=D. Resistance.=--A fourth characteristic to be noted regarding nervous matter is that a nervous impulse, or current, as it is transmitted through the system, encounters _resistance_, or consumes an amount of nervous energy. Moreover, when the nervous current, whether sensory or motor, involves the establishment of new connections between cells, as when one first learns combinations of numbers or the movements involved in forming a new letter, a relatively greater amount of resistance is met or, in other words, a greater amount of nervous energy is expended. On the other hand, when an impulse has been transmitted a number of times through a given arc, the resistance is greatly lessened, or less energy is expended; as indicated by the ease with which an habitual act is performed. =Education and Nervous Energy.=--It is evident from the foregoing, that the forming of new ideas or of new modes of action tends to use up a large share of nervous energy. For this reason, the learning of new and difficult things should not be undertaken when the body is in a tired or exhausted condition; for the resistance which must be overcome, and the changes which must take place in the nervous tissue during the learning process, are not likely to be effectively accomplished under such conditions. Moreover, the energy thus lost must be restored through the blood, and therefore demands proper food, rest, and sleep on the part of the individual. It should be noted further that nervous tissue is more plastic during the early years of life. This renders it imperative, therefore, that knowledge and skill should be gained, as far as possible, during the plastic years. The person who wishes to become a great violinist must acquire skill to finger and handle the bow early in life. The person who desires to become a great linguist, if he allows his early years to pass without acquiring the necessary skill, cannot expect in middle life to train his vocal organs to articulate a number of different languages.