Neural arch

“ The complex nature of our higher nervous centres, which is required to bring about the proper adjustments of the infinite variety of our present sensory experiences to appropriate co-ordination with a nearly equal variety of possible muscular combinations, is too common a subject of modern knowledge to need more than mention here. This vast central co-ordination of sensory paths to motor paths has itself become a special function of our neural organism of major importance. ”

“ They originate in nests of characteristic large cells located in the cerebral cortex and are regarded as paths, though not the only ones, through which volitional impulses are conveyed from the brain. They seem to control certain of the finer and more delicate movements of the body.
Great Complexity in Associations and More Neurons in the Brain of Man Than of Other Animals.—It has already been noted that as animals stand higher in the scale of life while the general plan of their neural elements remain the same, there is increasing complexity in the number and connections of the neurons. ”

“ Consequently, the neurons in retaining residua of the original process become, to a greater or less degree, organized into a functioning system corresponding to the system of ideas of the original mental process and capable of reproducing it. When we reproduce the original ideas in the form of memories it is because there is a reproduction of the physiological neural process. ”

“ HAVING thus summarily indicated those factors of evolution associated with genesis and which are essentially physiological, however much psychical phenomena may co-operate, we may touch upon the more purely psychical factors or those pertaining to the growth and use of mind, employing the term to express those neural phenomena traceable to the medium of the brain. Their importance in evolution increases with increasing cephalization and complexity of nerve system. ”

“ This imaginary construction can be made more concrete by imagining a solid with a large number of facets upon any one of which it can rest. If we try to balance it upon one of its coigns or ridges it settles down upon the nearest facet. In the case of the neural system we are trying to suggest each stable poise has been determined by a definite set, or better, context of conditions. Membership of this context is what corresponds to nearness to a facet. ”

“ 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. ”

“ This would seem to indicate that the greater mental activities of higher animals depend more on richness in complex associations than on mere increase in number of neurons. The latter, however, is by no means unimportant as may be seen in man, for instance, in whom it is estimated that the cerebral cortex, that is, that part of his brain in which his more complex mental processes transpire, contains some nine billion more nerve cells than does the corresponding region of the brain of an anthropoid ape. ”

“ It is just because of this community in their concepts that there actually is a certain “point to point” correspondence or parallelism between the psychical and the neural: as an organ a neuron is a unit; physically regarded, it ceases to be one. ”

“ It is hard to teach an old dog new tricks, because the old tricks employ familiar, much-used neural paths, while new tricks require the connecting up of groups of neurones not in the habit of working together; and the flow of nerve energy is more easily accomplished in the neurones accustomed to working together. ”

“ The study of the development of cortical neurones and association fibers makes it probable that every mental process modifies these nervous elements; so that education, whatever else it may be, is a matter of developing specific nervous organs through which the mind may work. Thus the study of mathematics means, on the neurological side, the building up of cortical neurones, with their association fibers, which shall constitute a mathematical nervous mechanism. So, likewise, the study of Latin or Greek means the building up of nervous structures specifically adapted for those languages. ”

“ It will help in understanding both the structure and the working of the nervous system to keep in mind that it contains but one fundamental unit of structure. This is the neurone. Just as the house is built up by adding brick upon brick, so brain, cord, nerves and organs of sense are formed by the union of numberless neurones. ”

“ Because of the greater plasticity of the neural mechanism in youth it is easier to open up and fix pathways of conduction than in later years. Moreover the earlier established lines of conduction become the more permanent.
Characteristic Arrangements of Nerve Cells Are as Subject to Inheritance as Other Structures of the Body.—That the main features of the nervous system [Pg 211] are inherited becomes obvious when we see that each kind of animal has its own distinctive numbers, arrangements and proportions of the various neural units. ”

“ We really see the skull pared off and the surface of the brain. The surface shows quite normal convolutions. Under a microscope some of them might, perhaps, show some differences in size and form, but now we can only see a common surface. We see, plainly also the well-known gray matter, composed of nerve-cells and deposited all along the surface in small convolutions,—and, for the benefit of laymen, I add that modern physiologists designate this gray matter as the seat of consciousness, thought, talent, and recollections. ”

“ That all we call mind is indissolubly connected with matter through the grey cells of the brain and other nervous ganglia. This is positive. If the skull could be removed without injury to the living organism, a skilful physiologist could play with his finger on the human brain, as on that of a dog, pigeon, or other animal, and by pressure on different notes, as on the keys of a piano, annihilate successively voluntary motion, speech, hearing, sight, and finally will, consciousness, reasoning power, and memory. But beyond this physical science cannot go. ”

“ But the association neurones are not only the most numerous in the vertebrates; they also constitute the basis of the most significant evolution. The central nervous organs that show the most conspicuous progressive changes in the vertebrates are the cerebellum and the cerebrum, particularly their cortical portions, and when it is remembered that few or no primary sensory or motor neurones contribute to these two organs, but that they are made up of association neurones almost exclusively, it will be seen how enormously important these neurones become. ”

“ 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. ”

“ In fact the convolutions of the average human brain give as much grey matter in a head of average size, as would be given by a head of four times the size if the brain were a plane surface. The extent of the convolutions is, therefore, a sure sign of the extent of intellect. They are more numerous and deeper in the European than in the negro; in the negro than in the chimpanzee; in the anthropoid ape than in the monkey or lemur. This grey nerve-tissue is the organ by which impressions from without are turned into perceptions, volitions, and evolutions of nerve force. ”

“ Each of the constant connections among the fibres of the cerebral masses answers to some constant connection of phenomena in the experiences of the race. . . . Those who contend that knowledge results wholly from the experiences of the individual, ignoring as they do the mental evolution which accompanies the autogenous development of the nervous system, fall into an error as great as if they were to ascribe all bodily growth and structure to exercise, forgetting the innate tendency to assume the adult form. ”

“ Such a process might be relied upon either to equip the cortical center of the accessory nerve for its new and more varied functions of control, or to prepare new paths of connection between this center and that which had formerly exercised exclusive control of the muscles of the face through the facial nerve. In a word, tin-building process in the brain, finding much of the old mechanism thrown out of its accustomed use, may have made ready another center and new tracts for the same use. ”

“ 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. ”

“ This curious state of things is probably only accidental to the present stage of development of the human mind, and may, at any time, be rectified by perhaps either a slight rearrangement of that slender network of nerves upon which depends our faculty of thinking, or the joining together of a few microscopical filaments attached to the cells in the grey cortical layer, or even a single bridge thrown across from one convolution to another of the brain; a very slight alteration would open up to our consciousness the present existence of the future. ”

“ 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 cortical neurone therefore collects impulses in the region of cortex just about its perikaryon and discharges them to other regions, some not cortical or even cerebral, but spinal, &c. One question which naturally arises is, do these cells spontaneously generate their impulses or are they stirred to activity by impulses which reach them from without? The tendency of physiology is to regard the actions of the cortex as reactions to impulses communicated to the cortical cells by nerve-channels reaching them from the sense organs. ”

“ We must now examine how new sense systems would join in upon the old, and, in consequence, what relationships the mental experiences mediated by the new would bear to those mediated by the old. Suppose a pair of eyes should develop in the worm's head. They would be of no use biologically unless joined in some way with the motor apparatus of the creature. Either a new set of motor nerves must be developed correspondent to the new optic fibres, or the new fibres must be grafted upon the old nervous centres. ”

“ The strongest adherents of computational neuroscience argue that not only do mental states depend on brain states, but that (as a methodological dictum) we ought to focus our scientific efforts on mapping neuronal firings only. That is, it’s not just necessary to understand the brain in order to get a grip on psychology—understanding how neurons work just is understanding psychology. ”

“ 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. ”

“ From the perspective of neurobiology, on the other hand, brains are emphatically not unanalyzed objects, but are rather composites of neural cells—neurobiologists make measurements that are designed to discern patterns in how chunks of the physical world consisting of neural cells (or clusters of neural cells) evolve over time. ”

“ In accordance with this, we find in different orders of animal life not merely analogies or resemblances in structure, but homologies or examples of complete identity of structure and function. Thus the brain, and the two sets of nerve lines, namely sensory and motor, are the same in nature and functions in all animals, from the frog to man inclusive, and they differ only in complexity of arrangement within the central organ, and extent of ramification of the nerve lines. Diversity of nature thus far appears in the relative complexity of organism. ”

“ The fact is that we do not inherit habits or associations as such at all, but the neurones of the grey matter in spine and brain which subserve, direct and control them. Though a fresh neurone or two in the brain of an early ungulate deliberating, so to speak, as to the life he shall take up, whether that of oxen or horses, may be trifling in itself as to immediate value to the animal, it may be to him as much a matter of fate to acquire those microscopic cells as it was to the undifferentiated organism that paused before it sealed its fate as plant. ”