RESPIRATION AND ENERGY IN MAN 129 



puscles, it is very important that their number be sufficient 

 and that they be kept in a healthy condition. To this end, 

 an abundance of sleep, exercise, fresh air, and nutritious foods 

 are the essential conditions. Every one is familiar with the 

 fact that the face looks pale after loss of sleep, or when food 

 and fresh air are insufficient, or during periods of physical 

 inactivity, and this appearance indicates a lack of red cor- 

 puscles. Habitual paleness, or a-nce'mi-a, is a disease re- 

 quiring medical treatment. It is frequently due to a want 

 of iron in the system ; hence, the value of spinach and other 

 vegetable foods containing this element. Fresh air, a mod- 

 erate amount of exercise, and good food are usually the 

 best remedies for anaemia. A good complexion is, therefore, 

 very largely dependent on healthy blood. Paint, powder, 

 and other cosmetics will not give such a complexion; and 

 besides cheapening the individual who uses them habitually, 

 they are often a source of permanent injury to the skin and 

 blood. 



III. THE PROCESS OF BREATHING 



185. Structure of the chest cavity. In the upper portion 

 of the trunk is the cone-shaped chest cavity, which is more 

 or less inclosed by the breastbone, the ribs, the collar bones, 

 and the spinal column. This bony framework is covered by 

 muscles that help to move the ribs, and by the outside cover- 

 ing of skin. The floor of the chest cavity is formed by 

 the tough sheet of muscle and connective tissue, the dia- 

 phragm. In this way there is formed an air-tight compart- 

 ment, which is completely filled by the heart, the blood 

 vessels, the gullet, and the lungs (Figs. 1 and 2). 



186. The pleura. The outer surface of each lung is covered 

 with a thin layer of membrane, and the walls of the chest cavity 

 are lined with the same kind of tissue. These two layers constitute 



