The ear is anatomically divisible into external, middle (tympanum or drum), and internal (labyrinth). The outer ear collects the vibrations, the middle ear conducts them, and the internal converts them into a special physiological condition of the hair-cells and the auditory nerve. This condition is communicated to the other links in the anatomical hearing chain, until the highest part of the brain, or cortex, is reached. Hearing, from the physiological point of view, is the outcome of a series of processes having their development in a corresponding series of centres, or collections of nerve-cells. The perceptions associated with the ear, in the mind of the musician, are those of the pitch, rhythm (and time), and quality of tones. The loudness of a tone is, of course, recognized by the ear also, but this is hardly a musical quality proper. In reality, like all that belongs to hearing, these perceptions are the result of a series of physiological processes, in which the ear takes an important but not the sole or even the chief part, which is to be referred to the brain. It is practically important to recognize that these various qualities are distinct perceptions, and that the "ear" for relative pitch may exist well developed and the color, clang, or quality of a tone be imperfectly recognized, and the reverse. The most comprehensive ear-training involves attention to each of the above characters of tones, and then uniting them in a musically perfect result. Lack of "ear" is often simply want of attention to the characters of sounds. The auditory messages are the most important of all the nervous impulses that reach the brain, for the musician, whether appreciation or execution be considered. They are the chief guides for the outgoing nervous impulses to the muscles. The good executant must, above all, be a good listener. CHAPTER XVIII. CONSIDERATION OF GENERAL AND SPECIAL HYGIENE AND RELATED SUBJECTS. Hygiene deals with the laws by the observance of which health is to be maintained and disease prevented; but as such laws must be based on physiological principles, hygiene follows from physiology. Accordingly, throughout this work our method has been to point out the correct way as soon as the physiological principle has been laid down, so that the reason for the recommendation made would be obvious. However, it may be well if now some of the more important tendencies, errors, bad habits, and dangers to be guarded against by the singer and speaker be pointed out afresh, briefly, with some additional observations that experience has shown to be of practical importance.