*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. In the center, and running the entire length of the axon, is a thread-like body, called the _axis cylinder_ (Fig. 126). 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. Surrounding the axis cylinder is a thick, whitish-looking layer, known as the _medullary sheath_, and around this is a thin covering, called the _primitive sheath_, or neurilemma. The medullary sheath and the primitive sheath are not, strictly speaking, parts of the nerve cell, but appear to be growths that have formed around it. Certain of the axons have no primitive sheath and others are without a medullary sheath.(97) *Form and Length of Axons.*—Where the axons terminate they usually separate into a number of small divisions, thereby increasing the number of their connections. Certain axons are also observed to give off branches before the place of termination is reached (Fig. 131). These collateral branches, by distributing themselves in a manner similar to the main fiber, greatly extend the influence of a single neuron. In the matter of length, great variation is found among the axons in different parts of the body. In certain parts of the brain, for example, are fibers not more than one one-hundredth of an inch in length, while the axons that pass all the way from the spinal cord to the toes have a length of more than three feet. Between these extremes practically all variations in length are found. *Arrangements of the Neurons.*—Nowhere in the body do the neurons exist singly, but they are everywhere connected with each other to form the different structures observed in the nerve skeleton. Two general plans of connection are to be observed, known as the anatomical and the physiological, or, more simply speaking, as the "side-by-side" and "end-to-end" plans. The side-by-side plan is seen in that disposition of the neurons which enables them to form the nerves and the ganglia, as well as the brain and spinal cord. The end-to-end connections are necessary to the work which the neurons do.