The author considers the summaries at the conclusion of the chapters of much importance. They not only furnish exact and condensed statements of the main facts and principles involved, but afford the reader a test of the extent to which the foregoing chapter has been comprehended. As the author has a horror of what is termed "cramming," he expresses the hope that no student will use these synopses, which have been prepared with much care, for so great a misuse of the mind as cramming implies.
Breathing is essential for life. The oxygen of the air is, of all food-stuffs, the most important. Without it a mammal will perish in less than three minutes; hence there is no need of the body so urgent as that of oxygen. It is also of great moment that the waste—the carbon dioxide, or carbonic acid gas—should be got rid of rapidly; nevertheless, it is not this gas which kills when the air-passages are closed, though it is highly deleterious. The body is a sort of furnace in which combustions are continually going on, and oxygen is as essential for these as for the burning of a candle, and the products are in each case the same.
Whether the voice-user respires, like others, to maintain the functions of the body, or whether he employs the breathing apparatus to produce sound, it is to be borne in mind that he uses the same physical mechanisms, so that the way is at once clear to consider the anatomy and physiology of the breathing organs.
It has been already pointed out that respiration is in all animals, in the end, the same process. The one-celled animal and the muscle-cell respire in the same way, and with the same results—oxidation, combustion, and resulting waste products. In the animal of complicated structure special mechanisms are necessary that the essential oxygen be brought to the blood and the useless carbon dioxide removed. The respiratory organs or tract include the mouth, nose, larynx, trachea, bronchial tubes, and the lung-tissue proper or the air-cells.
The mouth, nose, and larynx, in so far as they are of special importance in voice-production, will be considered later.
The air enters the trachea, or windpipe, through a relatively narrow slit in the larynx, or voice-box, known as the glottis, or chink of the glottis, which is wider when air is being taken in (inspiration) than when it is being expelled (expiration). Life depends on this chink being kept open. The windpipe is composed of a series of cartilaginous or gristly rings connected together by softer tissues. These rings are not entire, but are completed behind by soft tissues including muscle. It follows that this tube is pliable and extensible—a very important provision, especially when large movements of the neck are made, during vigorous exercise, and also in singing and speaking.
The bronchial tubes are the tree-like branches of the trachea, and extend to the air-cells themselves, which may be considered as built up around them in some such fashion as a toy balloon on its wooden stem, but with many infoldings, etc. ([Fig. 10]). The air-cells are composed of a membrane which may be compared to the walls of the balloon, but we are of course dealing with living tissue supplied by countless blood-vessels of the most minute calibre, in which the blood is brought very near to the air which passes over them.
Throughout, the respiratory tract is lined with mucous membrane. Mucous membranes are so named because they secrete mucus, the fluid which moistens the nose, mouth, and all parts of the respiratory tract. When one suffers from a cold the mucous membrane, in the early stages, may become dry from failure of this natural secretion; hence sneezing, coughing, etc., as the air then acts as an irritant.
At no time do we breathe pure oxygen, but "air"—i.e., a mixture of 21 parts of the former with 79 parts of an inert gas, nitrogen; and there is always in the air more oxygen than the blood actually takes from it in the air-cells.
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.