The air is expelled from the lungs principally by the action of the muscles of respiration; and when these relax, the lungs expand by virtue of their own elasticity. This may be exemplified by means of a sponge, which may be compressed into a small compass by the hand, but, upon opening the hand, the sponge returns to its natural size, and all its cavities become filled with air. The purification of the blood in the lungs is of vital importance, and indispensably necessary to the due performance of all the functions; for if they be in a diseased state,--either tuberculous, or having adhesions to the pleura, their function will be impaired; the blood will appear black; loaded with carbon; and the phlebotomizer will have the very best (worst) excuse for taking away a few quarts with a view of purifying the remainder! The trachea, or windpipe, after dividing into smaller branches, called _bronchia_, again subdivides into innumerable other branches, the extremities of which are composed of an infinite number of small cells, which, with the ramifications of veins, arteries, nerves, and connecting membranes, make up the whole mass or substance of the lungs. The internal surface of the windpipe, bronchia, and air-cells, is lined with a delicate membrane, highly organized with blood-vessels, &c. The whole is invested with a thin, transparent membrane--a continuation of that lining the chest, named _pleura_. It also covers the diaphragm, and, by a duplication of its folds, forms a separation between the lobes of the lungs. CIRCULATION OF THE BLOOD. The blood contains the elements for building up, supplying the waste of, and nourishing the whole animal economy. On making an examination of the blood with a microscope, it is found full of little red globules, which vary in their size and shape in different animals, and are more numerous in the warm than in the cold-blooded. Probably this arises from the fact that the latter absorb less oxygen than the former. When blood stands for a time after being drawn, it separates into two parts. One is called _serum_, and resembles the white of an egg; the other is the clot, or crassamentum, and forms the red coagulum, or jelly-like substance. This is accompanied by whitish tough threads, called _fibrine_. [Illustration: THE HEART VIEWED EXTERNALLY. _a_, the left ventricle; _b_, the right ventricle; _c_, _e_, _f_, the aorta; _g_, _h_, _i_, the carotid and other arteries springing from the aorta; _k_, the pulmonary artery; _l_, branches of the pulmonary artery in the lungs; _m_, _m_, the pulmonary veins emptying into the left auricle; _n_, the right auricle; _o_, the ascending vena cava; _q_, the descending vena cava; _r_, the left auricle; _s_, the coronary vein and artery. (See _Circulation of the Blood_, on the opposite page.)]