THE RESPIRATION 287 



measuring the circumference of the chest in expiration and 

 in inspiration. 



Even after the whole of the reserve air has been driven 

 out of the chest, a considerable quantity still remains in 

 the air vesicles, its amount depending upon the size of the 

 chest, but averaging about 2000 ccms. This is called the 

 residual air. 



This very important point must always be remembered, 

 that the air taken into the chest never fills the air vesicles, 

 and that air is never driven completely out of them. The 

 air in them is thus not changed by the movements of respira- 

 tion but by the process of diffusion. 



III. Interchange of air in the lungs by diffusion of gases. 



It has been shown that only the air in the trachea and bronchi 

 undergoes exchange in mass, but that the air of the vesicles is 

 not driven out of the chest. The renewal of co ^ o 

 this air depends upon the diffusion of gases. 



If two gases are brought into relationship 

 with one another they diffuse and tend to 

 form a mixture uniform throughout. But, 

 if at one point of a system, one gas is con- 

 stantly being taken away and another con- 

 stantly added, there will be a constant 

 diffusion of the former towards the part 

 where it is being taken up, and a constant 

 diffusion of the latter away from the point 

 from which it is being given off. Suppose a FIG. 135. Shows 

 tube (Fig. 135) containing oxygen and carbon * he diffusion of 



j- -j j ^tA / J ^ Oxygen into, and 



dioxide, and suppose that at one end of the of carbon Dioxide 

 tube oxygen is constantly being taken up and out of, the Air 

 carbon dioxide constantly given off, a diffu- Ves 

 sion of the gases in the direction indicated by the arrows 

 will continually go on, and thus a constant supply of oxygen 

 will be conveyed to the bottom of the tube, while carbonic 

 acid will constantly be cleared out. This is exactly the con- 

 dition in the lungs. 



The lower part of the tube corresponds to the air vesicles 

 the upper part to the air passage in which the air is con- 

 stantly being exchanged by the movement of respiration. 



t 



