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Updated: September 4, 2025
In all these modes of developing our efferent powers, we conceive that the race comes very close to the individual youth, and that ancestral momenta animate motor neurons and muscles and preside over most of the combinations. Some of the elements speak with a still small voice raucous with age. The first spontaneous movements of infancy are hieroglyphs, to most of which we have as yet no good key.
Only the brain itself is more closely and immediately an organ of thought than are these muscles and their activity, reflex, spontaneous, or imitative in origin. Whether any of them are of value, as Lindley thinks, in arousing the brain to activity, or as Mueller suggests, in drawing off sensations or venting efferent impulses that would otherwise distract, we need not here discuss.
Here it produces an effect, not of a psychical nature, but some minute chemical, or physical,—or purely mechanical change, which goes through many permutations within the nervous centre itself, unites there with the stored energies, and then, thus altered, returns by the efferent nerve paths to effect a muscle-contraction in some organ, a stretching of the hand, or a movement of the whole body.
From the sentient surface an afferent impulse passes along to the posterior root of the spinal cord, the nerve fibers of the posterior root ending in minute filaments among the small cells of this part of the cord . In some unknown way this impulse passes across the gray part of the cord to the large cells of the anterior root , the cells of this part being connected by their axis-cylinder with the efferent fibers . These convey the stimulus to the fibers of the muscle , which accordingly contract.
Some of them put religious liberty in the first place; others, political; but each had its inconsistency, or its shortcoming. None had gone quite to the root of the matter. What was that root? or, let us say, the mother lode, of which these were efferent veins? The Pilgrims and Puritans, heretics in Episcopalian England, had escaped from their persecution, but had banished heretics in their turn.
It irks them to occupy themselves with purely mental processes, so enslaved are they by what is near and personal, and thus they are impoverished in the best elements of language. It is as if what are sometimes called the associative fibers, both ends of which are in the brain, were dwarfed in comparison with the afferent and efferent fibers that mediate sense and motion.
For applied to our previous example of a flash of light, the incoming message corresponds to the sensuous report of the flash, the outgoing message to the closure of the eye, and the unfortunate central office girl has disappeared. The afferent nerve reports directly to the efferent, without passing the message through consciousness.
"The afferent nerves, when excited by some physical irritant, be this as gross in its mode of operation as a chopping axe or as subtle as the waves of light, convey the excitement to the nervous centres. The commotion set up in the centres does not stop there, but discharges through the efferent nerves, exciting movements which vary with the animal and with the irritant applied."
This circuit of communication from the cell over the afferent nerves to the nerve centers in the brain or spinal cord, and from these centers over the efferent nerves back to the cell or to other cells is called the reflex arc. Let us use an illustration: Suppose the fingers come in close contact with a hot iron. The cells in the finger tips experience a sensation of burning pain.
What ones contain both afferent and efferent fibers? In what respects is the nervous system similar to a system of telephony? In what respects is it different? Observe the convolutions of the cerebrum and compare these with the parallel ridges of the cerebellum. Observe the appearance and general distribution of the nerves and their connection with the brain and spinal cord.
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