94 THE STRUCTURE AND LIFE OF BIRDS chap. 



beyond dispute that the more rapid and complete 

 the change of air in the lungs, the more rapid and 

 thorough will be the oxidation of the blood. It is 

 easy to increase the warmth of the body by taking 

 long and rapid breaths for a minute or two, and the 

 increase in temperature is due to the fact that the 

 blood, carrying more oxygen, burns the tissues more 

 rapidly. 



The lungs proper are never penetrated by fresh 

 air. There is an amount of air in them which by no 

 effort can be exhaled. This is called residual air, and 

 in man averages from 75 to 100 cubic inches. There 

 is about an equal quantity which may be driven out 

 with effort, but which remains after an ordinary 

 expiration. This is called supplemental air. Only ■ 

 the tidal air (20 — 30 cubic inches) passes in and out 

 in ordinary breathing. Thus, taking the largest 

 estimates — residual air 100, supplemental 100, tidal 

 30 — rather less than -1- of the air is changed when an 

 ordinary breath is taken, and the fresh air penetrates only 

 the trachea and bronchi, and not the minute air-cells, 

 which open from them. The stationary air, residual 

 and supplemental, carries on respiration. It receives 

 the carbonic acid from the blood and gives it up to 

 the tidal air, since it is a law of nature that gases, 

 when in contact, diffuse into one another. Increase 

 the volume of the tidal air, and the more rapid will be 

 the interchange of oxygen and carbonic acid gas be- 

 tween it and the stationary air. If the inspired air is 

 very poor in oxygen or meagre in amount the process 

 will be much slower and the whole vitality will be 

 lowered. In fact the refreshing effect of exercise is due 



