THE EESPIRATORY SYSTEM 95 



forces most of the air out of them. The balance of the air normally 

 exhaled depends for its expulsion on the contraction of the internal 

 intercostal muscles, which help draw the ribs backward and down- 

 ward. When breathing is forced or labored, these and the abdomi- 

 nal muscles also come into play to aid in reducing the size of the 

 thorax. 



The air which is exhaled in normal, quiet respiration is known 

 as tidal air; the supplemental air is that which can be voluntarily 

 breathed out after a quiet expiration; the residual air is that which 

 the animal is unable to force out of the lungs. To show that resid- 

 ual air is present in considerable amount, the lungs of a recently 

 killed animal should be placed in water, in which they will be found 

 to float. Fetal lungs sink in water, for the air-cells have never been 

 filled with air. 



Internal respiration consists of the passage of oxygen from the 

 blood to the tissues, and the passage of carbon dioxid and other im- 

 purities from the tissues to tfte blood. Two factors are at work in 

 this interchange — the mechanical diffusion due to the difference in 

 oxygen pressure, and the vital action of the endothelial cells which 

 line the capillaries. The extent of the latter action is not known, 

 but it probably is important. True respiration and oxidation 

 really take place between the body cells and the lymph which bathes 

 them. Lymph contains both oxygen and carbon dioxid ; the blood 

 is constantly supplying the first and removing the latter. In other 

 words, breathing is not respiration, in the exact sense of the term, 

 though it is a necessary adjunct to the process. 



The respiratory center for regulating breathing is a collection 

 of nerve-cells located in the medulla oblongata. Special nerve- 

 fibers run from this center, by way of the vagus nerve trunk, to the 

 lungs and the muscles concerned in respiration. This permits 

 the lungs to be ventilated by reflex action. Among the things 

 which stimulate the respiratory center are carbon dioxid in the 

 blood, the presence of various products of metabolism in the blood, 

 and a low oxygen content of the blood, as occurs in high altitudes. 

 A perfect balance is maintained in health between the amount of 

 carbon dioxid and oxygen in the blood by the action of the respira- 

 tory center on the movements of respiration. 



