The success attending this method of treating consumptives suggests the proper mode of strengthening lungs that are not diseased, but simply weak. The person having weak lungs should spend as much time as he conveniently can out of doors. He should provide the most ample ventilation at night and have a sleeping room to himself. He should practice deep breathing exercises and partake of a nourishing diet. While avoiding prolonged chilling and other conditions liable to induce colds, he should take advantage of every opportunity of exposing himself fully and freely to the outside atmosphere. *Summary.*—The purpose of respiration is to bring about an exchange of gases between the body and the atmosphere. The organs employed for this purpose, called the respiratory organs, are adapted to handling materials in the _gaseous_ state, and are operated in accordance with principles governing the movements of the atmosphere. By alternately increasing and diminishing the thoracic space, air is made to pass between the outside atmosphere and the interior of the lungs. Finding its way into the smallest divisions of the lungs, called the alveoli, the air comes very near a large surface of blood. By this means the carbon dioxide diffuses out of the blood, and the free oxygen enters. Through the combined action of the organs of respiration and the organs that move the blood and the lymph, the cells in all parts of the body are enabled to exchange certain gaseous materials with the outside atmosphere. [Fig. 49] Fig. 49—Model for demonstrating the lungs. *Exercises.—*1. How does air entering the lungs differ in composition from air leaving the lungs? What purposes of respiration are indicated by these differences? 2. Name the divisions of the lungs. 3. Trace air from the outside atmosphere into the alveoli. Trace the blood from the right ventricle to the alveoli and back again to the left auricle. 4. How does the movement of air into and from the lungs differ from that of the blood through the lungs with respect to (_a_) the direction of the motion. (_b_) the causes of the motion, and (_c_) the tubes through which the motion takes place? 5. How are the air passages kept clean and open? 6. Describe the pleura. Into what divisions does it separate the thoracic cavity? 7. Describe and name uses of the diaphragm. 8. If 30 cubic inches of air are passed into the lungs at each inspiration and .05 of this is retained as oxygen, calculate the number of cubic feet of oxygen consumed each day, if the number of inspirations be 18 per minute. 9. Find the _weight_ of a day’s supply of oxygen, as found in the above problem, allowing 1.3 ounces as the weight of a cubic foot. 10. Make a study of the hygienic ventilation of the schoolroom. 11. Give advantages of full breathing over shallow breathing. 12. How may a flat chest and round shoulders be a cause of consumption? How may these deformities be corrected?