It is sympathetic vibration, manifesting itself in some instances in the chest and in the head cavities, and in other instances almost entirely within the latter, that gives to voices their peculiar timbre or tone-quality--their physiognomy. It is by timbre that we distinguish voices as we distinguish features. With instruments, differences in quality of tone--differences in timbre--are due to differences of shape; and in case of instruments of the same kind, for example, violins, to slight differences in form or to the grain, age and quality of the wood. In the same manner, there are minute differences in the structure of the vocal tract of different people; and it is especially the structural differences between the resonant cavities of individual singers that determine differences of timbre or quality. It is easy to distinguish between tones even of the same pitch that come from a harp, a violin, a trumpet, a flute or from the human voice. Between two violins of exactly the same make, played on by the same person, there would be greater difficulty in discovering differences in the quality of tone, although, even if made after the same pattern and about the same date, there probably would be minute structural differences that would differentiate their timbre to a musical ear; while if, of two violins, one of the instruments were new, and the other old, a musical ear probably would immediately detect differences in their tone-quality. It is easier to distinguish between voices even of the same range, than between instruments of the same kind, because there is strong individuality in voices. This is due to the fact that structural differences between the vocal tracts of individuals are far more numerous and far more minute than possibly can be introduced into instruments. Moreover, the vocal tract, being part of the human body, is subtly responsive to innumerable impulses and adjusts and readjusts itself in innumerable ways. Instruments are made of material, chiefly wood and metal, and, unlike the vocal tract, cannot change structurally. The cornet, for example, is made of brass. The lips of the player protruding into the cup can effect certain changes in shape, and changes also can be made in the tube between the mouthpiece and the bell of the instrument by pistons or valves. But these changes are absurdly small in number compared with the structural changes of which the vocal tract is capable, and commonplace in character compared with the refined and subtle minuteness of the latter. For this reason, while there is a distinct timbre for each kind of instrument, there are innumerable timbres of the human voice--as many as there are voices, and all due to the pliability of the vocal tract.