836 SPECIAL PHYSIOLOGY. 



stages of hibernation in animals, this also must be the mode of respi- 

 ration. Under ordinary circumstances, however, the necessity for air 

 cannot thus be relieved; but successive respiratory movements are 

 excited through the nervous system, in order to satisfy it. 



But the entrance of oxygen into, and the escape of carbonic acid 

 from, the blood of the pulmonary capillaries, from and into the air in 

 the air-cells, is not explicable by simple diffusion : for this double pro- 

 cess is one of moist or false gaseous diffusion. Both gases must be 

 dissolved as they pass in, or out of, the pulmonary tissues and capilla- 

 ries ; and the actual diffusion result depends, first, on the relative solu- 

 bility of the diffusing gases in the fluid of the natural moist septum 

 through which they pass, and secondly, on the special chemical affini- 

 ties of those gases for the blood. The simple diffusion volumes of dry 

 oxygen and carbonic acid, are in the proportion of 1174 to 1000, the 

 oxygen, or lighter gas, having a higher diffusion volume than the car- 

 bonic acid or heavier one. But this proportion does not agree with the 

 ratio of the oxygen absorbed to the carbonic acid evolved in respira- 

 tion, which, according to the Table in p. 8^3, is as 1255 to 1000. The 

 quantity of oxygen absorbed is, therefore, not only greater than that 

 of the carbonic acid evolved, but greater than that which the law of 

 the diffusion of dry gases would account for. Again, the relative solu- 

 bility of these gases in the water of the walls of the air-cells and capil- 

 laries, and in that of the blood, will not explain the difference; for 

 carbonic acid is nearly 30 times more soluble in water than oxygen, 

 and, accordingly, it exhibits a far greater diffusibility through a dead 

 moist membrane, instead of a less diffusibility, as occurs in actual res- 

 piration. It has been shown that recent blood, even at a temperature 

 of 32, retains from 16.8 to 19.8 volumes per cent, of oxygen ; whilst 

 water, at 60, dissolves not quite 3 volumes. Furthermore, fresh blood 

 deprived of its fibrin, at a temperature of 48, absorbs 178 volumes of 

 carbonic acid, whilst water takes up about 90 volumes. Hence, the 

 proportion between the oxygen absorbed, and the carbonic acid exhaled 

 from the blood in respiration, does not depend on the relative solvent 

 power of the blood for those two gases, which is about as 1 to 10. 



The remarkable affinity of blood for oxygen is also shown by an- 

 other calculation. The absolute quantity of carbonic acid which is 

 taken up by the blood, is larger than that of the oxygen. But in 

 comparison with water, the special affinity of blood for oxygen is 

 much stronger than that for carbonic acid ; for the quantity of oxygen 

 absorbed by the blood, in comparison with that absorbed by water, is 

 about as 18 to 3, or 6 to 1, whilst the quantity of carbonic acid ab- 

 sorbed by the blood, in proportion to that taken up by water, is only 

 about as 178 to 90, or less than 2 to 1. 



The total quantity of all the gases normally contained in 100 vol- 

 umes of blood, amounts to somewhat less than 50 volumes, i. e., about 

 half its own volume. This is less than it is capable of dissolving under 

 artificial pressure, or through other means. Of these 50 volumes, 

 about 12.5 are oxygen, 34.5 carbonic acid, and 3 nitrogen. Of 100 

 volumes of these mixed gases, the mean of several observations, how- 

 ever, gives 28.2 oxygen, 64.7 carbonic acid, and 7.1 nitrogen. Nitro- 



