292 THE BLOOD. [c HAP. XXVII. 



Thirdly, the physical analysis of the blood, and the characters of 

 its constituents. 



Fourthly, the chemical analysis of the blood. 



I. Of the Quantity of Blood in the Body. It is almost impossible 

 to obtain sufficiently accurate data upon which to found a calcula- 

 tion of the total quantity of the blood which circulates in the 

 vascular system. The various estimates which have been formed 

 have been guesses, based on trials by bleeding animals to death, 

 and comparing the weight of the blood drawn with that of the 

 animal's body : also, on ascertaining, in various cases of hemor- 

 rhage, or of venesection, the quantity of blood which had been lost 

 in a brief period, without destruction to life. 



Harvey estimated the weight of the blood as one-twentieth of 

 that of the body, and Haller at one-fifth. According to the former 

 estimate, a man of one hundred and fifty pounds weight would have 

 only about seven and a half pounds of blood, whilst the latter 

 would assign him thirty pounds. 



Valentin devised an ingenious method of estimating the quantity 

 of the blood. He first ascertains the amount of solid constituents 

 in a certain quantity of blood, withdrawn by venesection : this 

 is replaced by a certain quantity of distilled water, and then he 

 ascertains the amount of solid constituents in a quantity of the now- 

 diluted blood equal to that which had been first withdrawn. From 

 the data thus obtained he calculates the whole quantity of the fluid, 

 which admits of such a change in its specific gravity, by the sub- 

 stitution of a certain quantity of distilled water for the quantity of 

 the fluid itself previously withdrawn. The problem is, to deter- 

 mine the quantity of fluid of specific gravity B., which, on 

 removing from it say six ounces, and replacing those six ounces 

 by a certain quantity of distilled water, becomes reduced to the 

 specific gravity fi. Having by this method determined the quan- 

 tity of blood in dogs, he deduces the quantity of blood in 

 the human body by comparing the weight of men with that of 

 dogs. And thus he assigns about thirty-two pounds for a man 

 between thirty and forty years of age, and twenty-eight pounds for 

 the female.* 



On the whole, we have no right to infer that the quantity of 

 blood in the human body exceeds thirty pounds : and, for practical 

 purposes, we shall do well to form a much lower estimate of it, and 

 to learn from thence how important it is to avoid being prodigal 

 in the removal of a fluid, so essential to the phenomena of life, 

 * Valentin. Physiol. 



