GAS DISEASE IN FISHES. 347 
_ which a dissolved gas could gain access to their circulation. In warm- 
blooded animals the life processes depend upon the absorption of oxy- 
gen by the tissues and the elimination of carbon dioxid, and this 
interchange is effected through the medium of the blood. The liquid 
portion of the blood, the plasma, carries but a small portion of the 
total oxygen dissolved in the blood. This portion is in amount about 
what an equivalent volume of water would absorb, and is held in 
simple solution, as in water (Foster, 1895, p. 588). Most of the oxy- 
gen of the blood is carried by the red corpuscles, which are vehicles 
for this gas by virtue of the hemoglobin they contain, with which 
oxygen readily combines and from which it may readily be separated. 
The tissues of the body have a stronger affinity for the oxygen than 
that which exists between the hemoglobin and the oxygen, and they 
therefore take the oxygen from the hemoglobin of the corpuscle, and 
give in return carbonic acid, not to the corpuscle, but to the plasma of 
the blood. When the blood next reaches the lungs it gives up this 
carbonic acid to the external air, while the hemoglobin of the corpus- 
cle takes up a new supply of oxygen from the air. Though the blood 
does not come into direct contact with the atmosphere, the corpuscles 
come into Intimate relation with it and are separated from it only by 
a thin layer of epithelial cells, constituting the final subdivision of the 
lung. Through this membranous partition the interchange of gases 
takes place by diffusion, the process being known as osmosis, and the 
permeable membrane as an osmotic membrane. Osmosis is governed 
by laws analogous to those of simple diffusion of gases, or of the absorp- 
tion of gases by liquids, and depends therefore in part on the pres- 
sure exerted by each gas concerned. The blood side of the membrane 
is high in carbon dioxid and low in oxygen, while the air side is high 
in oxygen and low in carbon dioxid. Each gas exerts its pressure 
independently of the other, the carbon dioxid to pass out toward the 
air, the oxygen to pass in toward the blood. The tendency is to 
equalize each gas on the two sides of the membrane, when the pressure 
on both sides would be equal and osmosis would cease. Since in life 
this can never occur, because the carbon dioxid going out is continu- 
ously produced within and the oxygen coming in is continuously used 
up within, there is a continuous stream of these two gases passing in 
different directions, and at an osmotic pressure which does not vary 
greatly under usual conditions. Any increase of the proportion of 
oxygen in the atmosphere, or any increase of barometric pressure, 
would increase the osmotic pressure and more rapidly force the oxy- 
gen into the blood. The workman in the compressed-air caisson 
labors under a high osmotic pressure, which may seriously affect the 
respiratory process. 
The nitrogen of the air is normally taken up by the blood in amounts 
insignificant as compared with the oxygen, and is held in simple 
