Hindbrain

“ Figure 17a is in the region of the pharynx, and passes through the forebrain (fb) and posterior part of the hindbrain (hb) . In the thick walls of both of these structures histological differentiation has begun, so that even under low power an inner granular and an outer clear zone may be distinguished. Under greater magnification the presence [309] of short fibers may be made out among the cells. The cerebral hemispheres (ch) are well-marked structures, their asymmetry being of course due to the obliquity of the section. ”

“ The cerebro-spinal cavity is divided into the sub-cavities, called the cranial cavity and the spinal cavity.
In the cranial cavity we find the brain. The brain is the seat of the mind, and the functions which the brain performs distinguishes man from the other animals, as man becomes a conscious, intelligent, responsible being through the action of the brain. The brain is egg-shaped, soft and yielding, closely fitting the cranial cavity. The front and top of the brain is called the cerebrum, which is the center for intelligence, reason, and will. ”

“ In the lowest vertebrates, as in the amphioxus, the nervous system consists of nothing more. It may therefore be said to have only two parts—the cord and the sensory ganglia, and to have two functions—the automatic, attributable to the former, and the instinctive, attributable to the latter.
But as we advance from the low vertebrates upward in the animal scale, we begin to detect new organs; on the medulla oblongata a cerebellum, and on the sensory ganglia a cerebrum. Their intellectual apparatus. ”

“ The mid-brain gives rise to the third nerve, and has the optic lobes on its dorsal side, but these are hollow, and they are not subdivided by a transverse groove into corpora quadrigemina, as in the rabbit. In the hind-brain the cerebellum is a mere band of tissue without lateral lobes or flocculi, and the medulla gives origin only to nerves four to ten; there is no eleventh nerve, and the hypoglossal is the first spinal-- from which it has been assumed that the rabbit's medulla equals that of the frog, plus a portion of the spinal cord incorporated with it. ”

“ 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. ”

“ The nervous system on the dorsal surface of the notochord consists of the brain in the head and the spinal [75] marrow running down the back. The brain of all except the very lowest vertebrates consists of four portions: 1. The cerebrum, or cerebral lobes, or simply "forebrain," the seat of consciousness, thought, and will, and from which no nerves proceed. Whether the primitive vertebrate had any cerebrum is still uncertain. 2. The mid-brain, which sends nerves to the eyes, and in this respect reminds us of the brain of insects. ”

“ The lower centers are directly connected by nerves with the sense organs, glands and muscles, while the higher centers have direct connections with the lower and only through them with the sense organs, glands and muscles. In other words, the sensory nerves run into the cord or brain stem, and the motor nerves run out of these same, while interconnecting nerve strands extend between the lower centers in the cord and brain stem and the higher centers in the cerebrum and cerebellum. ”

“ The three main divisions of the brain may still be clearly seen from the surface. Posteriorly is situated the hind-brain, now consisting of the medulla oblongata and cerebellum. At the anterior part of the medulla is to be seen the thin roof of the fourth ventricle, and anteriorly to this again the roof becomes thickened to form the rudiment of the cerebellum. In front of the hind-brain lies the mid-brain, the roof of which is formed by the [Pg 294] optic lobes, which are still situated at the front end of the long axis of the embryo. ”

“ Intellect, Emotion, and Volition. Physiologists assign to the cerebrum its functions, and neurological, as well as phrenological writers, have located them as represented in Fig. 68. True, there is no structural division between the parts of the cerebrum to indicate this diversity of function, nor is there any perceptible limit between the sensory and motor filaments of the game nerve. As no one has any reason for denying that separate portions of the brain may manifest distinct functions of the mind, we shall assume it as a conceded proposition. ”

“ 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. ”

“ The disposition of the fibres in the brain is far too complex to be accurately followed. All that we can say is, that there are strands which connect one convolution with another, strands which connect one hemisphere with another, strands which connect cerebrum with cerebellum, and strands which connect the cerebrum with the lower ganglia. It is important to conceive this distinctly; for we shall hereafter see that the function of the Brain (by brain is here meant both Cerebrum and Cerebellum) is not that of innervation, but of incitation and regulation. ”

“ 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. ”

“ 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. ”

“ Are there any more internal and peculiar to vertebrates than the brain and nerves? Let us take the neurohæmal system itself, that which confessedly has no perfect counterpart in the articulate class. It is not suggested that any creature lower than cephalopod mollusks presents any thing exactly corresponding to the cerebral hemispheres, and the optic and auditory lobes of the vertebrate brain. But let us see if we cannot trace some homologues? Without this historical derivation from annulose segments there will remain many things in cerebral anatomy entirely inexplicable. ”

“ 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. ”

“ Its growth is faster than the rest of the neural tube and from its walls, by proliferation, develop the structures of the cerebrum, mid-brain, and hind-brain. It also gives off ganglionic masses from which grow sensor processes to form the afferent elements of the cranial nerves and contains, like the cord, motor nuclei, or nidi, from which motor axons grow toward the periphery to come into relation with definitely predetermined organs. ”

“ Sometimes, indeed, the two sets of filaments are combined into one nerve cord, but if traced to their origin these are found to arise from different brain centres. Thus it was clear that a hitherto unrecognized duality of function pertains to the entire extra-cranial nervous system. Any impulse sent from the periphery to the brain must be conveyed along a perfectly definite channel; the response from the brain, sent out to the peripheral muscles, must traverse an equally definite and altogether different course. ”

“ In all Craniates the dorsally-placed and tubular central nervous system has its anterior portion enlarged and otherwise modified to form a "brain," while the remaining portion, retaining a simpler and more uniform structure, forms the spinal cord. In the embryo the brain always consists of three successive sac-like enlargements known as the fore-, mid-, and hind-brain, and from {144}these are developed the various parts of the complex adult brain, which in the disposition and mutual relations of its parts conforms to a common plan in all the members of the group. ”

“ 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. ”

“ Psychologists would venture a step further, and say, “It is not the brain that feels, but the intelligent individual entity, the living soul or self, of whom the brain is only the material transmitting organ.”
It is at the extremity of this ganglion that the cords wrapped within that great bundle of nerve cords which constitutes the spinal cord cross each other and pass into opposite sides of the brain and of the body. The nerves that control the left side of the body pass into the right side of the brain, and those that control the right side of the body pass into the left side of the brain. ”

“ To these, in human anatomy, the designations of anterior, middle, and posterior lobes have been respectively given. In proportion, as this development has proceeded, the intellectual qualities have become more varied and more profound.
Action of the spinal cord alone.The relation of the cerebrum to the cranio-spinal axis is manifested by the circumstance that the latter can act without the former. In sleep the cerebrum is, as it were, torpid, but respiration, deglutition, and other reflex actions go on. If we touch the palm of a sleeping infant our finger is instantly grasped. ”

“ The manner in which the brain has formed itself in the evolution of the animal world will render the comprehension of its activities easier. Let us consider the simplest creatures, those possessing, so to speak, only a spinal cord. The nerves branching off from the upper part to the nostrils, eyes, ears, mouth, and elsewhere, were subjected during the long series of generations to more continuous stimuli than other nerves. The cells placed at the roots of these nerves were constantly excited by impressions from the external world ”

“ The nerves are simple conductors, while the central part perceives sensations and determines movement. Now, in this central part, the cerebro-spinal, inclosed by the cranium and vertebral column, as in a case, there are two distinct elements; an active element, and a conducting element. The white substance is the conducting: element; the gray substance the active. The gray substance forms a thin column, which is the central part of the spinal cord, and is continued into the brain where it enlarges. ”

“ 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. ”

“ The chains of neurones passing through the cortex are more complex and connected with greater numbers of associate complex chains than are those of the spinal centres. But just as the reflex centres of the cord are each attached to afferent channels arriving from this or that receptive-organ, for instance, tactile-organs of the skin, or spindles of muscle-sense, &c., so the regions of cortex whose function is to-day with some certainty localized seem to be severally related each to some particular sense-organ. ”

“ The frontal brain or cerebrum contains the centers for the sensory organs, also the motor centers which supply the driving impulses for the muscular activities of the body, and in the occipital and frontal lobes, the centers for the higher and highest qualities of mind and soul, which constitute the governing and restraining faculties on which depend the powers of self-control. ”