Nervous tissue

“ The fibres which emanate from them are very delicate, and, becoming mixed with others, do not admit of being further traced. Where the nervous tissue meets the enveloping epithelium it is connected with the latter from within, but differs widely from it; for the nerve-cells contain a longitudinally striated cylindrical or thread-like nucleus which carries on its peripheral end a delicate hair, while its central end is prolonged into a fine nerve-fibre. There are, besides these, two other kinds of cells which form a transition between the ganglion and the epithelium cells. ”

“ Now of all the tissues the nervous tissue is that which takes longest to repair when injured; and it seems a not improbable inference that it is a tissue which is slower in its histological development than others. If this be so, we may see why, in the embryos of the higher vertebrates, the central nervous system quickly grows large in comparison to the other systems—why by pre-adaptation the brain of a chick develops in advance of other organs so much more than the brain of a fish. ”

“ The NERVES that are spread over every part of the sensitive layer of the true skin, proceed from the spinal cord. As a proof of the great number of nervous filaments in the skin, no part of this tissue can be punctured with a fine needle without transfixing a nerve, and inducing pain. In some parts of the system, however, the nerves are more abundant than in others; where the sense of feeling is most acute, we find the greatest number of nerves, and those of the largest size. ”

“ In his opinion this distinction can only be made good by employing electrical stimulation upon the sub-umbrella tissue when this has lost its spontaneity under the influence of poisons: if a response ensues which does not ensue when the tissue is stimulated mechanically, he regards the fact as proof that the muscular tissue remains unaffected while the nervous tissue has been rendered functionless. ”

“ Muscular action, however, cannot take place without nervous action. These two tissues are dependent one on the other. Yet the muscular tissue may be considered as subordinate to the nervous tissue. While the muscular tissue may become totally inactive or incapacitated, or even removed, the brain tissue may retain its activity and continue to perform its functions. The very reverse takes place when the brain is either injured or removed. We know by experience, experiments, that injuries or other pathological changes will cause impairment to muscular tissue. ”

“ If perturbation of nerve-function—a disturbance quite different from mere exaltation of the normal development of nerve-force—be the essence of pain, how comes it that pains of the severest type may be produced by changes in structures which are usually described, for practical purposes, as lying outside the nervous system? We must, in the first place, remark that the externality of any bodily tissue to the nervous system is more apparent than real. ”

“ The deeply stained oblong nuclei indicate the nerve corpuscles within the neurilemma.
264. The Nervous System Compared to a Telegraphic System. In men and other highly organized animals, nerves are found in nearly every tissue and organ of the body. They penetrate the most minute muscular fibers; they are closely connected with the cells of the glands, and are found in the coats of even the smallest blood-vessels. They are among the chief factors of the structure of the sense organs, and ramify through the skin. ”

“ In the order of decreasing generality an account of organs should be preceded by an account of systems of organs. Some of these, as the muscular system and the osseous system, are co-extensive with tissues, but others of them are not. The nervous system, for example, contains more than one kind of tissue and is constituted of many different structures: besides afferent and efferent nerves there are the ganglia immediately controlling the viscera, and there are the spinal and cerebral masses, the last of which is divisible into numerous unlike parts. ”

“ With a little care it can be stripped off the brain without causing injury to its substance. At the base of the brain the pia mater is prolonged on to the roots of the cranial nerves. This membrane consists of a delicate connective tissue, in which the arteries of the brain and spinal cord ramify and subdivide into small branches before they penetrate the nervous substance, and in which the veins conveying the blood from the nerve centres lie before they open into the blood sinuses of the cranial dura mater and the extradural venus plexus of the spinal canal. ”

“ The muscular tissue, which acts independently of the will—as, for example, the stomach, intestines, womb, blood vessels, ducts, etc.—is called involuntary muscular tissue, and receives nervous stimulus from the sympathetic division.
The brain, spinal cord, and the ganglia are the central organs of the nervous system. The nerves conduct the nervous influence. The nerves terminate differently according to their function. ”

“ The system naturally groups itself under three heads, the nerve ring, the sensory clubs, and the motor plexus of fibres and ganglia that underlies the epithelium of the subumbrella. The general relations of the nerve ring and of the sensory clubs have been given before in the description of Charybdea Xaymacana, so that we may pass at once to the consideration of the finer details of the nervous tissues. ”

“ In the same point of view it becomes clear also that, as the inclosing of the lower system in the higher one is characteristic of animal organization, and the rudiments of the nerves always show this peripherous type, the higher animals, and man in particular,—in whom the spinal nerves encompass the common cavity of the trunk, somewhat similarly to the bending of the ribs,—possess a system of nerves appropriated to the vegetative structure, and perfectly analogous to the nervous system of the inferior animals, namely, the ganglionic system, or that of the great sympathetic nerve. ”

“ We should remember that the nervous system is one continuous structure, and it is only the necessities of nomenclature and the ingenuity of anatomists that have divided it into so many parts. It is as continuous as is an oak tree with its various branches. The spinal cord and the nerves are composed in varying proportions of the same materials as is that part of the system which we call the brain: injury and disease with them give rise to symptoms analogous to similar injury or disease to the brain. ”

“ Certain organs in which the cerebral nerves evidently enter, as the liver, the lungs, &c. can be irritated in animals, without seeming to give them much pain. 3d. The muscles of animal life, in the structure of which so many nerves enter, in which also the branches of these perform so great a part as it relates to animal contractility, do not occasion much pain when their texture is cut without entangling the nervous filaments that penetrate them. 4th. The ligaments, that no nerve enters, are the seat of acute pain when they are distended, as my experiments have proved. ”

“ By the separation of a main branch of one of the nerves of sensation, although all the operations of life will still proceed, the organ to which that nerve goes is dead to its particular sense. In like manner, if one of the nerves of volition is divided, the member will not obey the inclination of the brain. It is evident, therefore, although many of the great phenomena of vital force are dependent on the nervous system, and the paralysis of a member ensues upon the separation or the disease of a nerve, that the nerves are but the channels through which certain influences are carried. ”

“ 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 ”

“ As the cellular texture, the means of union, grows from the cut extremity of the nervous coat, as well as from that which is between the cords, it partakes of the nature of the nervous coat, and becomes a parenchyma of nutrition, whose mode of organic sensibility is analogous to that of the nerves, and whose vessels deposit there medullary substance, which gives a new appearance to the nervous cicatrix, and makes it resemble very nearly the texture of the nerves themselves. ”

“ All the operations of the nervous system are performed by the influence of the nervous fluid, which is generated in the gray portions of the brain and ganglia. Whenever a nerve is cut off from its connection with these nervous centres, its power is gone, and the part to which it ministered becomes lifeless and incapable of motion. ”

“ Anatomy.#--A nerve-trunk is made up of a variable number of bundles of nerve fibres surrounded and supported by a framework of connective tissue. The nerve fibres are chiefly of the medullated type, and they run without interruption from a nerve cell or _neuron_ in the brain or spinal medulla to their peripheral terminations in muscle, skin, and secretory glands. ”

“ 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. ”

“ So early as 1845, while still at Zürich, he supplied what was as yet still lacking, the clear proof that nerve-fibres are continuous with nerve-cells, and so furnished the absolutely necessary basis for all sound speculations as to the actions of the central nervous system. ”

“ These central nervous masses are in vertebrates all fused into one, of which the part which lies in the head is especially large and complex, because directly connected with particularly important and delicate receptive organs. The part of the central nervous organ which lies in the head has, in consequence of its connexion with the most important receptive organs, evolved a dominant importance in the nervous system, and this is especially true of the higher animal forms. ”

“ The tissue composing the nerves is entirely of the white variety, or, in other words, the nerve-fibres; the same as we have observed forming a part of the brain. But the nerves, instead of being soft and pulpy, as in the case of the brain, are dense in structure, being hardened and strengthened by means of a fibrous tissue which surrounds each of these delicate fibres, and binds them together in glistening, silvery bundles. ”

“ The function of nerve-cells is different, viz., that of accumulating nervous energy; and, at fitting times, of discharging this energy into the attached nerve-fibres. The nervous energy, when thus discharged, acts as a stimulus to the nerve-fibre; so that, if a muscle is attached to the end of a fibre, it contracts on receiving this stimulus. I may add that, when nerve-cells are collected into ganglia, they often appear to discharge their energy spontaneously; so that in all but the very lowest animals, whenever we see apparently spontaneous action, we infer that ganglia are probably present. ”

“ Yet, no one denies that each is a morbid affection of the central nervous system; this being granted, the rejoinder to those who insist on the extreme unlikeness is that the lesion in one is at the very bottom, in the other at the very top, of the central nervous system ”

“ Nerve cells and often large areas of gray matter are replaced by connective tissue (resembling scar tissue) , which grows and increases in what would otherwise be vacant spaces. All areas which contain this connective tissue, this filling which has no function, of course, cease to join with other parts of the brain in concerted action, and so the power of the brain is diminished, and certain of its activities are restricted or abolished. ”

“ This mass of nerve tissue is much smaller than the cerebrum. The white nerve tissue forms central nuclei which send out branches that ramify in every direction. The centre of the muscular sense is said to be located in this division of the brain. A second function is the maintenance of body equilibrium through its connection with the nerve of the middle ear. ”

“ 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. ”

“ Through his efforts, and with the invaluable aid of such other workers as Remak, Purkinje, Henle, Muller, and the rest, all the mystery as to the general characteristics of nerve tracts was cleared away. It came to be known that in its essentials a nerve tract is a tenuous fibre or thread of protoplasm stretching between two terminal points in the organism, one of such termini being usually a cell of the brain or spinal cord, the other a distribution-point at or near the periphery—for example, in a muscle or in the skin. ”