It is said that diseases of the lungs are the cause of four-fifths of all indispositions, ill health, and actual disease among civilized people, and that, between the age of fifteen and twenty-five, almost one-half of the mortality is due to pulmonary tuberculosis. This almost universal weakness of the lungs, as it may be styled, is a product of modern civilization, and is caused by our unhygienic mode of life. Too little importance is attached to physical development, and well-developed lungs can only be found in a well-developed chest; too little time is spent in outdoor exercise; and private houses, public conveyances, and public assembly rooms are not properly ventilated. Good development of the chest and lungs, and thorough and systematic ventilation of the lungs, are essential to a strong heart, a vigorous circulation, and power of the tissues to resist disease. =The Mechanics of Circulation and Respiration.=—In order to understand the mechanics of circulation and respiration, it is necessary to appreciate four fundamental facts—that the thorax is a distensible, air-tight cage; that it contains and is filled by the heart, lungs, and great blood-vessels; that the exchange of gases in the blood takes place in the lungs; and that, on the development of the chest and respiratory muscles, depend the development of the lungs and the force of the circulation. [Illustration: Fig. 7.—The bony thorax, anterior view (Ingals).] _The Thorax._—The chest or thorax is a cone-shaped, distensible cage, formed of bones, elastic cartilage, and muscles. The spinal column forms the fixed part of this living cage, and the ribs are attached to this in such a way as to allow of their being raised in inspiration, thus increasing the anteroposterior and the lateral diameters of the chest. The thorax is converted into an air-tight cavity by means of muscles. The base is made up of one huge muscle, the diaphragm. This is attached at its border to the ribs and posteriorly to the backbone. It is the diaphragm which separates the cavity of the thorax from that of the abdomen. When the diaphragm is relaxed, it has a concavoconvex form, the convexity being directed toward the chest, and the heart and lungs rest directly on it, while the concave surface covers or rests on the liver. _The Circulatory Apparatus._—This consists of a central force and suction-pump, the heart, and a series of elastic tubes that grow smaller the further from the heart they are situated; they divide and subdivide, like the branches of a tree. The smallest arteries, called capillaries, from their hair-like size, are so minute that they only allow the passage of a single corpuscle at a time and their walls are transparent.