378 COMPARATIVE PHYSIOLOGY. 



both invertebrate and vertebrate, excepting mammals, the blood 

 is freely exposed in the gills to oxygen dissolved in the water as 

 it is to the same gas mixed with nitrogen in terrestrial animals. 

 In the land-snail, land-crab, etc., we have a sort of intermediate 

 condition, the gills being kept moist. It is not to be forgotten, 

 however, that normally the respiratory tract of mammals is 

 never other than slightly moist. 



THE QUANTITY OF AIR RESPIREI}. 



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 ex- 

 piration, amounting to 1,500 cc. 4. Residual air, which can 

 not be voluntarily expelled at all, amounting to about 3,000 cc, 

 or 120 cubic inches. Although these quantities have been esti- 

 mated for man, probably si similar relation (proportion) between 

 them holds for the domestic animals. 



The mtal 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 suflScient 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 remain- 

 ing in the lungs after each breath being the sum of the residual 



