*Purpose of the Impulse. *—Though the nature of the nervous impulse is not understood, (103) its purpose is quite apparent. It is the means employed by the nervous system for controlling and coördinating the different parts of the body. The arrangement of the neurons enables impulses to be started in certain parts of the nervous system, and the property of conductivity causes them to be passed _as stimuli _to other parts. This enables excitation at one place to bring about action at another place. Acting as stimuli, the impulses seem able to produce two distinct effects: first, to throw resting organs into action and to increase the activity of organs already at work; and second, to diminish the rate, or check entirely, the activity of organs. Impulses producing the first effect are called _excitant_ impulses; those producing the second effect, _inhibitory_ impulses. *Functions of the Parts of Neurons.*—The _cell-body_ serves as a nutritive center from which the other parts derive nourishment. Proof of this is found in the fact that when any part of the neuron is separated from the cell-body, it dies, while the cell-body and the parts attached to the cell-body may continue to live. In addition to this the cell-body probably reënforces the nervous impulse. 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. An impulse in one neuron, however, is able to excite the neuron with which it makes an end-to-end connection, so that a series of impulses is produced along a given nerve path (Fig. 129). The special _function of the axon_ is to transmit the impulse. 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.