2. Fill the chest with special reference to its lower or its upper part, as desired, and very rapidly, letting the breath flow out slowly. SUMMARY. The primary purpose of respiration in all animals is the same--namely, to furnish oxygen and remove carbon dioxide (carbonic acid). The lowest animals, as the amoeba, breathe by the whole surface of the body. In all vertebrates the anatomical mechanism is essentially the same: a membrane (covered with flat cells) in which the blood is distributed in the minutest blood-vessels (capillaries). Respiration is finally effected in the tissues (cells) of the body. The more active the animal, or the higher in the scale, the more need of frequent interchange between the air, the blood, and the tissues. The respiratory organs in mammals are the mouth, nose, larynx, trachea, bronchial tubes, and lung-tissue or air-cells proper. The windpipe is made up of cartilaginous rings completed by membrane, muscle, etc. (behind). The bronchial tubes are the continuation of the windpipe, and branch tree-like until they become very fine. The air-cells are built round these latter. The lung-tissue is highly elastic. The lungs are made up of an elastic membrane, covered with flat cells, and very abundantly supplied with a mesh-work of the finest blood-vessels. The whole of the respiratory tract as far as the air-cells is lined by mucous membrane. The air consists essentially of 21 parts of oxygen and 79 parts of nitrogen, with a variable quantity of watery vapor. Only a small portion of the total oxygen of the air is removed before it is exhaled. The respiratory act consists of (1) inspiration, and (2) expiration; the latter is of a little longer duration than the former. The rate of breathing in man is from 14 to 18 per minute, in the resting state, or about one respiration to three or four heart-beats. The quantity of air inspired depends on (1) the size of the thorax, and (2) the extent of its movements. These are effected solely by muscular contractions, and give rise to an increase in all the diameters of the thorax. The lungs are closely applied (but not attached) to the inside of the chest wall, and remain so under all circumstances. When the chest cavity is enlarged by inspiration, the air, pressing down into the elastic lungs, expands them as much as possible, that is, as much as the chest walls will allow; but the lungs are never at any time either filled with or emptied of air to their utmost capacity. At most, the amount of expansion is very moderate. _The Quantity of Air in the Lungs._ 1. The quantity of air inspired in quiet breathing is about 20-30 cubic inches. 2. The quantity that can be added to this by a deep inspiration is about 100 cubic inches. 3. The quantity that can be expelled by a forcible expiration is about 100 cubic inches. 4. The quantity that cannot be expelled at all is about 100 cubic inches.