Cerebrum

“ But the power to alter, postpone, or entirely inhibit, nervous movements is but a part of the general work ascribed to the cerebrum as the organ of the mind. Numerous experiments performed upon the lower animals, together with observations on man, show the cerebrum to be the seat of the mental activities, and to make possible, in some way, the processes of consciousness, memory, volition, imagination, emotion, thought, and sensation. ”

“ The cerebellum may be regarded as the physiological and the cerebrum as the psychic brain, for the cerebellum is void of intelligence and volition, but has important influences on the body. It may be considered, like the spinal cord, an intermediate structure between the controlling and conscious brain and the corporeal organs. ”

“ For there is no reason to regard the cerebrum as an exception to the general principle that, while each part of the organism tends to form itself in accordance with the mode in which it is habitually exercised, this tendency will be especially strong in the nervous apparatus, in virtue of that incessant regeneration which is the very condition of its functional activity. It scarcely, indeed, admits of doubt that every state of ideational consciousness which is either very strong or is habitually repeated leaves an organic impression on the cerebrum ”

“ As the fibres of the medulla pass up through the pons to the great inferior ganglion, and the fibres of the corpus striatum pass outward and upward to form the cerebrum, this procession of the fibres is shown in the annexed engraving, in which we see the restiform bodies passing up to form the cerebellum, and the remainder of the medulla fibres passing through the pons, and then, under the name crus cerebri or thigh of the cerebrum, passing through the thalamus and striatum to expand in the left hemisphere of the cerebrum. ”

“ That the nervous tissue of the cerebrum should be thus the instrument of the mind need not surprise us when we think how the muscles and joints of the tumbler, the vocal organs of the singer, the finger-ends of the watchmaker; the palate of the tea-taster, grow to the uses they are steadily put to; and, much more, both in the case of the brain and all other organs, grow to the uses they are earliest put to. ”

“ What the facts indisputably prove is that the cerebrum has an important part in the mechanism by which the most complex psychical combinations are effected, and that the cerebellum has an important part in the mechanism by which the most complex muscular combinations184 are effected. The supreme importance of the cerebrum may be inferred from its dominating all the other centres, and from its preponderance in size. In man it stands to all the other cranial centres together in the relation of 11 to 3. ”

“ Although a large cerebrum is generally indicative of more gray matter than a small one, yet it is ascertained that the grey substance depends upon the number, and depth of the convolutions of the brain, and the deeper its fissures, the more abundant is this tissue. ”

“ In man, the cerebrum presents an immense preponderance in weight over other portions of the brain; in some of the lower animals, the cerebrum is even less in weight than the cerebellum. Another interesting point is the development of cerebral convolutions in certain animals, by which the relative amount of gray matter is increased. In fishes, reptiles, and birds, the surface of the hemispheres is smooth ”

“ The Cerebrum or Great Brain lies above the plane of the tentorium, and forms much the largest division of the encephalon. It is customary in human anatomy to include under the name of cerebrum, not only the convolutions, the corpora striata, and the optic thalami, developed in the anterior cerebral vesicle, but also the corpora quadrigemina and crura cerebri developed in the mesencephalon or middle cerebral vesicle. The cerebrum is ovoid in shape, and presents superiorly, anteriorly and posteriorly a deep median longitudinal fissure, which subdivides it into two hemispheres. ”

“ Throughout the lower orders—fishes, reptiles, and birds—the cerebrum, though constantly increasing in relative size from mere nodules of gray matter less in size than the optic ganglia, up to masses of much greater bulk than the sensorium, still retains the same general appearance—two smooth oval bodies, one on each side of the median line, and gradually approaching each other as they enlarge until they meet, then extending forward and backward so as more and more to cover in and hide the sensorium. ”

“ The superior peduncles, which descend from the corpora quadrigemina of the cerebrum, form connexions mainly with the corpus dentatum. The middle peduncles form a large proportion of the white core, and their fibres terminate in the grey matter of the foliated cortex of the hemispheres. It has been noticed that those fibres which are lowest in the pons go to the upper surface of the cerebellum and vice versa. ”

“ Since the cerebrum is the seat of memory and of our mental life in general, it is both the most interesting and the most important part of the whole body, and a very large amount of study has been given to it. Many years ago attempts were made to show that the brain is subdivided into a number of parts, each of which has control over certain mental characteristics. ”

“ 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. ”

“ But above all those is the cerebrum, or brain-proper, consisting of two large lobes of nervous substance, which in man are so large that, when viewed vertically, they cover and conceal the cerebellum. To this organ is specially assigned emotion, volition, and ratiocination. It is the assumed seat of the mind; and, in a truer sense than the skull—
The dome of thought, the palace of the soul ”

“ After the removal of the brain from the skull the abnormal dimensions of the cerebrum present a remarkable contrast to the normal size of the cerebellum, pons, and medulla oblongata, which also appear flat and broad from pressure. When a horizontal section is made through the hemispheres of the cerebrum and a little above the corpus callosum, the centrum ovale appears unusually large. The cavities of the ventricles are very narrow, their walls touching one another. There is no serum in the pia mater or in the ventricles. The substance of the brain is pale, bloodless, and dry. ”

“ In the average bird (Fig. 47) the cerebrum has grown so large that it extends backward, and partly covers the optic lobes. In the lower mammals (marsupials) , the brain is much the same in this respect,163 as in birds—i. e., the cerebrum only partly covers the optic lobes, so that, looked at from above, the whole series of ganglia are still visible. But in the average mammal (Fig. 48) the cerebrum is so enlarged that it covers entirely the optic lobes and encroaches on the cerebellum behind and the olfactive lobes in front. ”

“ This is an important intermediate structure between the cerebrum and the cerebellum, and in fishes is the largest part of the brain, but in man is the smallest portion, as will be explained hereafter, and is the origin of the optic nerve, as well as a commanding head for the spinal system, from which convulsions may be produced. ”

“ The basis of the middle lobe, which is the most physiological portion of the cerebrum, being devoted to the sensibility, appetites, and muscular impulses, is that which first presents itself, being the first outgrowth from the great inferior ganglion or summit of the spinal system. As human brains degenerate to a lower type they approximate this form. ”

“ Around the whole is wrapped a tough membrane (dura mater) , which lines the bony box of the skull, and separates the various parts of the organ by strong partitions. The brain consists of two parts—the cerebrum, and the cerebellum.
The CEREBRUM fills the front and upper part of the skull, and comprises about seven eighths of the entire weight of the brain. As animals rise in the scale of life, this higher part makes its appearance. It is a mass of white fibers, with cells of gray matter sprinkled on the outside, or lodged here and there in ganglia. ”

“ The cerebellum has its unconscious action in the processes of respiration and in the involuntary movements which are made in response to the senses, as in winking, starting back at a sound, etc. The cerebrum acts automatically in oases familiar to all. A large part of our mental activity consists of this unconscious brain work. There are many cases in which the mind has obviously reasoned more clearly and more successfully in this automatic condition, when left entirely to itself, than when we have been cudgeling our brains, so to speak, to get the solution. ”

“ 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. ”

“ In the human cerebrum, the vertex is extraordinarily high and is placed further back; so that the fall of the outline behind, is necessarily more sudden, and the depth of the posterior region is very characteristic. In the ape again, the shortness and shallowness of the anterior portion makes the curve of that part of the cerebrum more abrupt, and more equal to the hinder curve, than it is in man, in whom the elongated and deep, frontal region produces a much more gradual curvature from the vertex forwards, than exists backwards. ”

“ Its cerebral area includes the posterior and inferior portions of the cerebrum, the entire cerebellum, and that part of the medulla which connects with the spinal cord, all of which sustain intimate relations to vital conditions. Accordingly, such a development indicates good digestion, active nutrition, vigorous secretion, large heart and lungs, powerful muscles, and surplus vitality. The violent faculties, such as Combativeness, Destructiveness, and Hatred, are natural adjuncts, and their excess tends to sensuality and crime. ”

“ The extent of cerebral cavity behind c, which is a perpendicular erected on b at the point where the tentorium is attached posteriorly, indicates the degree to which the cerebrum overlaps the cerebellum—the space occupied by which is roughly indicated by the dark shading. ”

“ After the cerebrum has been cut transversely in this way the peduncles, pons, medulla and cervical cord are elevated on the index-finger of the left hand and also sectioned transversely and the cut surfaces examined. If the index-finger be placed beneath the medulla parallel with its long axis, and medulla and pons raised up the cerebellar lobes fall to the side out of the way. All transverse cuts are made from left to right and so deep that only a small portion of brain-tissue, or the basal meninges hold the parts together for future orientation. ”

“ From the twelve weeks embryo to those of twenty-one weeks and of seven months we trace a progressive development and a commencement of the furrows that form the convolutions.
In the brain of seven months, the right hemisphere is out open horizontally, showing the ventricle.
Thus we perceive in the essential plan of the brain its two organs, cerebrum and cerebellum, are hollow spheres which grow gradually into solid bodies, filling their interior cavities, of which the lateral ventricles in the cerebrum, which have been explained, are the remnants. ”

“ When the surface of the human brain is inspected, it is seen to present a very unequal appearance, due to the fact that its substance is thrown into a large number of folds or convolutions (see Fig. 1) , as they are technically named by the anatomist. The brain, or cerebrum, is in reality a double organ, formed of two similar halves or hemispheres, which are separated by a deep central fissure, but which are also connected together below by a broad band of nervous matter known as the corpus callosum. ”

“ From such simple beginnings as are seen in Ammocœtes, the higher forms of brain have been evolved, to culminate in that of man, in which the massive cerebrum and cerebellum conceals all sign of the dorsal membranous roof, those parts of the simple epithelial tube which still remain being tucked away into the cavities to form the various choroid plexuses. ”

“ There is reason to believe that, whatever it may do besides, one office of the cerebellum is to combine the action of the voluntary muscles for the purpose of locomotion. The corpora quadrigemina are four tubercles which connect the cerebrum, cerebellum, and medulla oblongata to each other. If one of the uppermost of these bodies be removed, blindness of the eye of the opposite side is the consequence. ”

“ Upon examining the contents of the cranium, the pia mater was considerably inflamed, and an extravasated blotch, about the size of a shilling, was seen upon that membrane, near the middle of the right lobe of the [Pg 49] cerebrum. There was no water between the membranes, nor in the ventricles, but a general determination of blood to the contents of the cranium. The medullary substance when cut into was full of bloody points. The consistence of the brain was natural. ”