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.