This doctrine is of so much importance in Vocal Science and in modern methods of instruction as to require a detailed explanation. The theory of breath-control may be stated as follows:[3] "In ordinary breathing the air is expelled from the lungs quietly, but rapidly; at no point of the breathing apparatus does the expired breath meet with resistance. In singing, on the contrary, the expiratory pressure is much more powerful, yet the expiration must be much slower. Furthermore, all the expired breath must be converted into tone, and the singer must have perfect control over the strength and the speed of the expiration. This requires that the air be held back at some point. The action of holding back the breath must not be performed by the muscles which close the glottis, for all the muscles of the larynx are very small and weak in comparison with the powerful muscles of expiration. The glottis-closing muscles are too weak to oppose their action to the force of a powerful expiration. If the vocal cords are called upon to withstand a strong breath pressure, they are seriously strained, and their proper action is rendered impossible. In the same way, if the throat be narrowed at any point above the larynx, so as to present a passage small enough to hold back a powerful expiration, the entire vocal mechanism is strained and forced out of its proper adjustment. The singer must have perfect control of the breath, and at the same time relieve the larynx and throat of all pressure and strain. To obtain this control the singer must govern the expiration by means of the muscles of inspiration. When the lungs are filled the inspiratory muscles are not to be relaxed as in ordinary breathing, but are to be held on tension throughout the action of expiration. Whatever pressure is exerted by the expiratory muscles must be almost counterbalanced by the opposed action of the muscles of inspiration. The more powerful the blast, the greater must be the exertion by which it is controlled. In this way the singer may have perfect control both of the speed and of the strength of the expiration." [Note 3: This statement of the doctrine of breath-control must not be construed as an endorsement of the theory of the vocal action embodied in this doctrine. On the contrary, both the theory of "opposed action" breath-control and the "breath-band" theory are held to be utterly erroneous. For a further discussion of this subject see Chapter II of Part II.] The exercises for acquiring command of this "opposed action breath-control" are easily understood; indeed, they will readily suggest themselves to one who has grasped their purpose. Most important of these exercises is a quick inspiration, followed by a slow and controlled expiration. Exercises for breathing and breath-control are usually combined; the student is instructed to take breath in the manner advocated by the teacher, and then to control the expiration.