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found in the rest of the nervous system. They differ not only in their mode of origin and in their subsequent development, but also in the connexions of their nerve-fibre processes.
Nerve-cells of the Brain and Spinal Medulla. The cells in the cerebrospinal axis are variable both in size and form. Some are relatively large, as, for example, certain of the pyramidal cells of the cerebral cortex and the motor cells in the spinal medulla, which almost come within the range of unaided vision; others are exceedingly minute, and require a high power of the microscope to bring them into view. The cell consists of a protoplasmic nucleated body, from which the axon proceeds, and the protoplasmic processes of Deiters, or the dendrites (Fig. 447).
The axon presents a uniform diameter and a smooth and even outline. It gives off in its course fine collateral branches, but does not suffer thereby any marked diminution in its girth. The most important point to note in connexion with the axon, however, is the fact that it becomes continuous with the axiscylinder of a nervefibre. The axon then is simply a nerve-fibre, and in certain circumstances it assumes one or two investingsheaths, of which more will be said later. The axon may run its entire course within the substance of the brain or spinal medulla, either for a short or a long distance (intercalated cells), or it may emerge from the brain or spinal medulla in one of the cerebral or spinal nerves as the essential part of an efferent nerve-fibre, and run a variable distance before it finally reaches the peripheral structure in relation to which it ends (efferent nerve-cells). The axon and the collaterals which spring from it appear to terminate either in small button-like swellings or knobs, or more frequently in terminal arborisations, the extremities of which seem to be furnished with exceedingly small terminal varicosities. In those cases where the axon or its collaterals end within the brain or spinal medulla, some of the terminal arborisations interlace with the dendrites of nerve-cells, whilst others are twined around the bodies of other cells. In the latter case the interlacement may be so close and complete that it almost presents the appearance of an enclosing basket-work. In cases where the axon emerges from the cerebro-spinal axis its terminal arborisation ends in relation to a muscle-fibre or some other tissue in the manner described below.
FIG. 447.-THREE NERVE-CELLS FROM THE ANTERIOR COLUMN OF GRAY
brain and spinal medulla, with the exception of the olfactory and optic, are formed of medullated fibres provided with a primitive sheath; whilst the entire mass of the white substance of the brain and spinal medulla, and also the optic nerves, are formed of medullated fibres devoid of a primitive sheath.
It is important to note that the distinction between the medullated and non-medullated fibres is not one which exists throughout all stages of development. As will be presently pointed out, every fibre is the prolongation of a cell, and in the first instance it is not provided with a medullary sheath. Indeed, it is not until about the fifth month of foetal life that those fibres which are to form the white substance of the cerebro-spinal axis begin to acquire their coating of myelin. Further, this coating appears in the fibres of different fasciculi or tracts at different periods, and a knowledge of this fact has enabled anatomists to follow out the connexions of the tracts of fibres which compose the white matter of the brain and spinal medulla.
Every nerve-fibre is directly continuous by one extremity with a nerve-cell, whilst its opposite extremity breaks up into a number of ramifications, all of which end in relation to another nerve-cell, or in relation to certain tissues of the body, as, for example, musclefibres or the epithelial cells of the epidermis. The length of nervefibres, therefore, varies very greatly. Some fibres are short and merely bring two neighbouring nerve-cells into relation with each other; others travel long distances. Thus, a fibre arising from one of the motor cells of the lower end of the spinal medulla may, after leaving the spinal medulla, extend to the most outlying muscle in the sole of the foot, before it reaches. its destination. But even when a fibre does not leave the central axis, a great length may be attained, and cells situated in the uppermost part of the brain give origin to fibres which pass down to the lower end of the spinal medulla.
FIG. 450. NERVE-FIBRE
It has already been explained that fibres which form the nerves may be FROM A FROG (after classified into two sets, afferent and efferent. Afferent nerve-fibres conduct impressions from the peripheral organs into the central nervous system; and as a change of consciousness, or, in other words, a sensation is a frequent result, these fibres are often called sensory. Efferent nerve-fibres carry impulses out from the brain and spinal medulla to peripheral organs. The majority of these fibres go to muscles and are termed motor; others, however, go to glands and are called secretory; whilst some are in- FIG. 451.-RAMÓN Y CAJAL'S IN
hibitory and serve to carry impulses which restrain or
TERPRETATION OF THE DEVELOP-
check movement or secretion.
Neuroblast with rudimentary axon, but no dendrites; b and c, The dendrites beginning to sprout out; d and e, Further development of the dendrites and appearance of collateral branches on the axon.
The dendrites, or protoplasmic processes of the nervecell, are thicker than the axon, and present a roughedged irregular contour. They divide into numerous branches, and these gradually, as they pass from the cell-body, become more and more attenuated until finally they appear to end in free extremities. The branching of the dendritic processes sometimes attains a marvellous degree of complexity (Fig. 449), but it is commonly supposed that there is no anastomosis between the dendrites of neighbouring cells, or between the dendrites of the same cell.
It is commonly believed that the neuroblast passes through stages analogous to those shown in the diagram (Fig. 451); that just as a seed gives off a root which strikes downward, and leaves which grow upward, so the neuroblast sprouts out an
axon (a) and subsequently develops a bunch of dendritic processes (b). In the case of the axon reasons have already been given for not accepting this view as the whole explanation; and in the case of the dendrites, although the appearance of microscopic sections seems to favour the view expressed in the diagrams, the fact that the neuroblasts are united into a continuous network or syncytium at an early stage of development (see p. 503) raises the possibility that the dendrites may be formed by the gradually drawing out of the existing bridges as the linked cell-bodies become moved apart.
The Ganglia of the Sensory Nerves.-The cells found in the ganglia of the cerebral nerves and on the posterior or dorsal roots of the spinal nerves have a different origin, and present many points of contrast with neurones in the gray matter of the brain and spinal medulla. As already indicated, the ganglia in question are derived from the neural crest. The cells forming these ganglionic masses are somewhat oval in form, and each extremity or pole becomes drawn out into a process, so that the neurones become bipolar. These processes are distinguished as central and peripheral, according to the direction which they take. The central processes
penetrate the wall of the neural tube. In the region of the spinal medulla they form almost the whole of the fibres which enter into the composition of the posterior roots of the spinal nerves. In the substance of the cerebro-spinal axis they give off numerous collaterals, and after a course of varying extent they end, after the manner of an axon, in terminal arborisations, which enter into relationships with certain nerve-cells in the cerebro-spinal axis. The peripheral processes proceed along the path of the particular nerve with which they are associated, and they finally reach the skin or other sensory surface. Thus, to take one example: the majority of the fibres which go to the skin break up into fine terminal filaments, which end freely between the epithelial cells of the epidermis. The two processes of a ganglion cell, therefore, form the afferent fibres of the cerebro-spinal nerves, and constitute the path along which the influence
FIG. 452.—THREE STAGES IN THE DEVELOPMENT of peripheral impressions is conducted to
OF A CELL IN A SPINAL GANGLION.
wards the brain and spinal medulla. The
body of the cell is, as it were, interposed in the path of such impulses. .
But the original bipolar character of these cells, with very few exceptions (ganglia in connexion with the acoustic nerve and the bipolar nerve-cells in the olfactory mucous membrane), gradually undergoes a change which ultimately leads to their transformation into unipolar cells. This is brought about by the tendency which the cell-body has to grow to one side, viz., the side towards the surface of the ganglion (v. Lenhossek). This unilateral growth leads to a gradual approximation of the attached ends of the processes, and finally to a condition in which they appear to arise from the extremity of a short common stalk in a T-shaped manner (Fig. 452). It is interesting to note that in fishes the original bipolar condition of these cells is retained throughout life, without change.
Both the central and peripheral processes of these ganglionic cells become the axis-cylinders of nerve-fibres, which, acquiring a medullary sheath, belong therefore to the medullated variety. From this it might very naturally be thought that the ganglionic neurone, with its two axons and no typical dendrites, is a nervous unit very different from a neurone in the gray matter of the cerebro-spinal axis. It is believed by some, however (van Gehuchten and Cajal), that the peripheral process, in spite of its enclosure within a medullary sheath, and though presenting all the characters of a true axon, is in reality a dendrite If this is the case, the
morphological difference between a dendrite and an axon disappears, and van Gehuchten's functional distinction alone remains characteristic, viz., that the axon is cellulifugal and conducts impulses away from the cell, whilst the dendrites are cellulipetal and conduct impulses towards the cell.
It is, however, more in accordance with the facts to regard the sensory neurones as genetically quite distinct from the rest of the nervous system (see p. 498).
Neuroglia. The neuroglia is the supporting tissue of the cerebro-spinal axis. It may be considered to include two different forms of tissue, viz., the lining ependymal cells and the neuroglia proper. We place these under the one heading, seeing that they have a common developmental origin.
The ependymal cells are the columnar epithelial cells which line the central canal of the spinal medulla and the ventricles of the brain. In the embryonic condition a process from the deep extremity of each cell, traverses the entire thickness of the neural wall, and reaches the surface. It is not known whether this process exists in the adult.
The neuroglia proper is present in both the white and the gray matter of the cerebro-spinal axis. It constitutes an allpervading basis substance, in which the various nerve elements are embedded in such a way that they are all bound together into a consistent mass, and are yet all severally isolated from each other. Neuroglia consists of cells and fine filaments. The fibrils are present in enormous numbers, and by their interlacements they constitute what appears to be a fine feltwork. At the points where the fibrils intercross may be seen the flattened glial cells. Whilst the neuroglia is for the most part intimately intermixed with the nerve elements, there are, in both brain and spinal medulla, certain localities where it is spread out in more or less pure layers. Thus, upon the surface of the brain and of the spinal medulla there is such a layer; likewise beneath the epithelial lining of the central canal and of the cavities of the brain there is a thin stratum of neuroglia.
FIG. 453. SECTION THROUGH THE CENTRAL
B, Neuroglial cell.
The ependymal cells are derived from the original neuro-epithelial cells of the early neural tube, and in all probability the neur- [Note that the dorsal (posterior) aspect is below.] oglia proper has a similar origin. They both, therefore, are products of the ectoderm. Summary.-1. The cerebro-spinal nervous system is composed of two parts, viz., (a) a central part, consisting of the brain and spinal medulla, with the efferent nervefibres which pass out from them; (b) the ganglionic part, with the afferent nerve-fibres.
2. Each of these parts has a different origin, and is composed of neurones which possess characteristic features.
3. The ganglionic neurones are derived from the primitive cells of the neural crest, and have each one process, which divides into two. Of these the central division enters the cerebro-spinal axis, whilst the peripheral division becomes connected with a peripheral part. The central fibres from the ganglionic cells in the region of the spinal medulla form the dorsal or posterior roots of the spinal nerves. The cells of origin of these posterior roots are outside the spinal medulla, and carry impulses into its substance.
4. The cerebro-spinal neurones are derived from the neuroblasts in the wall of the early neural tube. Certain of these furnish efferent nerve-fibres, which issue from the spinal medulla in separate bundles termed the anterior or ventral roots of the spinal nerves. In the case of the cerebral nerves, however, with the exception of the trigeminal and facial nerves, the efferent fibres are not thus separated from the afferent fibres at their attachment to the brain.