RESPIRATION 



523 



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 strensfth of the individual — but 

 on an average about 14,000 c.cms. 



This is called the complemental air. 



By forced expiration, an amount of air 

 much larger than the tidal can be expelled, 

 an amount usually about the same as the 

 complemental air, and called the reserve 



air. 



Vif-o/ 



* Copacihu 



Fig. 211.— The Amount 

 of Air Respired in 

 Ordinary Respira- 

 tion, and in Forced 

 Inspiration and 

 Expiration. 



The total amount of air which an 

 individual can draw into and drive out 

 of his lun^s 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 80,000 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 10,000 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. 



A fairly reliable conclusion as to the vital capacity may be 

 arrived at by measuring the circumference of the chest in 

 expiration and inspiration (Practiced Physiology). 



III. Interchange of Air in the Lungs by Diffusion of Gases. 



Since, in ordinary breathing in the horse, a residue of almost 

 24,000 c.cms. of air remains in the lungs while only 3000 c.cms. 

 pass into and out of them, the question whether any of this gets 

 to the air vesicles must be considered. The trachea and bronchi 

 have a capacity of probably about 1400 c.cms. and they constitute 

 a " dead space," so that, after filling these, about 1600 c.cms. are 

 available to reach the vesicles. But, so large is the capacity of 



