This section is from the book "Human Physiology For The Use Of Elementary Schools", by Charles Alfred Lee. Also available from Amazon: Human Physiology, for the Use of Elementary Schools.
1. The great superiority of the intellectual faculties of man over those of other animals, has led to a diligent examination, whether there be any thing in his anatomical structure which would seem to account for this superiority. Aristotle noticed the great size of the human brain compared with that of other animals, and laid it down as a general principle, that the faculties referred to this organ were in proportion to its size, compared with that of the whole body. Though this rule will hold true in relation to some of the domestic animals, yet it does not in relation to many others, for accurate researches have shown that while in man the ratio of the weight of the brain to that of the whole body is as 1 to 28, in the dog, it is as 1 to 160, in the horse, as 1 to 400, in the elephant, 1 to 500, in the canary bird, 1 to 14, and in one species of ape, 1 to 11.
2. It is now generally admitted by physiologists, that the perfection of the sensitive functions does not depend on the absolute size of the brain, nor on its proportion to the body .at large, but upon the proportion between the size of the brain, and the aggregate bulk of the nerves that proceed from it ; in other words, between the sensorial and nervous organs. For example, the absolute size of the brain of the horse is only about half the size of the human brain, while the mass of the nerves of the horse at their origin is no less than ten times larger than that of man. Extensive observations prove that though most of the inferior animals have larger nerves, and possess some of the nervous functions in a much more acute state than man, yet man decidedly exceeds them all in the comparative size of the brain, and in the perfection of his intellectual faculties.
3. The brain then, is the organ of the intellectual and moral faculties, the material instrument of the mind. This is proved not only by comparative anatomy, and experiments on animals, but by the history of injuries of the brain, compared with those of other organs. We know that if the nerves supplying any limb are severed, the will has no longer any influence over it; it is to all useful purposes a dead portion of matter. The same is true of the spinal marrow. If this be compressed by fracture or dislocation of the spine, the whole body below that point, is deprived both of sensation and motion, but the mind loses none of its powers any more than if a limb had been amputated.
4. If the brain is not the seat of the intellectual faculties, neither are any of the other organs of the body. The lungs, the liver, the spleen, the intestines, or the kidneys, may be affected with gangrene, and still the mind remains clear; though from sympathy, inflammation of any organ may cause delirium, or other mental affections. Neither is the heart the seat of the mind ; for cronic disease of that organ, does not impair the mental faculties. Besides the functions of all these organs are known and eannot be mistaken. If the brain is not the seat of the intellectual faculties, they cannot be said to have any seat in the body.
5. But the effects of injuries of the brain are very different, for every cause which disturbs its action suddenly or slowly, affects at the same time the mind. Inflammation of the brain is always attended with delirium, or stupor ; pressure on it, whether produced by depressed bone, foreign bodies, a tumour, serum, blood or pus, always gives rise to similar symptoms, and often destroy both sensation and motion. In cases of apoplexy, where a person falls in a fit, and becomes insensible, we find pressure on the brain from effusion of blood or serum. In cases of lunatics, we find in nearly all cronic cases, structural changes in the brain ; but if the case be recent, these changes though they probably exist, yet may escape our imperfect means of investigation. Alcohol, opium, and other narcotics affect the mind and the nerves through their influence upon the brain. Whenever in fact, we see a person become stupid and insensible, we may be certain that the brain has suffered some physical change; and where in cases of sickness, we see the mental faculties unimpaired to the last, we may be equally sure, that the brain is not affected.
6. The following facts also show that the brain is the organ of the mind. A man received a blow on his head, and immediately lost his mental faculties, and his bodily power. His appetite and digestion were good ; the blood circulated freely ; and his breathing and pulse were natural. He continued in this state more than a year, when a surgeon raised up a piece of bone which had been driven in upon the brain. His reason was immediately restored ; the next day he spoke, and in a short time he recovered entirely; but he could recollect nothing of what had happened since the accident. Not long since a beggar exhibited himself in Paris, who had lost a portion of his skull; his brain was only covered by the skin and membranes. For a trifling sum he would allow any one to press on this exposed part. As soon as any pressure was made he became wholly unconscious; but his intellect was immediately restored when the pressure was taken off.
7. It has been objected to the brain's being considered the seat of the mind, that in some cases, considerable disease has been found affecting an entire hemisphere without the mental faculties having suffered ; but experiments on animals show that a sudden lesion of one hemisphere only, does not immediately produce complete stupor, and that this effect does not follow until both are removed ; so that it appears that one hemisphere aids the other, and compensates for its inaction in the operations of the mind.
8. But though it is almost universally admitted, that the brain is the seat of the higher intellectual faculties, yet some physiologists, like Bichat, contend that the passions are seated in the thoracic and abdominal viscera. It is, however, to be remarked, that the passions, by means of the change which takes place in the brain, affect the whole nervous system. We find, for example in nervous females, that the exciting passions give rise to spasmodic action of certain muscles, especially those supplied by nerves belonging to the respiratory system ; hence the crying, sobbing, sighing, and the spasmodic distortion of the features. In the depressing passions, such as fear and terror, the muscles of the body lose their tone, because the supply of nervous influence from the brain is cut off; the limbs are not able to support the body ; the features lose their expression ; and the loss of power may be so great as to cause complete paralysis of the whole body.