384 COMPARATIVE PHYSIOLOGY. 



the blood has passed through the capillaries and reached the 

 veins, the color has changed to a sort of purple, characteristic 

 of venous blood. Putting these two facts together, we are led 

 to suspect that the change has been caused in some way by 

 oxygen. Exact experiments with an appropriate form of blood- 

 pump show that from one hundred volumes of blood, whether 

 arterial or venous, about sixty volumes of gas may be obtained : 

 that this gas consists chiefly of oxygen and carbonic anhydride, 

 but that the proportions of each present depends upon whether 

 the blood is arterial or venous. 



The following table will make this clear: 



Arterial blood . 

 Venous blood. . 



from 100 volumes of blood at 0° 0. and 760 millimetre pressure. 



Arterial blood, then, contains 8 to 12 per cent more oxygen 

 and about 6 per cent more carbonic dioxide than venous blood. 

 It is not, of course, true, as is sometimes supposed, that arterial 

 blood is " pure blood " in the sense that it contains no carbonic 

 axihydride, as in reality it always carries a large percentage of 

 this gas. 



The Conditions nnder which the Gases exist in the Blood.— 

 If a fluid, as water,be exposed to a mixture of gases which it 

 can absorb under pressure, it is found that the amount taken up 

 depends on the quantity of the particular gas present independ- 

 ent of the presence or quantity of the others ; thus, if water be 

 exposed to a mixture of oxygen and nitrogen, the quantity of 

 oxygen absorbed will be the same as if no nitrogen were pres- 

 ent — i. e., the absorption of a gas varies with the partial press- 

 ure of that gas in the atmosphere to which it is exposed. But 

 whether blood, deprived of its gases, be thus exposed to oxygen 

 under pressure, or whether the attempt be made to remove this 

 gas from arterial blood, it is found that the above-stated law 

 does not apply. 



When blood is placed under the exhaustion-pump, at flrst 

 very little oxygen is given ofP ; then, when the pressure is con- 

 siderably reduced, the gas is suddenly liberated in large quan- 

 tity, and after this comparatively little. A precisely analogous 

 course of events takes place when blood deprived of its oxygen 

 is submitted to this gas under pressure. On the other hand, 

 if these experiments be made with serum, absorption follows 



