EESPIEATION 303 



In an ordinary respiration in the horse about 3000 c.cms. 

 of air enter and leave the chest. That is called the tidal air. 

 Its amount varies with the size and muscular development of 

 the chest. 



By a forced inspiration a much larger quantity of air may 

 be made to pass into the lungs a quantity varying with the 

 size and strength of the individual but on an average about 

 14,000 c.cms. 



This is called the eomplemental air. 



By forced expiration an amount of air much larger than 

 the tidal can be expelled, an amount usually about the same 

 as the eomplemental air, and called the reserve air. 



The total amount of air which an individual can draw into 

 and drive out of his lungs is a fair measure of the size and 

 muscular development of the thorax, and it has been called 

 the vital capacity of the thorax, and in the horse it amounts 

 to something like 25,000 to 30,000 c.cms. 



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 c.cms. This is called the residual air. 



This very important point must always be remembered, 

 that the air taken into the chest never Jills 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 respiration 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 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 constantly being 

 taken away and another constantly 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 to which it is being added. Suppose a tube 

 (fig. 139) containing oxygen and carbon dioxide, and suppose 



