IS THE BODY A MACHINE 1 51 



while if the pressure be below this point the 

 union is at once destroyed, and the oxygen 

 leaves the haemoglobin to become free. All of 

 this is a purely chemical matter, and can be 

 demonstrated at will in a test tube in the labora- 

 tory. But this union and disassociation is just 

 what occurs as the foundation of respiration. 

 The blood coming to the lungs contains haemo- 

 globin, and since the oxygen pressure in the air 

 is quite high, this haemoglobin unites at once 

 with a quantity of oxygen while the blood is 

 flowing through the air-vessels. The blood is 

 then carried off in the circulation to the active 

 tissues like the muscles. These tissues are con- 

 stantly using oxygen to carry on their life pro- 

 cesses, and constantly at all times use up about 

 all the oxygen within their reach. The result is 

 that in these tissues the oxygen pressure is very 

 low, and when the oxygen-laden haemoglobin 

 reaches them the association of the haemoglobin 

 with oxygen is at once broken up and the 

 oxygen set free in the tissue. It passes at 

 once to the lymph, from which the active 

 tissues seize it for the purpose of carrying on 

 the oxidizing processes of the body. This whole 

 matter of supplying the body with oxygen is 

 thus fundamentally a chemical one, controlled 

 by chemical laws. 



Removal of Waste. The next step in this life 

 process is one of difficulty. After the food and 

 oxygen have reached the tissues it is seized by 

 the living cell. The food material is now oxi- 

 dized by the oxygen, and its latent energy is 

 liberated, and appears in the form of motion or 



