416 THE NERVOUS SYSTEM. 
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 fibres carry impulses out from the brain and cord 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 inhibitory and 
serve to carry impulses which restrain or check movement or secretion. As 
previously stated, the spinal nerves are each attached to the cord by a ventral or 
anterior root, and a dorsal or posterior root; the fibres composing the former are 
efferent; whilst the fibres of the posterior root are almost entirely afferent. 
Nerve-cells.—The nerve-cells constitute the active and all-essential elements 
of nerve-tissue. At the very start it is necessary to draw a broad distinction 
between the ganglionic cells, which are found in the spinal ganglia, and the cells 
which are so plentifully scattered through the gray matter of the brain and cord. 
They differ not only in their mode of origin and in their subsequent development, 
but also in the connexions of the nerve-fibres to which they give origin. 
Nerve-cells of the Brain and Cord.—The cells in the gray matter of the 
cerebrospinal axis are variable both im 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 gray matter of the cord, 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 certain processes proceed. One process is termed the axis-cylinder process 
or axon; and as a rule it is easily distinguished from the others, which are collectively 
called the protoplasmic processes of Deiters, or the dendrites. 
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 axis-cylinder of a 
nerve-fibre. The significance of this is obvious, and will become more striking 
when the development of the nerve-cells is studied. The axon then is simply a 
nerve-fibre, and in certain circumstances it assumes, as already stated, one or two 
investing sheaths. The axon may run its entire course within the substance of 
the brain or cord either for a short or a long distance, or it may emerge from 
the brain or cord in one of the cranial 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. The axon and the col- 
laterals which spring from it terminate either in small button-like swellings 
or knobs, or more frequently in terminal arborisations, the extremities of which 
are free and are furnished with exceedingly small terminal varicosities. In 
those cases where the axon or its collaterals end within the brain or cord, 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 already referred to. In all cases, however, it 
must be clearly understood that the terminal branches of the axon, no matter how 
complicated the connexion may appear, are free, and that the connexion is simply 
one of contact or contiguity, and not one of continuity. 
Held maintains that, although at first the terminal arborisations of an axon are quite 
free, in the process of growth and development they exhibit a tendency to become fused 
with the dendrites and even the bodies of other nerve-cells. 
The dendrites, or protoplasmic processes of the nerve-cell, are thicker than the 
axon, and present a rough-edged irregular contour. They divide into numerous 
branches, and these gradually, as they pass from the cell-body, become more and 
more attenuated until they finally end in free extremities. The branching of the 
dendritic processes sometimes attains a marvellous degree of complexity, but it is 
now satisfactorily established that, except in exceptional circumstances, there is no 
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