Central nervous system

“ Central Nervous System.—The neural tube shows in very early stages an anterior dilated portion which forms the rudiment of the brain in contradistinction to the hinder, narrower part which forms the spinal cord. This enlargement of the brain is correlated with the increasing predominance of the nerve centres at the anterior end of the body which tend to assume more and more complete control over those lying behind. ”

“ The next thing which we have to consider is how knowledge of the external world reaches the central nervous system. Sensations of touch, temperature, and pain are fairly easy to understand, since the nerves which convey such impressions have numerous endings in the skin. End organs of nerves in the joints and muscles doubtless enable the animal to perceive and estimate strain and resistance in moving or lifting things. ”

“ The central neurons become either quite separated from certain of the peripheral neurons, or at least the connections are not made with that nice adjustment necessary for the proper passage of nerve impulses. The shock communicated to the nervous system by fright is sufficient, however, to restore these connections, and consequently to enable the patient once more to exercise motor functions that have been in abeyance for some time. ”

“ It is perhaps a truism to state that upward progress is necessarily accompanied by increased development of the central nervous system; but it is necessary to lay special stress upon the importance of the central nervous system in all problems of evolution, because there is, in my opinion, a tendency at the present time to ignore this factor to too great an extent. ”

“ Rabies - viral disease of mammals usually transmitted through the bite of an infected animal, most commonly dogs; virus affects the central nervous system causing brain alteration and death; symptoms initially are non-specific fever and headache progressing to neurological symptoms; death occurs within days of the onset of symptoms. ”

“ The anatomy and physiology of the last century, however, prepared the way for the successful elucidation of the subject, and the first great step forward was made by Sir Charles Bell about the year 1825, in establishing the double action of the nervous system, or that impressions from the external world pour in upon the brain through one set of nerve-lines, while all the mandates of volition controlling human activity are transmitted outward along another system of nerve-lines. ”

“ 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. ”

“ When the cerebro-spinal nerves refuse longer to perform their function, fatigue and sleep ensue, and the current of blood leaves the brain and seeks the intestines. While the cerebro-spinal system rests, the sympathetic system takes up its task of directing the renewal of tissue and supplying the nerve sheaths through the lymph vessels, which draw their material from the digestive canal, with a new supply of phosphatic oil. Thus the brain and spinal nervous system are prepared for another day's work. ”

“ One includes the brain, spinal cord, the cranial and spinal nerves, which together make up the cerebro-spinal nervous system. The other division is called the sympathetic nervous system and has to do with those bodily functions which are beyond our control. Every group of cells in the body that has work to do (excepting the floating cells of the blood) is directly influenced by these nerves. Our bodily comfort is dependent upon their directive work. The organs which put us in touch with our surroundings are naturally at the surface of the body. ”

“ The nerve structures along which it runs in its trajectory are spoken of as a nervous arc. The whole purpose of the central nervous organ is therefore to bring afferent neurones into touch with efferent neurones. The whole purpose of reflex arcs is to bind one part of the organism to another part in such a way that what the environment is doing to the organism at one place may appropriately call forth or restrain movement or secretion in the muscles or glands possessed by the organism. ”

“ Cerebral Control over Lower Nervous Centres.—There accrues to the brain, especially to the fore-brain of higher Vertebrates, another function besides that of grafting psychical qualities upon the reactions of the nervous system. This function is exhibited as power to control in greater or less measure the pure reflexes enacted by the system. These pure reflexes have the character of fatality, in the sense that, given a particular stimulus, a particular reaction unvaryingly follows; the same group of muscles or the same gland is invariably thrown into action in the same way. ”

“ The higher functions of the nervous system, which make up our conscious mental life, are merely more complex modifications of this fundamental property of nerve substance. Closely related to this doctrine is the view that the nervous organism acts as a whole, that particular mental operations cannot be referred to definitely circumscribed regions of the brain, and that the hypothesis of nervous activity passing in the centre by an isolated pathway from one nerve-cell to another is altogether illusory. ”

“ So uniformly is a separate instrument provided for every additional function, that there is strong reason to regard the muscular nerves, although running in one sheath, as in reality double, and performing distinct functions. Sir Charles Bell, in his work on the Nervous System, endeavors to show, 352 that one set of nervous fibres conveys the mandate from the brain to the muscle, and excites the contraction ”

“ Every animal that has distance sense organs shows this feature of possible delay in the movement of the nervous disturbance over the nervous pathway, and in every one the particular part of the nervous system in which this occurs is known as the brain. In fact no other nerve cells except some of the nerve cells of the brain have this ability of stopping the nervous disturbance and holding it for a time before sending it on. ”

“ And in fact there have been evolved in the animal a number of structures called receptive organs which are selectively excitable by different environmental agencies. Connected with these receptive organs lies that division of the nervous system which is termed afferent because it conducts impulses inwards towards the nervous centres. This division consists of elongated nerve-cells, in man some two million in number for each half of the body. ”

“ It must be remembered, also, that the brain is not only an organ of mental function. It is likewise the center of the entire nervous system, and its volume, therefore, must correspond with the size and number of nerve trunks under its control. In man, as in animals, the brain elements are to a great extent but sensori-motor delegates whose function is the regulation and government of every part of the body. ”

“ As the separate wires are massed together to form cables, the neurons are massed to form the gross structures of the nervous system. The nervous system, however, is so radically different from anything found outside of the animal body that no comparison can give an adequate idea of it. We now pass to a study of the gross structures observed in the nerve skeleton. ”

“ Now, in the waking state, the varied stimuli, which constantly rain upon all the sense-organs, maintain the whole cerebrum in a state of sub-excitement, keep all the cerebral neurones partially charged with free nervous energy. ”

“ That what has been called nervous system consists of two parts, one purely sensorial, the other conveying impressions.
That the three powers are combined in the most perfect animals. That the muscular may be destroyed through the nervous system, and the nervous through the sensorial; and though each is not strictly dependent on the others, they are so connected that no one can exist long without the others. ”

“ The nervous system at its beginning is a neural canal, not only as regards its bony covering, but in its interior; a wide space lined by ciliated epithelium and filled with fluid extends along the axis of the spinal cord, and expands into a series of chambers in the brain. ”

“ Thus, for instance, when the nervous continuity of the spinal cord is interrupted, so that a stimulus applied to the lower extremities is unable to pass upwards to the brain, the feet will be actively drawn away from a source of irritation without the man being conscious of any pain; the lower nervous centres in the spinal cord respond to the stimulation, but they do so without feeling the stimulus. ”