GAS DISEASE IN FISHES. 347 



which a dissolved gas could gain access to their circulation. In warm- 

 blooded animak the life processes depend upon the absorption of 0x3'- 

 gen b}' the tissues and the elimination of carbon dioxid, and this 

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

 lX)rtion of the blood, the plasma, carries but a small portion of the 

 total oxj'gen dissolved in the blood. This portion is in amount about 

 what an equivalent volume of water would absorl), and is held in 

 simple solution, as in water (Foster, 1895, p. 588), Most of the 0x3^- 

 gen of the blood is carried by the red corpuscles, which are vehicles 

 for this gas b}^ virtue of the hemoglobin they contain, with which 

 oxygen readil}' combines and from which it ma}^ readilj^ be separated. 

 The- tissues of^the bod}^ have a stronger afhnitj^ for the oxj^gen than 

 that which exists between the hemoglobin and the 0x3" gen, and the3' 

 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 suppl}^ of ox3^gen from the air. Though the blood 

 does not come into direct contact witli the atmosphere, the corpuscles 

 come into intimate relation with it and are separated from it onl3^ by 

 a thin layer of epithelial cells, constituting the final subdivision of the 

 lung. Through this membranous partition the interchange of gases 

 takes place b3' diffusion, the process being known as osmosis, and .the 

 permeable membrane as an osmotic membrane. Osmosis is governed 

 b3^ laws analogous to those of simple diffusion of gases, or of the absorp- 

 tion of gases b3' liquids, and depends therefore in part on the pres- 

 sure exerted b3^ each gas concerned. The blood side of the membrane 

 is high in carbon dioxid and low in ox3^gen, while the air side is high 

 in oxvgen and low in carbon dioxid. Each gas exerts its pressure 

 independentl3" of the other, the carbon dioxid to pass out toward the 

 air, the oxvgen to pass in toward the blood. The tendenc3^ 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- 

 ousl3^ produced within and the ox3'^gen coming in is continuousl3^ 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 

 greatl3^ under usual conditions. Any increase of the proportion of 

 oxygen in the atmosphere, or an3^ increase of barometric pressure, 

 would increase the osmotic pressure and more rapidl3^ force the 0x3^- 

 gen into the blood. The workman in the compressed-air caisson 

 labors under a high osmotic pressure, which mav seriously' affect the 

 respiratory process. 



The nitrogen of the air is normally taken up ]>y the blood in amounts 

 insignihcant as compared with the ox3'gen, and is held in simple 



