878 ANIMAL PHYSIOLOGY. 
gotten, however, that normally the respiratory tract of mam- 
mals is never other than slightly moist. 
THE QUANTITY OF AIR RESPIRED. 
We distinguish between the quantity of air that usually is 
moved by the thorax, and that which may be respired under 
‘ special effort, which, of course, can never exceed the capacity 
of the respiratory organs. 
Accordingly, we recognize: 1. Tidal air, or that which 
passes in and out of the respiratory passages in ordinary quiet 
breathing, amounting to about 500 cc., or thirty cubic inches, 
2. Complemental air, which may be voluntarily inhaled by a 
forced inspiration in addition to the tidal air, amounting to 
1,500 cc., or about 100 cubic inches. 3. Supplemental (reserve) 
air, which may be expelled at the end of a normal respiration 
—i.e., after the expulsion of the tidal air, and which represents 
the quantity usually left in the lungs after a normal quiet 
expiration, amounting to 1,500 cc. 4. Residual air, which can 
not be voluntarily expelled at all, amounting to about 2,000 cc., 
or 120 cubic inches. 
The vital capacity is estimated by the quantity of air that 
may be expired after the most forcible inspiration. This will, 
of course, vary with the age, which determines largely the elas- 
ticity of the thorax, together with sex, position, height, and a 
variety of other circumstances. But, inasmuch as the result 
may be greatly modified by practice, like the power to expand 
the chest, the vital capacity is not so valuable an indication as 
might at first be supposed. 
It is important to bear in mind that the tidal air is scarcely 
more than sufficient to fill the upper air-passages and larger 
bronchi, so that it requires from five to ten respirations to re- 
move a quantity of air inspired by an ordinary act. Very 
much must, therefore, depend on diffusion, the quantity of air 
remaining in the lungs after each breath being the sum of the 
residual and reserve air, or about 3,500 cc. (220 cubic inches). 
Considering the creeping slowness of the capillary circulation, 
it would not be supposed that the respiratory process in its 
essential parts should be the rapid one that a greater move- 
ment of the air would imply. ‘ 
