Popular Science Monthly/Volume 23/May 1883/Why are We Right-Handed?

From Wikisource
Jump to: navigation, search
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 developement of the left hemisphere as to weight, bulk, and number of convolutions; as a consequence, when there is need of muscular action, the child naturally uses those muscles which possess the more powerful nerve-supply, for muscles are only strong in proportion to their nerve-supply; as a consequence, the nervous energy is dispatched, in those cases where there can be a choice, from the left hemisphere; as a consequence, the right hand and right leg will be the more likely used, since this side of the body is innervated by the left hemisphere. Thus, predisposition primarily, and use afterward, influencing and strengthening each other, fix upon us a habit almost unchangeable—how firmly, let those who ever attempted to break the habit in a left-handed boy testify.

But this leads to another question. Why are there left-handed people? Before we answer this question we will again look at the diagram. We have seen that, as the aorta rises from the heart, it arches from right to left, and the first large artery it gives off is the innominate, which in turn is divided into the right common carotid and right subclavian arteries. Farther on, we find the left common carotid and the left subclavian arteries arising separately from the aorta. Now, in making their dissections, anatomists have found that in a certain proportion of their subjects the aorta arches from left to right, in which cases the innominate is on the left side, and the common carotid and subclavian separate on the right. This arrangement would favor the growth of the right hemisphere, and would predispose to the use of the left hand.

Unfortunately, there have been no post-mortem examinations made for the purpose of observing whether this arrangement of blood-vessels and the use of the left hand really do occur in the same individual, nor is it necessary that it should be found in every case, for there are other anomalies in vessel-branching which would favor the growth of the right hemisphere. Apropos of speaking of the preponderance of the right over the left hemisphere, it might not be amiss to mention here that recent investigations have shown this condition of the brain to be characteristic of certain forms of insanity. This does not prove, however, that because a person is left-handed he is necessarily in any degree insane, as some dexterous reader may superciliously infer. Now, if the reason of our choice of a hand is due to an organic cause, how unwise is it to fight against nature, unless we commence at the beginning, and trust that habit will overcome the predisposition to the use of the left hand! Undertaken later, the result is often to spoil the skill of the left hand, without training the right to do its work as well. In conclusion, from what we have seen above, in answer to the question, Why are we right-handed? it might be said, because we are left-headed.