Cerebral hemisphere

“ The adaptation of the neural connexions of the two organs in this respect is therefore in accord with expectation.
The earliest cerebral cortex is formed in connexion with the neurone-chains coming into the central nervous organ from the patch of olfactory cells on the surface of the head. The region of cerebrum thus developed is the so-called olfactory lobe and hippocampal formation. The greater part of the cerebral hemisphere is often termed the pallium, because as its development extends it folds cloak-wise over the older structures at the base of the brain. ”

“ MODERN investigations have shown us that certain parts of the brain, situated in the region of the temples, have a predominant share in the formation of articulate language; or, to express it in a short phrase, that the majority of men speak by means of the third frontal circumvolution of the left cerebral hemisphere. All who have occupied themselves a little with physiology know also that the nervous fibers cross each other in the brain, so that the movements of the left arm are commanded by the right hemisphere, while those of the right arm depend on the left hemisphere. ”

“ When complete development is attained, the cerebral hemispheres completely cover the other parts of the brain, besides which they themselves are covered over with a multiplicity of folds constituting the convolutions. If we take a cross-section of the hemispheres, we find that they consist of an outer layer of gray matter formed of nerve cells, and of a central mass of white matter, formed of fibres. ”

“ The cervical cord passes insensibly into the medulla, the dorsal furrow becoming wider and more shallow as it merges into the fourth ventricle.
A dorsal view of the brain is shown in Figure 30, A. The most prominent structures here seen are the cerebral hemispheres, VH, whose combined transverse diameter is greater than their longitudinal. The tapering, cephalic end of each hemisphere forms an olfactory tract, I, which extends cephalad to form the olfactory bulb, B. ol. Lying between the caudal ends of the hemispheres is a small conical body, G.p., called by Bronn and others the pineal body. ”

“ And, first, it would seem that the cerebral hemispheres, including their integuments, are largely or completely devoid of the capacity of self-feeling. It has been known for some time that the substance of the brain is insensitive to pain ”

“ The general structure of the nervous system in all vertebrates from the frog to the man may be thus described: The cerebral hemispheres, or upper brain, a large double central nerve-mass which has no distinct nerves; the lower brain, which is also situated within the cranium, but below the cerebral hemispheres, and is provided with afferent and efferent nerves; and a spinal cord, which springs out of the lower brain and has an abundant supply of nerves connecting it with all parts of the body. ”

“ A peculiar fact relating to these convolutions is observed by all anatomists: mental [pg 99] development is always accompanied by an increasing dissimilarity between their proportional size.
The cerebral hemispheres may be injured or lacerated without any pain to the patient. The effect seems to be one of stupefaction without sensation or volition. A well-developed brain is a very good indication of intelligence and mental activity. That the cerebrum is the seat of the reasoning powers, and all the higher intellectual functions, is proved by three facts. ”

“ This is especially striking in the case of the fore-brain, where the cerebral hemispheres, which on embryological grounds we may conclude to have been hardly differentiated as distinct parts of the fore-brain in the most primitive types now extinct, gradually become more and more prominent, till in the highest Mammalia they constitute a more important section of the brain than the whole of the remaining parts put together. ”

“ Just as primitively the medullary fibers which arise in the dorsal zone pass into the ventral zone, so in the specialized cerebral hemispheres and in the cerebellum there arise very numerous nerve fibers, but these still obey the primal law and take their courses into the portions of the brain representing the ventral zones, and thence the fibers are distributed to their various destinations. ”

“ The name cerebrum is also applied to this portion of the brain; frequently, however, the name cerebrum is used as signifying the entire mass of the brain craniad of the rhombencephalon,—therefore including mesencephalon, diencephalon, and telencephalon. The term will here be used as synonymous with telencephalon.
The cerebral hemispheres arise as two lateral outgrowths from the cranial end of the primitive forebrain. They have undergone great increase in size in the course of evolution, so as to form the larger part of the brain ”

“ Compared with the cerebral hemispheres, these nerve centers are relatively more important in other vertebrates than in man, and are in more constant proportion to the size of the body, the muscular, glandular and sensory elements of which they supply. When the intelligence is about the same, they are, therefore, compared with the cerebral hemispheres, much more developed in the larger than in the smaller animals. ”

“ In fishes the cerebellum and cerebral hemispheres are minute, and the intermediate or optic lobes very large: in the reptiles the cerebral hemispheres exceed the optic lobes, while the cerebellum is smaller. In birds the cerebellum becomes complex and the cerebrum greatly increases. In mammals the cerebellum increases in complexity or number of parts, the optic lobes diminish, 156while the cerebral hemispheres become wonderfully complex and enlarged, bringing us to the highest development, in man. ”

“ The number of convolutions is about the same as one finds in Europeans of average intelligence; but the structure, as a whole, is, perhaps, a little less complicated and less tortuous than we are accustomed to see in our own sort. The large cerebral hemispheres completely cover the cerebellum. ”

“ But if stimulation of the brain does not paralyze, it must increase the tonicity of the vaso-motor centre, and hence the force and regularity of the circulation. Up to a certain point, these characters do indeed increase, with increase of pressure in the cerebral blood-vessels. They increase also during intellectual operations, unattended by emotion, in which a similar increase of pressure must take place, on account of the afflux of blood to the cerebral hemispheres, when these are aroused to activity. ”

“ The man is in a condition absolutely parallel to that of the frog, and no doubt, when he is in this condition, the functions of his cerebral hemispheres are at any rate largely annihilated. He is very nearly—I don't say wholly, but very nearly—in the condition of an animal in which the cerebral hemispheres are not entirely extirpated, but very largely damaged. And his state is wonderfully interesting to me, for it bears on the phenomena of mesmerism, of which I saw a good deal when I was a young man. ”

“ And, similarly, it is perhaps possible to conceive that the principles of mechanism might be more and more extended in their effects, until, in so marvellously perfected a structure as the human brain, all the voluntary movements of the body might be originated in the same mechanical manner as are the compensating movements of a watch; for this, indeed, as we have seen, is no more than happens in the case of all the nerve-centres other than the cerebral hemispheres. ”

“ In other words, there is every reason to believe that numerous "centres," in this restricted sense, exist in the brain that have as yet eluded the investigator. Indeed, the current conception regards the entire cerebral cortex as chiefly composed of centres of ultimate co-ordination of impressions, which in their cruder form are received by more primitive nervous tissues—the basal ganglia, the cerebellum and medulla, and the spinal cord.
This, of course, is equivalent to postulating the cerebral cortex as the exclusive seat of higher intellection. ”

“ Other Senses and Localization in the Cortex Cerebri.—Turning now to the connexion between the function of the cortex and the senses other than those of the great distance-receptors just dealt with, even less is known. Disturbance and impairment of skin sensations are observable both in experiments on the cerebrum of animals and in cases of cerebral disease in man. ”

“ Above, and on either side of the eyes, I saw the shadow of something white, soft, and spongy, in which I fancied I could detect a distinct likeness to a human brain, only on a large scale. There were the cerebral lobes, or largest part of the forebrain, enormously developed and overhanging the cerebellum, or great lobe of the hindbrain, and completely covering the lobes of the midbrain. On the cerebrum I even thought I could detect—for I have a smattering of anatomy—the usual convolutions, and the grooves dividing the cerebrum into two hemispheres. ”

“ When for any reason the supply of oxygen is insufficient, as in hemorrhage, producing cerebral anæmia, or in impregnation of the red blood corpuscles with carbonic oxide or chloroform, or other substances capable of excluding oxygen from the hemoglobin of the corpuscle, the cerebral tissues are imperfectly renovated. The explosive constituents of the cortical protoplasm are then inadequately renewed after mental activity, and the sensitive portions of the brain are no longer fitted to manifest the highest forms of intelligent activity. ”

“ With disease or willful destruction of the cerebral tissue the personal initiative is affected-- the animal becomes more distinctly a mechanism; the cerebellum is probably concerned in the coordination of muscular movements; and the medulla is a centre for the higher and more complicated respiratory reflexes, yawning, coughing, and so on. The great majority of reflex actions centre, however, in the spinal cord, and do not affect the brain. ”

“ But these volitional, emotional and visual centers are widely spread over the cortical area. About their special connections with one another, and especially with the center of the accessory nerve, under normal conditions, we are much in the dark. And how they could go to work to solve, in any length of time and even partially, the problem of readjusting their functional relations to the new and abnormal conditions, offers a problem as yet quite unanswerable by cerebral physiology. ”

“ Neither medicine nor physiology opposes the use of the clinical method in cerebral localization. But only circumscribed lesions that have little or no tendency to become general, or to act at a distance by compressing the brain, or otherwise, can come to the aid of our theory. When there is a lesion of the cortical region of the brain which fulfills these conditions, the resulting symptoms may be of two orders—either stimulative or paralytic of the true function. ”

“ From its small size, and from the imperfect development of the convolutions, this brain was, most likely, taken from a very young animal; the cerebral membranes have been removed; the vertex is somewhat flattened; the orbital surfaces have lost their characteristic concavity; the middle lobes have sunk asunder; and the cerebellum has, undoubtedly, been a little displaced backwards. ”

“ Lee, the big thing missing in that summation is the seemingly impossible task of synthesizing nerve tissue, especially that of the cerebral cortex. It's been approximated, at any rate closely enough to give us good enough results to allow an artificial tissue to respond to brain signals about ninety-eight per cent as well as the original would. ”

“ Caudad to the cerebrum are seen the end of the paraphysis, G.p., the optic lobes, MH, and the cerebellum, HH. From the cerebral peduncles (ventrad to the optic lobes) arises the oculomotor nerve, III, and dorsocaudad to this, between the optic lobe and the cerebellum, arises the trochlear nerve, IV. From the middle zone (in a dorsoventral direction) of the medulla, ventrad to the cerebellum, arises the very large trigeminal nerve, V; while from its usual place, on the ventral surface of the medulla, the abducens nerve, VI, takes its origin by several roots. ”

“ Objectively considered what is observable is hypertrophy of the cellules and nerve-tubes, excessive cerebral plethora and vascularity due to the great efflux of blood and superabundant nutrition of the encephalon. Inflammation of the meningeal covering, and of the brain itself, is, sooner or later, a further result of the chronic congestion. ”