by sound. When it is aroused by impulses coming from the ears, the sensation of sound occurs; but when it is aroused by nerve-currents, not from the ears, but from other parts of the brain, we have only the memory of sound. For a word to be understood, the auditory center alone is not sufficient. The sound must awaken the memories of other sensations. The word "orange," for instance, has a meaning because the auditory center, when the word is heard, arouses in the visual center the memory of the color and form of an orange; in the centers for touch, temperature, posture, and muscular sense, the memory of the sensations which occur when the fruit is grasped by the hand; in the centers for smell and taste, the memory of its peculiar odor, flavor, and tartness. These sensations are said to be associated with the sound of the word, and together with it they constitute the concept "orange." The nerve-currents passing from one center to another are called association impulses. If we have often eaten oranges, and at the same time heard the name, the auditory center whenever it perceives or remembers the sound will send vigorous impulses to the other centers, and the idea will be vivid. But if our experience of oranges has been very limited, or if the name has been rarely heard, or if instead of the correct name a merely similar sound has been heard, the association impulses will be sent slowly, feebly, and uncertainly, so that the idea will be vague. Prompt and strong associations should be cultivated as a means of securing clear and vivid ideas. The auditory center is the first language center to be developed. A child hears soon after birth, and toward the end of its first year the sounds of a few words are understood. Up to this time no words have been uttered, but the child now begins to imitate the sounds it understands and soon can use them. This requires the co-operation of the motor speech center, M, which by connecting fibers and the currents they carry combines the simpler movements of the vocal organs to form words. In the case of Gambetta, who was a loquacious politician and very successful orator, this center was excessively developed, though the brain as a whole was not remarkable. On the contrary, a study of the brain of the distinguished statistician, Bertillon, who was diffident and reticent, showed a high degree of general development, with an almost rudimentary motor speech center.
In speaking, the guidance of the auditory center is necessary. The sound of a word must be remembered when it is uttered. On the other hand, the effort of the motor speech center to utter a word reacts upon the memory of the sound, causing it to be more vivid. In the main the two centers work and develop together, but the auditory center is the more independent and fundamental. If a child becomes deaf, even as late as the tenth or eleventh year,