PLATE VII
Venus de Milo.
The Muscular System.—The bony skeleton forms simply the framework of the body, and, while it determines the general outlines and height for the most part, the weight and general size of the body depend upon the muscular development and the amount of adipose tissue. The bones are not only padded about with muscles, but the muscles are inserted into the bony sheaths in such a way that a development of the muscles causes a development of the bones as well. Again, the stature is increased by the erect position of the spinal column, and this can only be attained by great strength of the muscles which hold the spine erect. The inequalities of the muscles are filled out with adipose tissue, giving a pleasing contour to the face and figure.
Fig. 21.—The skeleton (Lewis).
There are some five hundred muscles in the human body; these muscles vary in size and form, according to their situations in the body and the functions which they are called on to perform.
Nearly all the muscles in the body are arranged in two different or antagonistic sets, and are placed on the opposite sides of the part, so that in acting—that is, by their contractions or shortening—they move the limb in opposite directions, and it is by the alternate contraction, or shortening and relaxation of the two sets of muscles, that the movements of the body are accomplished. The muscles which bend the joints are called flexors, while those that extend the joints are called extensors, so that in order to perform their work, which is that of contraction, the muscles must exert enough force to elongate the opposing muscles, overcome the tonicity of the antagonizing muscles, and lift the weight of that portion of the limb into which they are inserted. It is by the action of the muscles that the body is held upright.
Fig. 22.—Muscular fibers, highly magnified.
