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On account of this property, an excitation, or disturbance, in any part of a neuron is conducted or carried to all the other parts. Thus a disturbance at the distant ends of the dendrites causes a movement toward the cell-body and, reaching the cell-body, the disturbance is passed through it into the axon. This movement through the neuron is called the nervous impulse.
*Structure of the Axon.*—The axon, or nerve fiber, has practically the same structure in both classes of neurons, being composed in most cases of three distinct parts. The axis cylinder is present in all axons and is the part essential to their work. It may be considered as an extension of the protoplasm from the cell-body.
The dendrites serve two purposes: first, they extend the surface of the cell-body, thereby enabling it to absorb a greater amount of nourishment from the surrounding lymph; second, they act as receivers of stimuli from other neurons. The same impulse does not pass from one neuron to another. The special function of the axon is to transmit the impulse.
The central thread in the axon is the axis cylinder. The neurons are in all respects cells. They differ widely, however, from all the other cells of the body and are, in some respects, the most remarkable of all cells. They are characterized by minute extensions, or prolongations, which in some instances extend to great distances.
By its length, structure, and property of conductivity it is especially adapted to this purpose. The axis cylinder, however, is the only part of the axon concerned in the transmission. The primitive sheath and the medullary layer protect the axis cylinder, and, according to some authorities, serve to insulate it. The medullary sheath may also aid in the nourishment of the axis cylinder.
Horrocleave, his elbow on the table, leaned his head on his hand and began to examine the book. Suddenly he looked up at Louis, who could not move and could not cease from agreeably smiling. Said Horrocleave in a still more peculiar tone "Just ask Axon whether he means to go fetch wages to-day or to-morrow. Has he forgotten it's Saturday morning?"
If Axon happened to be a subject of a conversation and there was any uncertainty as to which Axon out of a thousand Axons he might be, the introducer of the subject would always say, "You know sandy-haired fellow." This described him hair, beard, moustache. Sandy-haired men have no age until they are fifty-five, and Axon was not fifty-five.
Their protoplasm, like that of the cell-body, is more or less granular. The dendrites increase greatly the surface of the cell-body, to which they are related in function. The axon, or nerve fiber, is a long, slender extension from the cell-body, which connects with some organ or tissue.
On the other hand, Axon Skiadas, the popular barber of the Hôtel Grande Bretagne, who has just been called to the colors, assures me no patriot would again plunge this country into conflict. The diplomats also disagree, especially as to which of them is responsible for the failure of Greece to join the Allies. The one who is to blame for that never is the one who is talking to you.
It was at one time described as a distinct nervous element, but later study has shown it to be an outgrowth from the cell-body. The mon-axonic neurons are so called from their having but a single axon. *Di-axonic Neurons.*—Neurons belonging to this class have each a well-defined cell-body and two axons, but no parts just like the dendrites of mon-axonic neurons.
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