=Neuron Theory.--=According to the prevailing modern conception the entire nervous system is made up of a series of units called _neurons_. Each neuron is a single cell with all its processes. The latter consists typically of short branching processes on the one hand, known as _dendrites_, and of a single process on the other, known as the _axon_, which extends from the cell to become a nerve fiber (Fig. 32, p. 209). The various neurons, with possibly a few exceptions, are not anatomically continuous but contiguous. They communicate with one another apparently by contact only. The axon of each neuron ends in an elaborate series of fine branchings which lie in contact with the dendrites of another neuron, or in some cases with the body of the other cell (Fig. 32, p. 209). Thus the nervous impulse passes from one neuron to the other at these points of contact. An impulse is supposed to travel normally only in one direction through a neuron, the dendrites being the receiving and the axon the discharging terminals. There are various types of neurons. Some, particularly within the brain, have their main processes so provided with branches and brushes that they may come into physiological connection with a number of other neurons. [Illustration: FIG. 32 A--Diagram to illustrate neurons and their method of connection; _a_, axon; _d_, dendrite; _s_, synapse. To simplify the diagram the medullary sheathes of such fibers as would have them have been omitted. The arrows indicate the direction in which the impulse travels. The lower series shows diagrammatically how from the same neuron in the cortex two subordinate neurons may be affected, the one excited to cause contraction of a certain group of muscle fibers, the other inhibited so that the antagonistic fibers may relax and thus not hinder the movement of a given part. B--Section of a region of the cerebral cortex (after Cajal). The cells have been blackened with chrome-silver and are much less highly magnified than the diagrams in A. The numerals refer to certain characteristic layers of the cortex in this region.] =Establishment of Pathways Through the Nervous System.--=It is believed that more or less resistance to transmission of stimuli prevails at the point of contact (_synapse_) between two neurons but that this resistance is lessened by repetition of conduction. The frequent traversing of a given pathway by similar impulses finally results in an automatic occurrence of the transmission, or, in other words, the action becomes habitual. Education consists largely in establishing such routes through the nervous tissue. Because of the greater plasticity of the neural mechanism in youth it is easier to open up and fix pathways of conduction than in later years. Moreover the earlier established lines of conduction become the more permanent.