58 THE BLOOD. 



escaped on decapitation. Two experiments of this kind gave 

 precisely similar results. 



The most reliable of these various means for estimating the 

 quantity of blood in the body yield as nearly similar results 

 as can be expected, when the sources of error unavoidably 

 present in all, are taken into consideration; and it may be 

 stated that in man, the weight of the whole quantity of blood, 

 compared with that of the body, is from about 1 to 8, to 1 to 10. 



It must be remembered, however, that the whole quantity 

 of blood varies, even in the same animal, very considerably, 

 in correspondence with the different amounts of food and drink, 

 which may have been recently taken in, and the equally vary- 

 ing quantity of matter given out. Bernard found by experi- 

 ment, that the quantity of blood obtainable from a fasting 

 animal is scarcely more than half of that which is present 

 soon after a full meal. The estimate above given, must there- 

 fore be taken to represent only an approximate average. 



Coagulation of the Blood. 



When blood is drawn from the body, and left at rest, cer- 

 tain changes ensue, which constitute a kind of rough analysis 

 of it, and are instructive respecting the nature of some of its 

 constitutents. After about ten minutes, taking a general 

 average of many observations, it gradually clots or coagulates, 

 becoming solid like a soft jelly. The clot thus formed has at 

 first the same volume and appearance as the fluid blood had, 

 and, like it, looks quite uniform ; the only change seems to be, 

 that the blood which was fluid is now solid. But presently, 

 drops of transparent yellowish fluid begin to ooze from the 

 surface of the solid clot; and these gradually collecting, first 

 on its upper surface, and then all around it, the clot or " cras- 

 samentum" diminished in size, but firmer than it was before, 

 floats in a quantity of yellowish fluid, which is named serum, 

 the quantity of which may continually increase for from twenty- 

 four to forty-eight hours after the clotting of the blood. 



The changes just described may be thus explained. The 

 liquor sanguinis, or liquid part of the blood (p. 55), consists of 

 a thin fluid called serum, holding fibrin in solution. 1 The 

 peculiar property of fibrin, as already said, is its tendency to 

 become solid when at rest, and in some other conditions. When, 

 therefore, a quantity of blood is drawn from the vessels, the 

 fibrin coagulates, and the blood-corpuscles, with part of the 



1 This statement has been left unaltered in the text ; but, as will 

 be seen farther on, it requires modification. (ED.) 



