the breast (see Fig. 9), in the arm-pit, on the back, etc. The skin-muscles are well developed in those of the mammalia which have loose skins, as, for example, the hedgehog, porcupine, and porpoise. In the hedgehog, when the skin-muscles contract, the animal becomes rolled up as in a bag of muscles. The sportive gambols of a school of porpoises are effected by an abundant supply of these skin-muscles; in the horse the skin-muscle is called the panniculus carnosus, and every one who has seen a horse twitching its skin to get rid of troublesome flies will easily understand how serviceable it is to that animal.

In all human beings there is a small muscle going from a hooked process (coracoid) on the upper end of the shoulder-blade to the inner side of the arm-bone about the junction of its upper and middle third. Sometimes this muscle is continued down to the lower end of the arm-bone; or, again, it may be quite short, and attached to the bag of fibrous tissue covering the shoulder-joint. On referring to the anatomy of the lower animals, it is found that both these varieties