Page:Popular Science Monthly Volume 23.djvu/96

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THE POPULAR SCIENCE MONTHLY.

WHY ARE WE RIGHT-HANDED?
By W. C. CAHALL, M. D.

THE reader has no doubt often wondered why people almost invariably use their right hand in preference to the left. Is it not remarkable that, through all time and in all lands, man has been a right-handed being? The individual exceptions only prove the rule. What is the reason? It can not be simply imitation or heredity, for in those children who are disposed to use the left hand these influences will not avail in changing the inclination, even, in many instances, when supplemented by persuasion or force.

In my belief, there is a physical cause for this uniform habit—a cause that is demonstrable by anatomical and physiological facts. These, for the sake of brevity, are expressed in the following statements:

  1. The brain (cerebrum) is divided into two hemispheres.
  2. The nerve-force and nerve-fibers which produce muscular action on the one side of the body have their origin in the opposite hemisphere of the brain.
  3. The left hemisphere, from the earliest period, is larger and heavier than its counterpart, and the convolutions of gray matter—the PSM V23 D096 Left carotid artery versus the right.jpg1, 2, 3, aorta.
    1. Ascending part of aorta.
    2. Transverse part and arch of aorta.
    3. Descending part of aorta.
    4. Innominate artery.
    5. Right subclavian artery.
    6. Right common carotid artery.
    7. Left common carotid artery.
    8. Left subclavian artery.
    reservoirs of nervous energy—are more numerous on this side than on the right.
  4. This superior development of the left hemisphere as to weight, size, and richness of convolutions, may be attributed to a peculiar arrangement of the blood-vessels, by means of which a greater blood-supply is distributed to the brain-substance of this side.
  5. The arrangement of the blood-vessels to which I refer is the manner of origin of the right and left common carotid arteries. The carotid artery is a branch of the innominate artery on the right side, while it springs direct from the aorta on the left.

This directness of communication, in addition to a larger caliber of the left carotid, gives the left hemisphere a decided advantage in the race of development.

To reverse these statements we would have: as a consequence of the greater capacity of the left carotid the left hemisphere of the brain has a greater blood-supply; as a consequence, there is a greater de-