This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The Cerebral Hemispheres are the parts of the brain connected with the higher operations of intelligence. The experiment just alluded to, as performed on fowls, can bo performed less easily on mammals; but the result in both cases is the same, namely, that the larger the part of the hemispheres taken away, the less intelligence remains. The effect of removal of the hemispheres from a pigeon is graphically described by Dalton, the American physiologist. " The bird remains sitting motionless on his perch, or standing upon the ground, with the eyes closed, and the head sunk between the shoulders. Occasionally, the bird opens his eyes with a vacant stare, stretches his neck, perhaps shakes his bill once or twice, or smooths down the feathers upon his shoulders, and then relapses into his former apathetic condition. This state of immobility, however, is not accompanied by the loss of sight, of hearing, or of ordinary sensibility. All these functions remain, as well as that of voluntary motion. If a pistol be discharged behind the back of the animal, he at once opens his eyes, moves his head half round, and gives evident signs of having heard the report; but he immediately becomes quiet again, and pays no further attention to it. . . . Longet has found that by moving a lighted candle before the animal's eyes in a dark place, the head of the bird will often follow the movements of the candle from side to side, or in a circle.....The limbs and muscles are still under the control of the will; but the will itself is inactive, because, apparently, it lacks its usual mental stimulus and direction".
It is difficult, however, to say how far that part of the nervous system extends on which the existence of consciousness is dependent. Possibly it reaches over a greater area in the lower than the higher animals. A decapitated frog can be made to leap, and will thrust objects aside when irritated; and although these movements are sometimes said to be reflex, it is not easy to understand how they can be so. But, undoubtedly, all the higher manifestations of consciousness which constitute intelligence, depend on the cerebral hemispheres. On comparing different kinds of mammals, it is found that increased development of the hemisperes and complexity of the convolutions into which their surface is thrown are associated with increased intelligence. In rodent animals, such as rabbits, the hemispheres are small and smooth, while in apes, they are proportionally larger, and are more highly convoluted than in any animal but man. Even in the higher races of men, the convolutions are more complex than in the lowest. The circumstance that disease of the grey matter of the hemispheres is liable to be accompanied with intellectual derangement, likewise points to a connection between the hemispheres and intelligence.
But if the sensorium, or seat of consciousness, be confined to the encephalon, the question arises: How do we become aware of impressions made at the surface of the body? The old physiologists believed in the diffusion of consciousness through a sensorium commune extending thronghout the nervous system; but the loss of all sensation in parts whose nervous communication with the brain has been severed puts an end to that theory. At the present day, it is customary to say that the mind refers impressions received at the brain to the extremities of the nerves by which they have been conducted. But it is perfectly certain that there is no separate nervous communication between the brain and each point of the surface of the body in which sensations can be distinguished. The question is of the utmost interest psychologically, but is still unsettled. Personally, I believe that the only tenable theory yet put forward is that which I have elsewhere broached, viz., that while consciousness is dependent on the encephalon, the sensorium extends thence, so far as there is, at any moment, unbroken continuity of nerves in the active or impressed condition.
164. It is frequently supposed that different parts of the hemispheres are connected with different faculties of the mind; and the opinion that the frontal, parietal, and occipital regions are devoted respectively to the intellectual powers, moral faculties and appetites, or in some other way differ in function, is not confined to the believers in the system founded by Gall, and commonly known as phrenology. But there is no foundation for any such supposition. On the contrary, the evidence points to an opposite conclusion. Serious damage to the hemispheres, of a limited description, often occurs without loss of life, and such cases may occur without any apparent interference with intellectual functions. A tumour may press on some particular part of a hemisphere without producing any disturbance, and bodies have penetrated the brain, and portions of brain protruding from wounds have been removed by surgeons, without any obvious impairment of the mental faculties. Not only so, but the kind of disturbance which is liable to occur from such injuries does not vary according to the site of lesion, so as to affect, in one instance, the faculties of perception, in another the disposition, and in a third the powers of volition, as might be expected, were phrenological theories true.
In recent years a little colour has seemed to be given to the theory of separate organs in the cerebral hemispheres, by the discovery that imperfections of speech, constituting a condition called aphasia, arise from disease of a limited portion of the brain, immediately outside the island of Reil, particularly on the left side. In these cases the affections of speech are very various, but always depend on mental deficiency and not on paralysis of the tongue. In some instances there is total dumbness, in others, incapability of clearly uttering any word; and in a larger number of cases, certain words and phrases are pronounced perfectly, but they are not the words which convey the idea which the patient wishes to express. Perhaps some word is repeated on all occasions; and even when the right expression is suggested to the patient, he is unable to employ any other than that which he keeps repeating, although quite conscious of his blunder. The very variety, lowever, of these cases of aphasia, shows that they do not arise from damage to the organ of a specific faculty. It is more rational to recognise in them the result of a lesion which interferes with the consentaneous action of the different parts of the hemisphere, by attacking the fibres where they emerge from the corpus striatum to proceed to every part. And we may well believe that consentaneous action of the hemispheres is especially required in so complex a process as conversation, which requires a number of distinct mental operations to be carried on at one time.
155. A consideration of the different mental operations required in talking, is exceedingly instructive. The attention of the speaker is directed principally to the idea which he wishes to express. And, if engaged in a continuous discourse, he must at the same time think of the sequence of his utterances, and especially what is immediately to succeed the sentence with which he is engaged. The choice of words will also occupy his mind to a recognisable degree; but apart from this intentional choice, there is the choice of the simpler grammar and names of objects, which are so habitual that we fail to separate them in our thoughts from the ideas which they express. Still less attention is devoted to the complex movements of the organs of voice and speech; although they are all performed in the service of the mind, and it was with mental effort in infancy that we learned, by means of observation and imitation, to accomplish them. Gesture also accompanies speech without attention being directed to it; and, except in exceedingly rare cases of mental absorption, the speaker during all these mental actions is able to note what is going on around him.
The exactitude of the tongue in speech furnishes but one of many instances of complex movements performed under mental stimulus, without perceptible attention being given to them; the movements of the limbs in walking afford another example; and these are the kinds of acts which are sometimes, although erroneously, called unconscious cerebration. They are precisely like other voluntary movements, only the effect of habit on the mind is such that the mind gives the stimulus to the nervous system to accomplish them, without expenditure of attention. But the mind not only may initiate commands without devoting attention to them; it may receive impressions in like manner; and it often happens that an impression received without perceptible attention will lead to a customary act. Thus, a rider, without conscious effort, accommodates himself to the movements of his horse, and a sailor balances himself on board a ship. An act so performed may well be called automatic; but the term acquired reflex action sometimes given to it is of more doubtful propriety; for in true reflex action there is an unbroken sequence of physical changes, while, in such actions as these, a physical cause produces a psychical effect, and psychical change is the stimulus to the movement.