Physiological Researches on Life and Death

by Xavier Bichat

Let us suppose for instance one hemisphere of the brain to be better organised, and therefore susceptible of livelier affections than its fellow; in such case the perception of the individual would be confused, for the brain is to the soul what the senses are to the brain; it transmits to the soul the impressions conveyed to it by the senses, as the senses convey to the brain the impressions made upon them by external objects. But, if the defect of harmony in the external senses confuse the perception of the brain, why may not the soul perceive but confusedly, when the two hemispheres of the brain are unequal in power, and incapable of blending into one the double impression, which is made upon them?

The memory is the faculty of re-producing former sensations, the imagination that of creating new ones, now in the act of remembering or imagining, each hemisphere of the brain appears to re-produce, or to create a sensation of its own. If both do not act alike, the perception of the mind, which ought to be the result of the two sensations united, will be inexact and irregular. But, it is evident, that there will be a disparity in the two sensations, if there be a disparity in the two halves of the brain, in which they have arisen, and since the general foundations of the judgment are made up of the faculties of perception, memory, and imagination, if these be confused, the judgment itself must be confused also.

We have now supposed an inequality of action in the hemispheres of the brain, and inferred, that the functions would in this supposition be imperfect; but what as yet is only supposition, in a variety of instances can be proved to be a fact; for nothing is more common than to find in consequence of compression on either hemisphere by blood, pus, or exostosis, a variety of alterations in the intellectual functions.

Even when all appearances of actual compression have vanished, if in consequence of that which has been experienced, a part of the brain remain enfeebled, the same alterations of mental power will be found to be prolonged. If both hemispheres of the brain, however, be affected equally, the judgment though weaker, will be more exact.[10] Perhaps it is thus, that we should explain those observations so frequently repeated, of an accidental stroke upon one side of the head having restored the intellectual functions, which had long remained dormant in consequence of a blow received upon the other side.

I now conceive myself to have proved, that with inequality of action in the hemispheres, there must be confusion of intellect. I have also pointed out some states of disease, in which such confusion is evidently the effect of inequality of action so occasioned; here we see the effect and its cause; but may we not from analogy, infer a similar cause where we see a like effect? when the judgment is habitually incorrect, and all the ideas wanting in precision, may we not be induced to believe, that there does exist a defect of harmony in the action of the two hemispheres of the brain? We see inaccurately if nature have not given to both eyes an equal power; we perceive and judge inaccurately in like manner, if the two sides of the brain are naturally dissimilar. The most correct mind, and the soundest judgment, pre-suppose in the hemispheres a perfect harmony of action; and what a multiplicity of shades do we not behold in the operations of the understanding? it is probable that they all of them correspond to so many varieties in the proportions of power in the hemispheres. Could we squint with the brain as we do with the eyes—that is to say, could we receive impressions on one hemisphere only, and form from thence our determinations, we might then command at will, a precision in our intellectual operations; but such a power does not exist.

To the functions of the brain succeed those of locomotion and the voice. The first of these would appear almost to form an exception to the general law. In considering the two vertical halves of the body, we shall perceive that the one is constantly more powerful than the other with respect to the strength and number of its movements. The right half is that, which from custom, is most made use of.

To comprehend the reason of this difference; we must make a difference between strength and agility; strength depends upon the perfection of the organization, on the energy of the nutritive process, on the plenitude of life in the muscular fibre; agility, on the contrary, is the result of habit and frequent exercise.[11]

At present we shall observe, that this disparity of action in the locomotive organs, does not consist in the difference of their actual strength, but in that of the agility, with which these motions are executed.—All is equal in the size, in the number of fibres, and nerves both of the one and the other of the superior, or inferior extremities; the difference of their vascular systems is scarcely any thing. From hence it follows that the discordance does not exist in nature, but that it is the effect of our social habits, which by multiplying our movements on one side of the body, increase their address without much adding to their power. Such in fact are the wants of society, as to call forth a certain number of general movements, which must be performed by all in the same direction, in order to be understood. It is generally agreed, that this direction shall be from left to right. The letters, which form the writing of most nations, are in this way directed; such circumstance occasions the necessity of our using the right hand to form them in preference to the left, the former being as much better adapted to this method, as the latter would be to the contrary one; of this we may convince ourselves by experiment.

The direction of the letters from left to right, imposes on us the necessity also of casting our eyes upon them in the same direction. From this habit acquired in reading, arises that of examining objects in the same manner.