THE MECHANICS OF THE LIVING MACHINE 313 



the glands, or the brain, it finds a condition where there is prac- 

 tically no free oxygen. Here, since the oxygen pressure becomes 

 reduced, the haemoglobin at once lets go its hold upon the 

 oxygen which it has seized in the lungs. The oxygen then 

 passes off rapidly into the tissues and the blood is carried back 

 again to the lungs to get a fresh supply. There is a similar 

 relation between carbon dioxid and the blood; when the pressure 

 of carbon dioxid is high the blood will absorb it, and when the 

 pressure is low, the blood will let go its hold upon the carbon 

 dioxid it has absorbed. In the active tissues and cells, carbon 

 dioxid is present in considerable quantity, as the result of the 

 activity of the tissues. When the blood flows through these 

 tissues, it therefore absorbs carbon dioxid, and then goes back 

 to the lungs loaded with this gas. In the lungs, however, it 

 comes in contact with the air, in which the carbon dioxid is 

 present in very small quantities only. Under these circum- 

 stances the blood can no longer hold the carbon dioxid. This 

 gas passes into the lungs and is exhaled in the next breath. 

 These two processes are purely chemical; they will take place 

 just as well in a laboratory as in the lungs, and are quite 

 independent of any vital factors. 



Up to this point in the study of the activity of the living 

 body, there is no special difficulty in reaching the following 

 conclusions: (1) So far as relates to the general problem of 

 the transformation of energy, the body neither creates nor 

 destroys energy, but simply transforms one kind into another. 

 (2) So far as concerns the functions now considered, the 

 laws of chemistry and physics furnish for them an adequate 

 explanation. 



It is necessary, however, to question further a function of 

 life in which the mechanical relation is less obvious. The 

 nervous system controls all the operations of the body as an 

 engineer controls an engine. Is it possible that this phase of 

 living activity can be included within the conception of the 

 body as a living machine? 



