Vj THE RESPIRATORY ORGANS 69 



diaphragm, ami by the elevation of the ribs through the con- 

 traction of the intercostal muscles. In the reverse process of 

 expiration the ribs are let fall to their former position and the 

 diaphragm becomes extended again. Expiration requires little 

 or no muscular effort : 



The amount of air inspired in ordinary quiet breathing in 

 man is about 500 c.c. (tidal air). By a forced inspiration another 

 laOOc.c. may be taken m (complemental air). Following upon 

 an ordinary expiration an additional 1500 c.c. may be expelled by 

 a forced expiration (supplemental air). The maximum amount 

 of air which can be expelled after the deepest possible mspiration 

 (i.e. tidal + complemental + supplemental air) is defined as the 

 vital capacity. In man this quantity is on an average about 

 3500 c.c. (or from 3 to 3-8 litres). In the horse the vital capacity 

 is from 25 to 30 litres. The air which remains in the lungs after 

 a forced expiration (residual air) is about 1-5 litres for a man, or 

 from 7 to 17 litres in a horse. 



The number of respirations or breaths per mmute varies in 

 different animals, and as a general rule the smaller the animal 

 the greater is the frequency'. The following are average numbers 

 for man and the domestic animals in a state of rest : 



That the blood in the lungs absorbs oxj-gen and excretes 



waste carbonaceous matter in the form of carbon dioxide can 



be proved by various experiments. Thus, by comparing the 



composition of expired air with inspired rair we get the following 



result : ^ 



N O CO., 



Inspired air 79 20-96 -04 



Expired air 79-4 16-50 4-10 



4-46 406 



The total quantity of air expired is slightly less than that of 

 air inspired since some of the oxygen taken instead of going 

 to form carbon dioxide unites with hydrogen to form water. 

 This accounts for the smaller percentage of nitrogen in the 

 inspired air 



