QUANTITY OF BLOOD. 101 



he employed the following process. He took first a small 

 quantity of blood from an animal for purposes of comparison ; 

 then injected into the vessels a known quantity of a saline solu- 

 tion, and taking another specimen of blood some time after, 

 he ascertained by evaporation the proportion of water which 

 it contained, compared with the proportion in the first speci- 

 men. He reasoned that the excess of water in the second 

 specimen over the first would give the proportion of the water 

 introduced, to the whole mass of blood; and as the entire 

 quantity of water introduced is known, the entire quantity 

 of blood could be deduced therefrom. Suppose, for example, 

 that the excess of water in the second specimen should be 

 one part to ten of the blood, it would show that one part of 

 water had been mixed with ten of the blood ; and if we 

 had injected in all five ounces of water, we would have the 

 whole quantity of blood ten times that, or fifty ounces. 



This method is open to the objection that it is impossi- 

 ble to take note of the processes of imbibition and exhalation 

 which are constantly in operation. Taking it for what it is 

 worth, the estimates, applied to the human subject, give the 

 weight of blood as -ffo that of the body. 



Blake estimated the quantity of blood by an analogous 

 process, injecting a known quantity of sulphate of alumina 

 into the vessels, estimating its proportion in a specimen of 

 blood, and from that deducing the entire quantity. He gives 

 the proportion of blood in dogs as from one-ninth to one- 

 third the weight of the body. The objection we have men- 

 tioned applies also to these experiments. 



The following process, which is, perhaps, least open to 

 sources of error, was employed by Lehmann and Weber, and 

 applied directly to the human subject, in the case of two 

 decapitated criminals. These observers estimated the blood 

 remaining in the body after decapitation, by injecting the 

 vessels with water until it came through nearly colorless. 

 It was carefully collected, evaporated to dryness, and the dry 

 residue assumed to represent a certain quantity of blood ; the 



