94 THE STRUCTURE AND LIFE OF BIRDS cuyap, 
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 4 of the air is changed when an 
ordinary breathistaken, 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 
