38 THE STORY OF THE LIVING MACHINE. 



drance 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 ex- 

 panded. 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 black- 

 smith'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 ques- 

 tion 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 haemo- 

 globin has peculiar relations to oxygen. It can 

 be separated from the blood and experimented 

 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 combina- 

 tion, since it is readily broken up. If the oxygen 

 is above a certain rather low pressure, the union 

 will take place ; while if the pressure be below this 

 point the union is at once destroyed, and the oxy- 

 gen 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 



