50 THE STORY OF LIFE'S MECHANISM. 



it is not indeed in actual contact with the air, 

 but is separated from it by only a very thin 

 membrane so thin that it forms no hindrance 

 to the interchange of gases. These air-cells are 

 kept filled with air by simple muscular action. 

 By the contraction of the muscles of the thorax 

 the thoracic cavity is enlarged, and as a result 

 air is sucked in in exactly the same way that it 

 is sucked into a pair of bellows when expanded. 

 Then the contraction of another set of muscles 

 decreases the size of the thoracic cavity, and the 

 air is squeezed out again. The action is just as 

 truly mechanical as is that of the blacksmith's 

 bellows. 



The relation of the air to the blood is just as 

 simple. In the blood there are various chemical 

 ingredients, among which is one known as haemo- 

 globin. It does not concern us at present to 

 ask where this material comes from, since this 

 question is part of the broader question, the 

 origin of the machine, to be discussed in the 

 second part of this work. The haemoglobin is 

 a normal constituent of the blood, and, being 

 red in colour, gives the red colour to the blood. 

 This haemoglobin has peculiar relations to oxygen. 

 It can be separated from the blood and experi- 

 mented upon by the chemist in his laboratory. 

 It is found that when haemoglobin is brought in 

 contact with oxygen, under sufficient pressure 

 it will form a chemical union with it. This 

 chemical union is, however, what the chemist 

 calls a loose combination, since it is readily 

 broken up. If the oxygen is above a certain 

 rather low pressure, the union will take place ; 



