The intaking of air is termed by physiologists _inspiration_, and its expulsion _expiration_, the whole process being _respiration_. Expiration takes a very little longer than inspiration, and the rapidity of respiration depends on the needs of the body. The more active the exercise, the more rapidly vital processes go on, the more ventilation of the tissues is required and the more is actually effected. When one is at rest breathing takes place at the rate of from 14 to 18 inspirations and expirations in the minute; but of all the processes of the body none is more variable than respiration, and of necessity, for every modification of action, every movement, implies a demand for an increased quantity of oxygen. It is not surprising, therefore, that the very exercise of singing tends in itself to put one out of breath. [Illustration: FIG. 12. In the above, the shaded outlines indicate the shape of the bony cage of the chest during inspiration, and the lighter ones the same during expiration. The alterations in the position of the ribs and in the diameters of the chest, giving rise to its greater capacity during inspiration, are evident.] [Illustration: FIG. 13. This figure is intended to indicate, in a purely diagrammatic way, by dotted lines, the position of the diaphragm (1) when inspiration is moderate, and (2) when very deep. The unbroken curved line above the dotted ones indicates the position of the diaphragm (only approximately, of course) after expiration.] Attention will now be directed to some facts that it is of the utmost importance to clearly understand, if one is to know how to breathe and the reasons for the method employed. The lungs are contained in a cavity the walls of which are made up of a domed muscular (and tendinous) structure below, and elsewhere of bony and cartilaginous tissues filled in with soft structures, chiefly muscles. This cage is lined within by a smooth membrane which is kept constantly moist by its own secretion. The lungs are covered by a similar membrane, both of these fitting closely like the hand to a glove, so that there are two smooth membranes in opposition. It cannot be too well remembered that these two, the inner surface of the chest walls and the outer surface of the lungs, are in the closest contact. This is so whatever the changes that take place in the size and shape of the chest. The lungs are concave below, and so fit accurately to the fleshy partition between the chest and the abdomen which constitutes the lower boundary of the chest, if we may use the term "chest" somewhat loosely. Above, suiting the shape of the chest, the lungs are somewhat conical.