THE BLOOD. Ill 



this system of constant interchange it is impossible that a 

 perfect balance should always be maintained. The blood 

 has, consequently, no fixed normal and typical composition. 

 We may even, in any given movement, distinguish several 

 different kinds of blood, especially arterial and venous blood. 

 Any analysis of the blood can therefore be considered only 

 as approximative. 



Quantity of the Blood. It seems, at first, easy to decide 

 on the quantity of blood contained in the body, but this also 

 presents great practical difficulties. It is now generally ad- 

 mitted that the human system contains at least from five to 

 six litres of blood. In order to measure this fluid mass, the 

 attempt has been made to bleed an animal white (Herbst, 

 Haidenhain) ; but a certain quantity of blood, which it is 

 impossible to measure, will always remain in the vessels. 



A complete injection of the vascular system, for the pur- 

 pose of measuring its capacity, has been found equally 

 unsatisfactory. A simpler, and at the same time more 

 ingenious, method is that employed by Valentin. It con- 

 sists in calculating the quantity of blood by means of the 

 dilution which it undergoes after the injection of a definite 

 quantity of water, the proportion of solid and liquid which 

 it contained at first being known. Let us suppose, for in- 

 stance, that the blood of an animal contains, at a given 

 moment, four parts of liquid to one of solid, this proportion 

 having been previously settled by analyzing the blood ob- 

 tained by blood-letting. We then introduce into the vascu- 

 lar system a quantity of water equal to that of the blood 

 which has been withdrawn, and then bleed the animal again, 

 by doing which we naturally obtain a bloody liquid, more 

 diluted than the first. Ifj for example, the first bleeding 

 produced ten grammes, and, after the injection of ten 

 grammes of water, the second bleeding produces blood con- 

 taining twice as much water, it is easy, by a simple computa- 

 tion, to calculate the quantity of blood which the animal 

 contained at first. 



There are great objections to this method also, on account 

 of the rapid change which takes place in the blood, and in 

 the tissues that it bathes, even in the short interval between 

 the two bleedings: the blood has, in fact, a tendency to 

 return immediately after bleeding to its original condition, 

 by borrowing this fluid substance from the surrounding tis- 

 sues. 



A still better method is that of washing, as employed by 



