The Body-Machine and Its Work S39 



automatic messages to the muscles of the ribs and the midriff, 

 or diaphragm. At each intake of breath twelve pairs of muscles 

 work harmoniously in expanding the chest, and then other 

 muscles pull the "bag" together again and expel the air. But 

 how can the air extract the carbonic acid from the blood in so 

 short a space? All such difficulties are provided for in the body- 

 machine. You breathe out only a fifth of the air in your lungs 

 every time. The little air-chambers automatically close if, by a 

 strong effort, you try to empty your lungs. The exchange of 

 gases is going on all the time. If, on the other hand, the muscles 

 are working hard, and need more oxygen, the increased carbonic 

 acid in the blood stimulates the medulla, and nerve-messages from 

 it rain upon the lung-muscles until you "pant for breath." 



A man or woman engaged in sedentary work gets into a 

 way of using the lungs to only about a tenth of their capacity. 

 You understand the pale scholar and the anaemic girl in the 

 cash-box. They provide too little oxygen, and the blood will 

 not provide red corpuscles which are not needed. If such people 

 will go for a good swinging walk, in air that is rich in oxygen, 

 the blood will stream through their medulla, the nerve-centre at 

 the base of the brain, and the lungs will open out. 



The real "breathing" is, of course, deep inside the body. 

 The little air-chambers have walls almost as thin as soap-bubbles, 

 and a rich supply of similarly thin blood-vessels (capillaries) 

 outside them. Through these thin walls the red corpuscles some- 

 how extract the oxygen from the air, and the blood also gives 

 off the carbonic acid. Then the blood streams back to the left 

 chamber of the heart to be pumped through the body in the 

 way we have described. When this blood finds itself in the thin- 

 walled capillaries amongst the organs of the body, the red cor- 

 puscles yield their oxygen, and the blood returns to the heart 

 with a new load of carbonic acid. 



We have seen that this union of oxygen and food in the 

 tissues may roughly be compared to what goes on in a steam- 



