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The nerve-cell whose body is in the top of the brain may have an extension or arm which may reach practically to the end of the spinal cord, and there make communication with another cell whose arm, in turn, may reach as far as the toe. Such nerve arms or extensions constitute the nerve-fibres, and bundles of these nerves, or nerve-trunks.
It is the vehicle which conveys the message of music from soul to soul through the medium of the human ear with its matchless harp of nerve-fibres and its splendid sounding-board, the eardrum. Music is the mirror which most perfectly reflects man's inner being and the essence of all things. Ruskin saw clearly that he alone can love art well who loves better what art mirrors.
The effect upon the latter is probably due to the spinal nerve-fibres contained in the plexus, and upon which the ganglionic excitement acts like the galvanism in Schiff's experiment, already described. Direct stimulation of the vaso-motor nerves, alone, as has been said, contracts the blood-vessels. Stimulation of the spinal fibres associated with them exercises the contrary effect.
The latter are the peripheral and ubiquitous nerve-fibres; with their terminal apparatus, the sense-organs, etc., they constitute the conducting marrow or peripheral nervous system. Some of them the sensory nerve-fibres conduct the impressions from the skin and other sense-organs to the central marrow; others the motor nerve-fibres convey the commands of the will to the muscles.
Each of these nervous masses contains nerve-cells as well as nerve-fibres, and is capable of generating nerve-force. Each, therefore, acts like a minute brain; and, in fact, the entire ganglionic system of nerves is analogous to the nervous system of certain among the lower animals the crustacea and mollusks.
The nerve-fibres might vibrate as often as they pleased, millions and millions of times in a second; they would never produce the sensation of red if there were no Self as the receiver and illuminator, the translator of these vibrations of ether; this Self, that alone receives, alone illumines, alone knows, and of which we can say nothing more than what the Indian philosophers call sak-kid-ânanda, that it exists, that it perceives, and as they add, that it is blessed, i.e. that it is complete in itself, serene and eternal.
Usually nerve-fibres make connection with the cells of an organ by a special modification of structure known as a nerve-ending. Nearly all central impulses, we now know, arise because of the peripheral ones. One may illustrate this important relation by a telegraph system.
The retina changes this picture into an affection of a number of the fibres of the optic nerve; the fibres of the optic nerve affect certain parts of the brain; the brain, in consequence, affects those particular fibres of the seventh nerve which go to the orbicular muscle of the eyelids; the change in these nerve-fibres causes the muscular fibres to change their dimensions, so as to become shorter and broader; and the result is the closing of the slit between the two lids, round which these fibres are disposed.
The auditory nerve proceeds first to the lowest or hindermost portion of the brain, known as the bulb, or medulla oblongata; thence a continuation of the nerve tract passes forward to a central region, the posterior corpora quadrigemina, then, by a new relay of nerve-fibres, to the highest and most important part of the brain, that most closely associated with consciousness, the cortex of the temporal lobe, where there is situated the most important of all the centres of hearing.
To this intricate part of the brain, these centres, converge the nerve-fibres collected in the spinal cord and medulla oblongata, and from them radiate other fibres that pursue a divergent course, and finally terminate in the gray matter of the cerebral hemispheres.
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