Page:Popular Science Monthly Volume 45.djvu/398

382 recorded in bold characters, in another may leave scarcely a visible mark; and it is obvious that a lean face will betray the story of emotional experience more readily than one covered with a mask of fat and smooth skin.

If we look at an anatomical representation of the human face with its integument removed, we see at once that the various groups of muscles are generally so arranged as to balance one another. Thus there is one set of muscles for opening the eyes or the lips, and another set for closing them; one group raises and another depresses the angles of the mouth, and so on. All these muscles, even when the features are quiescent, maintain a certain tone; for it is found that if one part of the face is paralyzed, the sound muscles near it draw it toward them and retain it there even when they are at rest. If one of the muscles or groups is stronger in proportion than its opponents, it will cause a marked change of expression, as is plainly seen in partial paralysis. It is a familiar fact that all muscles become larger and stronger through exercise; but the reason why they so increase is not such a simple matter. The vitality of muscular, and indeed of all other living tissue, is strangely under the influence of the nervous system. If the nerves which supply a limb are totally destroyed, it shrinks with extraordinary rapidity, although the main blood supply remains perfect. At the same time a limb may be paralyzed as to motion and yet undergo but little wasting, because certain nerve fibers (called trophic, because they have to do with nutrition) are left intact. In bedridden patients, again, in spite of a total want of exercise, the muscles often do not shrink in any great degree. Hence we see that nervous currents or impulses may influence the growth of a muscle apart from actual exercise.

Let us take an instance, the too visible results of which every one is familiar with. Persons who squint (with certain exceptions I need not here specify) are always "far-sighted"—that is, the convex lens of that marvelous living camera, the eye, is not quite convex enough; and in consequence its focus is too long to permit rays from a near object to form a clear image on the retina. If the retina could be pushed back away from the lens the difficulty would be overcome; but we can not, as in the case of an opera glass or a photographic apparatus, lengthen the space between the lens and the spot upon which the image is to be projected to any great extent, so Nature has provided a focusing apparatus in the crystalline lens itself. By a muscular effort the elastic lens can be made more convex, and in this way the focus is shortened to the required length. In long-sighted or flat-lensed persons this is constantly being done when they are reading or looking at some near object.

Now, it so happens that one of the little muscles which move