Page:Popular Science Monthly Volume 65.djvu/371

Rh times. That this sign-language of primitive man was dextral is not to be questioned, as about 98 per cent, of babies are now clearly righthanded before they are one year old. The protection of the heart by the shield would constitute sufficient reason for the institution of dextrality in counting and sign-making, and custom and uniformity of habit especially in early times, would result in almost a universality. But not an absolute one, for one or two per cent, are now sinistral. And the Bible story of the Benjamite tribe illustrates how the habit would not be absolute. There is in all this one noteworthy neurologic fact: In view of the long continuance and vast preponderance of dextrality it seems strange that the brain preserves all the preformed mechanisms, plastic and ready to make a sinistral child, and the outworking of sinistrality is as prompt, the result as dextrous, as if dextrality had been chosen. The wonder at this is, however, lessened when one notes that all the functions of completed dextrality are at the same time and in the same person now possible to the sinistral; there is a mere difference in the degree not in the kind of expertness. Besides this a number of left-handed acts in the dextral, e. g., those of the violinist, gunner, etc., are far more expertly and finely coordinated than those of the right, etc.

If the foregoing explanation of the origin and perpetuation of dextrality is adequate, it remains to explain the origin of sinistrality. Why are there about two out of a hundred naturally left-eyed and left-handed? Fundamentally, of course, because the speech-center is located in the right cerebral hemisphere, and the contributing and executing centers of vision and motion act in better unity if they are in close connection and contiguity than if connected by long commissural fibers to and from the opposite sides of the brain. The dextrocerebrality of two per cent, of sinistral exceptions to the usual law appears explainable, perhaps in part by persistence of original sinistral types, but more certainly they are due to accident, injury, disease, etc., of dextral organs, in the young of our ancestors. Especially in savage life would these accidents be more numerous than now. The loss of even one dextral finger might compel the education of the undeveloped speech center on the right side. Injury to the right hand and arm, even of the right foot or leg would do the same. Deafness of the right ear would compel a turning of the left ear forward and might work out complete sinistrality. But more important than all these causes combined would be the more frequent greater ametropia, amblyopia,