2. The principle that guides in all use of the muscles, all exercise, is that it be taken under the most favorable circumstances and short of fatigue, even of weariness; hence the question whether the student should practise five minutes or one hour is one that he himself, and he alone, can determine, provided he is old enough and observant enough to know when he begins to feel weary in his vocal mechanism, whether it be in the respiratory organs, the larynx, or the resonance-chambers. With some there is a weak spot, and this settles the question for all other parts. As a rule, beginners will do well not to practice, at first, for longer at one time than five minutes, not only because of the possible weariness, but because at the outset it is difficult to keep the attention fixed. The ear and brain tire as well as the muscles. Naturally, the condition of the student at the time has much to do with the length of a practice, but all things are determined by the sensible application of that principle which science and experience alike show to be a safe guide. Naturally, as in other exercises, the duration of an exercise may be gradually lengthened with experience. One singer may find an hour a day sufficient, if she be already perfectly trained in every respect--be "in good form," or "fit," as the athletes say--and have only light or _coloratura_ parts to sing; but would this suffice to form a singer to sustain the heaviest dramatic parts for hours together before a large public audience? The training of a hundred-yards sprinter should not be the same as that prescribed for a long-distance runner or a wrestler. [Illustration: FIG. 54. The above is a diagrammatic representation of a highly magnified section (or very thin slice) through the outermost or most superficial part of the great brain (cortex cerebri), and is inserted to help the reader to form some idea of the complexity of structure of the most important part of the brain so far as the highest mental processes are concerned. This complexity is greater in man than in other animals.] [Illustration: FIG. 55. A nerve-cell from the outer rind of the great brain (cortex cerebri), much magnified. (Schaefer.)] 3. In all practice it is ever to be borne in mind that the end, even in an exercise, is artistic. Tones of that quality only which is the best possible to the singer at the time are to be produced, and everything else must yield to this.