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Page:Popular Science Monthly Volume 65.djvu/364

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THE theories that have been advanced as to the origin of dextrality and sinistrality are:

1. A natural provision. (Sir Charles Bell, and others.)
2. The left-sided location of the heart. (Referred to by Wilson.)
3. A greater supply of nerve force to the muscles because of an earlier and greater development of the brain upon one side. (Professor Gratiolet.)
4. Obstruction to the flow of blood in the vena cava, by the pulsation of the aorta. (Dr. Barclay.)
5. Inspiration produces, mechanically, a superior efficacy of the muscles of the right side. (Professor Buchannan.) This theory is based upon the observation of the anatomic peculiarities of the liver, lungs, etc., and their supposed influence upon the center of gravity of the body. (So far as pertains to the center of gravity, the theory has been adopted by Dr. Struthers and by Dr. Allis.)
6. The center of gravity theory. The influence of the weight of the viscera of the two sides of the body, upon the position of the center of gravity. (Dr. Struthers, accepted by Buchanan, Allis, etc.)
7. The origin of the subclavian arteries, the left before the right in the-left-handed, with superiority of blood-supply to certain structures. (Professor Hyrtl.)
8. The development of one cerebral hemisphere more than the other. (Wilson.)
9. The Topsy theory—'just growed.'

These theories merit little argument in rebuttal. No. 3 and No. 8 are essentially the same, and, of course, are mere avoidances of an explanation. No. 2, No. 4, No. 5, No. 6 and No. 7 are not based upon facts, and contain fallacies of observation, rendering them at least of insufficient reach and validity. No. 9 is almost as good as any or all of the rest, and we are left with the frank confession of Dr. Struthers, that the mystery 'has baffled satisfactory explanation.' Carlyle said it was 'a question not to be settled, not worth asking except as a kind of riddle.' It is, however, of great and practical importance in medicine and in social life.

In a large way and notwithstanding a certain number of exceptions, it is an illuminating truth of biology that 'the ontogeny repeats the