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Page:Popular Science Monthly Volume 65.djvu/369

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365
DEXTRALITY AND SINISTRALITY.

are dual in nature, requiring dual centers of coordination and innervation in the two halves of the brain. But speech, being a single function, can have but one center, and that, of course, must be located, not in any median place, because there is no such place, but in one or the other side. We also know by physiology and pathology that in the dextral it is in the third left frontal convolution, and in the sinistral it is in the corresponding position on the right side. We know, furthermore, that it is the intellectual act of writing, rather than the grosser acts and functions, which localizes the speech center. A man may be left-handed for everything but writing and the judgments issuing in the correlations of spoken words are formed and innervated from Broca's convolution. Or vice versa in the case of I;he sinistromanual writer who is dextromanual for all other acts.

The reason why dextromanuality, dextrocularity, etc., must coexist with sinistrocerebrality becomes manifest. The function of speech or writing is the method whereby judgment or volition passes into action. The initial, dominating and guiding motility to vocal organs, to hand, and even to foot, springing from closely contiguous, and hence more quickly and accurately acting, cerebral centers, will be better correlated and certain than if the centers were in opposite cerebral hemispheres. The indicator of all action, the very creator of intellect, is vision. Hence all right-handed people are also right-eyed.[1] The centers for right vision, right motion, and for speech are thus in close relationship and upon the same side of the brain. As I have said (Science, April 8, 1904):

The unification and perfection of innervation and cerebration must be better if initiated and executed with the cerebral centers mainly upon one side of the brain, than if the unity is gained by means of the longer and more distant commissural fibers extending between the two sides of the brain. In the right-handed the speech center is in the left side of the brain, as is also the motor center for the right hand, and the optical center of the right eye. The dependence of all motion upon a perfect correlation of vision and judgment needs only to be mentioned. That all intellect is psychologically the product of vision is less recognized, but is not less absolute truth. The right hand writes, possibly because the right eye looks down upon the writing more accurately than would the left; both depend upon the synchronous and

  1. One of the best tests of predominant dextrality or sinistrality is the 'sighting' of a stick to see if it is straight, or the sighting of a gun or pistol. Dextrocularity is largely a dictator of general dextrality. And of dextropedality also, for the dextral is right-footed also. Errors of judgment, however, have been frequent as to the function of the feet. The 'spade-foot' is the left, naturally, because the right leg and foot are the directing ones, in the dextral, who also, as the masonic ritual directs, steps off with the left foot first. The dextral must spring from the right foot. It has been said that the oblique line of the body of the dog in trotting is due to incipient right or left-footedness. But all soft-footed animals avoid 'interfering' by this obliquity of progression. The much discussed knockout blow of the pugilist with the left is, I suspect, because of the better spring from the firmer right foot.