RESPIRATION RESPIRATION RESPIRATION, a part of the life of all organisms, animal and vegetable. It is a series of chemical changes, the first of which is the absorption of oxygen into the body, and the last of which is the excretion of carbonic acid. The associa- tion of this intake of oxygen and excre- tion of carbonic acid with the same organs, the lungs, is due to the fact that both the food stuff and the waste stuff are gases, and not to any immediate con- nection between them. Necessarily any organ adapted to the diffusion of a gas from the air into the blood must also be adapted for the diffusion of a gas from the blood into the air; that is, supposing that the living membrane, of which the lung essentially consists, which separates the air from the blood, acts, so far as the diffusion of gases is concerned, as a dead membrane; even if it has any effect aris- ing from the fact of its being a living membrane it is probable that it will be- have in a similar way to both the ingoing and outgoing gases. The respiration of plants comes under the head of vegetable physiology, and the general relation of the function of respiration to the other bodily functions, under physiology. In all animals in which the parts of the body are nourished by the circulation of a stream of food material, the blood, there must always be two distinct sets of proc- esses to consider : ( 1 ) the maintenance of the blood in a normal state, by the supply of fresh food matter from time to time, and by the elimination of waste matter; (2) the nutrition of the individual tissues and cells of the body by the blood stream. Applying this to the function of respira- tion, we shall have to consider (1) the manner in which oxygen is supplied to the blood and carbonic acid gas removed from it; (2) the manner in which the cells are able to take oxygen out of the blood and cast into it their useless carbonic acid; also the changes that take place within the cells between the intake of the oxygen and the output of the carbonic acid. These two sets of processes are usually described as the outer and the inner, or tissue, respiration. Some writers include in the term outer respiration the absorption of oxygen by the cells from the blood, and the excretion of carbonic acid into the blood, and restrict the term inner respiration to the actual changes that take place within the cells. It is evident that outer respiration corresponds to the processes of digestion and absorp- tion to which food materials other than gaseous are subjected. Structure of Respiratory Mechanism. — This mechanism consists of the lungs, a series of minute air chambers with a net- work of capillaries in the wall, the air passages from the air chambers of the lungs to the outer air, and the chest walls with their muscles, which act like bellows and change the air in the lungs. The essentials of structure that a lung must possess have already been emphasized. The simplest lung that we can imagine would be an elastic membranous bag, well supplied with blood vessels, and with a pipe connecting it with the air ; the most complicated that exist are essentially of that construction, the complications that occur having for their object merely the enlarging of the surface exposed to the air. Let us begin with the air passages. There are first the nose and mouth; these join the upper part of the gullet, known as the pharynx. From the pharynx arises the windpipe (trachea) ; this passes through the voice box (larynx) into the chest cavity; there it divides into two passages (the bronchi) ; the bronchi go on dividing again and again, generally into two; the ultimate divisions (the bronchioles) open into clusters of air chambers. _ The air chambers are about %oo inch in diameter. It has been esti- mated that there are some 725,000,000 of them, and that their total surface is about 2,000 square feet. The walls of the air chambers are formed of a thin membrane in which the blood and lymph capillaries ramify. Minute openings lead from the air chambers into the lymph spaces of the membrane. The membranous walls are partly formed of elastic tissue. It is this that gives to the lungs their elasticity. The larger air passages (trachea and bronchi) are kept open by horseshoe- shaped plates of cartilage; muscles stretch between the poles of the horse- shoe, complete the ring, and permit the size of the passages to vary, at the same time resisting over-distention when the internal pressure rises. These larger air passages are lined by a mucous mem- brane, containing mucous glands; the in- nermost layer is a ciliated epithelium ; the cilia lash upward, and thus keep the pas- sages free from mucus and remove for- eign particles. As the passages become smaller they lose their cartilages, and the muscles form a continuous circular layer. The lungs are invested by a membrane (the visceral pleura). At the root of the lungs this membrane is continuous with a membrane which lines the chest cavity (the parietal pleura). The space between the two is the pleural cavity ; it is in real- ity a large lymph space, and communi- cates with the lymphatics of the pleura. Owing to the air pressure within the lungs the two pleurae are closely pressed together, the lungs entirely filling the chest cavity. The ordinary respiratory movements differ in the two sexes and at different periods of life. In young children the
