Mesencephalon

“ 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 ”

“ The medulla oblongata and spinal cord innervate certain parts; the mesencephalon innervates others; the cerebrum rises above the whole. If after removing one limb, then another, we continued truncating the organism till we left only the head, should we call that the man? Clearly not. Should we even suppose that the intact brain—the supposed seat of sensation and volition—still felt, and willed? Clearly not. There is absolutely no evidence, however faint, of the isolated head manifesting any sensational and volitional phenomena ”

“ The brain, like the rest of the nervous system, is developed from the ectoderm or outer layer of the embryo by the formation of a groove in the mid-dorsal line. The lips of this medullary groove unite to form a canal beginning at the place where the neck of the embryo is to be. The part of the neural canal in front of the earliest union forms the brain and very early becomes constricted into three vesicles, to which the names of prosencephalon, mesencephalon and rhombencephalon are now usually given. ”

“ III., Lower aspect.
a, Medulla; b, corpora restiformia; c, cerebellum; d, lobi optici; e, hypophysis; f, fissura cerebri magna; g, nervus opticus; g’, chiasma; h, hemispheres; i, lobus olfactorius; k, sinus rhomboidalis (fourth ventricle) .
As regards external configuration, the brain of Lepidosteus and Amia approach still more the Teleosteous type. The prosencephalon, mesencephalon, and metencephalon are contiguous, and the cerebellum lacks the prominent transverse commissure at its upper surface. The sinus rhomboidalis is open. ”

“ A valvula cerebelli occurs also in ganoids, while in the Crossopterygians a similar extension of the cerebellum projects backwards into the IV. ventricle or cavity of the hind-brain (fig. 30) .
The mesencephalon is a conspicuous structure in the fishes from its greatly developed roof (tectum opticum) which receives the end pencils of the optic nerve. Normally it projects upwards as a pair of large optic lobes, but in the Dipnoans (fig. 29, C) the lateral thickening is not sufficiently great to cause obvious lateral swellings in external view. ”

“ This conclusion is confirmed by pathology. Disease of the striatum is followed by paralysis on the opposite side of the body. Disease of the thalamus, while not always so uniform in its testimony, does still, sometimes, give striking evidence of a sensory significance of the organ. It should be distinctly borne in mind, when speaking of these bodies, that paralysis of sensation and of voluntary motion may be produced by lesions in the cell-matter of the cerebrum apart from any injury to the thalamus and striatum ”

“ 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 coming to a conclusion respecting the function of this organ, we find that it can not be directly connected with sensation or volition. The sensations appear pronounced and the movements vigorous after destruction of the cerebellum. The marked feature in all deep cerebellar disorders is the maladjustment of muscular actions to the preservation of equilibrium and to harmonious movements. The consciousness of a normal relation between the person and the external world seems overthrown, the violent activities which ensue being plainly attempts to restore this lost feeling. ”

“ I take up my correspondent’s address at his desire, in the hope of learning something, and this sentence is among the first to catch my eye: ‘Voisin says that in simple insanity he finds certain alterations in the gray matter of the cerebrum, consisting of minute apoplexies, effusions of hæmatin and hæmatosin into the lymphatic sheaths, infarctions, atheroma, capillary dilatations, and necrosis of vessels, and certain changes of cerebral cells.’ Quite so. It may be all very true; but I can offer no suggestions as to medical treatment based [Pg 97] upon these remarkable assumptions. ”

“ 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 ”

“ He can predict all the movements that will take place as certainly as a chemist knows in advance the reaction he will obtain from mixing certain bodies.
Pathology confirms our conclusion respecting these higher functions of the cerebrum. Loss of cerebral substance, in man, is followed by a weakening of the intellectual powers. They make a very childish mistake who attempt to deal with the physiological materialism of our day by citing the American crow-bar case, or any number of cases of brain-loss unaccompanied by marked intellectual enfeeblement. ”

“ 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. ”

“ The cerebrum, or fore brain, is divided into two hemispheres, each of which is further divided into numerous lobes consisting of grey and white nervous substance and millions of nerve cells and fine blood vessels. The hemispheric ganglia are the instruments through which the intellectual powers manifest themselves. If these instruments have become imperfect in structure through retarded development by wrong environment and education, or have been damaged in any manner by severe shock, violence or disease, the manifestations of intelligence are affected in a corresponding degree. ”

“ The momentous announcement of modern physiology, that the cerebrum is the organ of consciousness and mental action in man and the higher mammals, is illustrated and confirmed by the pathological study of its diseases. When parts of the cortex are destroyed by disease their respective functions are affected, and thus we are enabled, to some extent, to localize the activities of the brain; when certain parts of the area are diseased, that portion of thought and consciousness disappears which depends on those particular sections. ”

“ Somehow or other, the nervous tissue of the cerebrum 'grows to' the thoughts that are allowed free course in the mind. How, science hardly ventures to guess as yet; but, for the sake of illustration, let us imagine that certain thoughts of the mind run to and fro in the nervous substance of the cerebrum until they have made a way there: busy traffic in same order of thoughts will always be kept up, there is the easy way for them to run in. Take a child with an inherited tendency to a resentful temper: he has begun to think resentful thoughts ”

“ Edinger [2] has shown that to the primitive division of the brain of the vertebrate (or the paleencephalon) , which is the bearer of the reflexes and the instincts, there has been gradually added with peculiar adjustments a newer division (the neencephalon) , which corresponds to the cerebrum, in which alone is centered the power of association, and of forming memory images. In proportion as the cerebral cortex increases, the primitive brain recedes, while in a corresponding ratio intelligence rises, and purblind instinct wanes. ”

“ The gray matter of the cerebellum, the medulla, and the cord may receive impressions from the senses and respond to them with movements, but their response is in all cases wholly automatic and unconscious. A person45 whose hemispheres had been injured in such a way as to interfere with the activity of the cortex might still continue to perform most if not all of the habitual movements of his life, but they would be mechanical and not intelligent. He would lack all higher consciousness. ”

“ At the same time the nervous centres, the cerebrum, the cerebellum, the spinal marrow, experience a very powerful impression. As the state progresses, consciousness is lost, and the subject is as it were in a state of delirium. The will is suspended, and the muscles are not controlled by it, but by the nervous centres which are so much irritated. Thus the trunk and limbs are agitated by involuntary motions and chills. ”

“ On removing the bones of the skull the brain appears to be normal macroscopically in a few instances, but in most cases the veins and capillaries of the meninges of the cerebrum, cerebellum, and occasionally the medulla is distinctly dilated and engorged, and in a few cases there are pronounced lesions of a leptomeningitis. An excessive quantity of cerebrospinal fluid is present in most of the cases. On the floor of the lateral ventricles of several brains there was noted a slight softening caused by hemorrhages into the brain substance. ”

“ With an excessive quantity, the functions of the cerebrum are suspended, and complete unconsciousness results. By an extension of the poisonous influence to the nervous centres governing respiration and circulation, these functions may cease, and death result. ”

“ While turning around in a circle has been already noticed there remains, in certain cerebral forms, the peculiar phenomenon of the animal rotating rapidly on its longitudinal axis. The patient falls on its side and rolls over and over. Among the brain lesions with which it has been experimentally identified are injuries to the middle peduncles of the cerebellum, or of the superoexternal portion of the cerebral peduncles, or of the posterior part of the encephalon, or of different parts of the hemispheres. ”

“ 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. ”

“ No organ possesses a vascular system so delicate and complex as the brain's, nor is there any so sensitive to the action of causes that modify the circulation. For this reason, disorders seated in the brain are peculiarly amenable to electric treatment, and, when carefully applied, it is remedial in brain-fevers, mental delirium, headaches, and sleeplessness. Physicians who first employed the current were quite aware of this benign influence of the galvanic fluid over brain-disorders, and even had the idea of utilizing it in the treatment of insanity. ”

“ It is an established fact that drugs do not affect brain structure; and it is equally well known that, in most instances, insanity causes no deterioration in nerve tissue. In these facts lies strong collateral proof that the sources of mental disease are to be sought elsewhere than in the brain. Injuries and ailments that involve change in brain substance will necessarily interfere with brain function, and, in softening of nerve tissue or in any inflammation, there are organic alterations that may be seen and noted. ”

“ As the spinal cord developed greater ganglia at one of its extremities, the more important of the psychic manifestations were passed on to these superior ganglia. Out of these developed the hind-brain, the mid-brain, the fore-brain, the total cerebrum in man, so that, as Goethe guessed, the skull is only the greatest of the vertebræ, and to the cellular contents of this crowning structure were at last transferred those neuroses which accompany human consciousness.
Consciousness is never strictly simple. ”

“ If, under the light of these facts, it be difficult to maintain that the seat of consciousness is not diffused through the central parts of the cerebrum and of the spinal cord, the pathological fact that in the human being the optic thalamus or the corpus striatum may be fundamentally changed in consistence and structure by disease, without loss of consciousness, is a barrier against the acceptance of Dr. ”