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 1)}' the tissues and the elimination of carbon dioxid, and this 

 interchange is effected through the medium of the blood. The liquid 

 lX)rtion of the blood, the plasma, carries ])ut 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 ma}^ readily be separated. 

 The tissues of the body have a stronger athnitj' for the ox3^gen than 

 that which exists between the hemoglobin and the oxygen, and they 

 therefore take the 0x3' gen 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 witii 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 

 b}" laws analogous to those of simple diff'usion 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 oxj^gen, 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 ox^^gen 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 oxj^gen 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 

 ox^^gen in the atmosphere, or any increase of l>arometric pressure, 

 would increase the osmotic pressure and more rapidly force the 0x3^- 

 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 l>y the blood in amounts 

 insignificant as compared with the oxygen, and is held in simple 



