586 Anatomy, Physiology, and Pathology of the 



and from the lungs to the heart. The atmospheric air which 

 enters the lungs at each inspiration through the medium of the 

 windpipe, contains by weight 77 parts of nitrogen and 23 of 

 oxygen ; but the expired air is deficient of oxygen, its place being 

 supplied by carbonic acid gas.* An interchange of the oxygen 

 of the air and the carbonic acid of the system is thus effected, 

 these gases displacing each other by permeating the thin walls of 

 the capillaries, which are everywhere spread out as a minute net- 

 work on the surface of the air-cells. The immediate result of 

 this change is the conversion of the blood from a dark Modena 

 red to a bright scarlet colour, and the fitting of it again for the 

 chief purposes of life. -j- Thus we see that the blood in circulat- 

 ing through the system becomes unsuited to life, and that it is 

 re-invigorated by its passage through the lungs. The function 

 of respiration is therefore no less important than that of circula- 

 tion. Many operations of the animal system may be suspended 

 for a considerable time with but little ill consequence, but re- 

 spiration must be continued, or life quickly ceases. It is true 

 that respiration may be increased or diminished even at plea- 

 sure, but it cannot be altogether arrested ; for if we attempt 

 to hold the breath for a long time, we experience so much 

 inconvenience, that irresistibly we are compelled to resume 

 the act of breathing. This without doubt depends on the cir- 

 cumstance that during its suspension there is an accumula- 

 tion of carbonic acid in the system, the continuance of which 

 would produce asphyxia or suffocation. We have here another 

 proof of the wisdom and design of the omnipotent Creator in 

 making respiration, like the circulation, independent of our will. 



As the circulation has its central organ, the heart, so has re- 

 spiration — namely, the lungs : and both these are situated within 

 the same cavity, where they are secure from external injury or im- 

 pediment to their function. We may in this place observe that the 

 osseous portion of the body of an animal forms three important 

 cavities: an anterior, called the skull, which contains the brain and 

 the nerves of special sense ; a middle, the thorax, in which is lodged 

 the heart and lungs (see fig. 8) ; and a posterior, the pelvis, holding 

 the uterus and its appendages, the chief of the female organs of 

 generation. The thorax, however, is constructed on a different plan 

 from the other cavities, for there is an all-important necessity that 

 its area should be capable of being enlarged and diminished, in 

 accordance with the altered volume of the lungs during respira- 



* This fact was illustrated by breathing into some pellucid lime-water, by which it 

 was rendered turbid; the carbonic acid uniting with the lime, and forming thereby 

 an insoluble carbonate of lime. 



t The experiment of pouring some dark, or carbonized, blood into a vessel of oxygen 

 gas was introduced, and immediately it became of a bright red colour. 



