Nerve fiber

“ Just as every solid is built up of molecules, so the nervous system is built up of nerve-threads and nerve-corpuscles. We owe to Mr. Lewes our very best thanks for the stress which he has laid on the doctrine that nerve-fiber is uniform in structure and function, and for the word neurility, which expresses its common properties. And similar gratitude is due to Dr. Hughlings Jackson for his long defense of the proposition that the element of nervous structure and function is a sensori-motor process. In structure, this is two fibers or bundles of fibers going to the same gray corpuscle ”

“ And it is fairly well understood how, by the action of this craftily balanced apparatus of nerve-fibers and nerve-centers, the supply of blood to the various tissues is adjusted to their ever-changing needs. But when we ask ourselves what happens in one of the nerve-cells which compose the nervous centers when it discharges an impulse? what that impulse which flies at the rate of a hundred miles an hour along the nerve-fiber really is? ”

“ The nerve fiber, which is connected with the nerve cell from which it arose, is capable of growing out again. It can regenerate itself, and that is a well-known phenomenon, and in many surgical cases it becomes a phenomenon of very great importance.
Let us go back to another familiar figure. Here is represented the lining layer of the œsophagus with the cells composing it, the upper bones being horny, the lower ones those which are capable of active growth. We are rather dull. We do not often stop to think about things. We buy a new horse which comes from the country, has never seen a train ”

“ There appear very early certain cells, which soon become recognizable as young nerve cells (neuroblasts) because of their size and pointed shape; the pointed end now elongates into a very delicate thread, the nerve fiber, which is at first very short but rapidly lengthens almost like a growing root; the growing fiber takes its course for a certain distance, varying according to circumstances, within the wall of the medullary tube, but ultimately passes outside the tube into the neighboring tissues together with other nerve fibers of similar origin. ”

“ A, a medullated nerve fiber, showing the subdivision of the medullary sheath into cylindrical sections imbricated with their ends, a nerve corpuscle with an oval nucleus is seen between the neurilemma and the medullary sheath; B, a medullated nerve fiber at a node or constriction of Ranvier, the axis cylinder passes uninterruptedly from one segment into the other, but the medullary sheath is interrupted. ”

“ The nerve-fibers in the trunks of the nerves remain perfectly distinct and disconnected from one another, and seldom, or never, divide throughout their entire length. However, where the nerves enter the nerve-centers, and near their outer terminations, the nerve-fibres often divide into branches, or at least gradually diminish in size, until, finally, the axis-cylinder, and the sheath with its fluid contents, are no longer distinguishable. The investing membrane is continuous from the origin to the termination of the nerve-trunk. ”

“ There is diminution in the number of fibers of the branches and roots of the ramus superior and ramus medius of the eighth nerve, and the fiber bundles are very loosely bound together.
5. Similarly the number of fibers in the inferior branch (the cochlear nerve) of the eighth nerve is very much reduced.
6. There is moderate reduction in the size of the two vestibular ganglia as a result of the unusually small number of nerve cells. ”

“ Imbedded in the tissues all over the body, or highly specialized and grouped together in separate organs, such as the eye or ear, we find large numbers of nerve-endings—that is, small lumps of protoplasm from which a nerve-fiber leads away to the spinal cord and so up to the brain. These nerve-endings are designed for the reception of the different kinds of vibration by which energy presents itself to us. As the largest example of these nerve-endings, let me here show you one of the so-called Pacinian bodies, or, more correctly, Marshall's corpuscles, for Mr. ”

“ This change, as I have seen in my practice of medicine, sometimes passes up the nerves toward the centers, and occasions a more or less constant irritation of the nerve-fibers, producing neuralgia, which is usually referred by the brain to that part of the lost limb to which the affected nerve belonged. This pain keeps the brain ever mindful of the missing part, and, imperfectly at least, preserves to the man a consciousness of possessing that which he has not. ”

“ The nerve fibers represent a conducting apparatus and serve to place the central nervous organs in connection with peripheral end organs. The nerve cells, however, besides transmitting impulses, act as physiological centers for automatic, or reflex, movements, and also for the sensory, perceptive, trophic, and secretory functions. A nerve consists of a bundle of tubular fibers, held together by a [Pg 211] dense areolar tissue, and inclosed in a membranous sheath—the neurilemma. Nerve fibers possess no elasticity, but are very strong. Divided nerves do not retract. ”

“ The non-medullated nerve fibers are axones of the sympathetic neurones and control the caliber of the vessels. These fibers are called the vasomotor nerves. They form plexuses in the adventitia, from which are given off branches which penetrate the media and terminate on the muscle cells. The medullated nerves are the axones of the spinal nerves. The larger fibers are found in the connective tissue outside the adventitia and give off branches to the media where they divide repeatedly, lose their sheath, and terminate in the media and at times in the intima. ”

“ Last is a layer of fine nerve fibers, g, and gray, ganglionic cells, h, like the gray matter of the brain, whence filaments extend into i, the fibers of the optic nerve. (See p. 354.)
The layer of rods and cones is to the eye what the bristles, otoliths, and Cortian fibers are to the ear. Indeed, the nerve itself is insensible to light. At the point where it enters the eye, there are no rods and cones, and this is called the blind spot. A simple experiment will illustrate the fact. ”

“ Just as primitively the medullary fibers which arise in the dorsal zone pass into the ventral zone, so in the specialized cerebral hemispheres and in the cerebellum there arise very numerous nerve fibers, but these still obey the primal law and take their courses into the portions of the brain representing the ventral zones, and thence the fibers are distributed to their various destinations. ”

“ Thus, if a nerve-fibre in connection with a muscle at one end and a motor nerve-cell at the other be stimulated, although the stimulus is conducted to both ends of the fibre, the effect of the stimulus can only be exhibited at the end in connection with the muscle, causing the muscle to contract. Or if a fibre in connection with a peripheral organ of touch be stimulated, we can only recognize the effects of such stimulation by changes in the nerve-cells at its central end which give rise to a sensation. ”

“ Furthermore, it is not too speculative to consider that the different kinds of nerve-fibres may be liable separately or in different degrees to morbid conditions, so that when mixed nerves are the seat of neuritis, motor, sensitive, or trophic symptoms may have a different prominence in different cases in proportion as one or other kind of fibres is most affected. ”

“ These have been separately cut and irritated in the living animal, and it has been found that the posterior root contains only sensory fibres, and the anterior root has only motor fibres. So that the nerves of a limb may be injured in such a way that it will retain power of motion and yet lose sensation; or the reverse condition, feeling without motion, may exist. Between these two sorts of fibres, no difference of structure can be found; and where they have joined to form a nerve it is impossible to distinguish one sort from the other. ”

“ 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. ”

“ In saying that no action at all can be propagated by the agency of the nervous system, we do not make proper allowance for the vital properties of nerve-fibre. The vital nature of common nerve-fibre is such that two actions can be conducted along it;—an impulse producing sensation, or an impulse producing muscular action, at a distant point. It is known that an impression on the terminal extremity of a sensory nerve is capable of producing either sensation or motion at a distance, by what is called reflex nervous action. ”

“ 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. ”

“ We have recently seen that the brain is wholly insensitive, and may be cut or pinched without pain. The same is true of the nails, hair, the scarf-skin or external covering of the body, and a few other structures. In these parts no nerves are found. On the other hand, the sensibility of the true skin, and of mucous membranes, as of the eye and nose, is exquisite, these organs having a large supply of sensory nerve-fibres. The bones and tendons have less of these fibres, and are only moderately sensitive. ”

“ Just above is a transverse section of the same fibre, showing the same arrangement from another point of view, and also illustrating the curious phenomenon, that when nerve-fibres are treated with carmine the centre takes up the colouring matter, while the sheath remains white as before. The best way of studying nerves is to decapitate a frog, and cut off a piece of one of the nerves, which, like fine silk threads, come out between the joints of the spine inside the abdomen. ”

“ The nerves also undergo division until they terminate as fine thread-like structures in all parts of the body. The distribution of nerve terminations, however, is not uniform, as might be supposed, but the skin and important organs like the heart, stomach, and muscles are the more abundantly supplied. On many of the nerves are small rounded masses, called ganglia, and from many of these small nerves also emerge. At certain places the nerves and ganglia are so numerous as to form a kind of network, known as a plexus. ”

“ We find, however, upon the 205capillaries evidence of the same kind of fine nucleated fibrils, which we find to be present in the fine nerve twigs, as far as we can follow them in their course towards their final distribution. To explain the action of nerves on the process of secretion we would have to assume either that the nerves accompanying the capillaries also influence the gland cells, or that the glands are stimulated to activity by some changes in the condition of the capillary wall furnishing a greater amount of oxygen and nutritive fluids. ”

“ In any nervous system there are two portions anatomically distinct. They are, 1st, the fibrous; 2d, the vesicular. It may be desirable to describe briefly the construction and functions of each of these portions. Their conjoint action will then be intelligible.
Structure of a nerve fibre.1st. A nerve fibre consists essentially of a delicate thread—the axis filament, as it is called—enveloped in an oil-like substance, which coagulates or congeals after death. This, in its turn, is inclosed in a thin investing sheath or membranous tube. Many such fibres bound together constitute a nerve. ”

“ There is no adequate warrant for the assumption that all nerves have their origin in ganglia, all fibres in cells. Such evidence as at present exists is against that supposition, and in favor of the supposition that both cell and fibre are differentiations of a common neuroplasm, sometimes directly,303 sometimes indirectly continuous. Fibres, and plexuses of fibres, interspersed with cells irregularly distributed—now singly, now in small groups, now in larger and larger groups—constitute the figured elements of nerve-tissue ”

“ That which can be done in the way of producing severe pain by these severe affections of the peripheral portions of nerves, or of tissues lying outside them, we might a priori expect would be effected by slighter but continuous changes in the nutrition of the more important portion of the nerve itself—its central gray nucleus. ”

“ From this double interlacing of fibres results a {165}crossed action between the original and terminal extremity of all nerve-fibres which pass through the medulla; namely, those of all the spinal nerves. Consequently, if the right hand be hurt, the left side of the brain feels the pain; and if the left foot move, it is the right hemisphere which dictates its movement. For the same reason, when a loss of sensation and power of motion affecting the right side of the body alone is observed, the physiologist understands that the brain has been invaded by disease upon its left side. ”