THE BLOOD. 131 



colored, and these are added to the previously withdrawn blood; lastly 

 the whole animal is finely minced with saline solution. The fluid ob- 

 tained from the mincings is carefully filtered, and added to the diluted 

 blood previously obtained, and the whole is measured. The next step 

 in the process is the comparison of the color of the diluted blood with 

 that of standard solutions of blood and water of a known strength, until 

 it is discovered to what standard solution the diluted blood corresponds. 

 As the amount of blood in the corresponding standard solution is known, 

 as well as the total quantity of diluted blood obtained from the animal, 

 it is easy to calculate the absolute amount of blood which the latter con- 

 tained, and to this is added the small amount which was withdrawn to 

 make the standard solutions. This gives the total amount of blood 

 which the animal contained. It is contrasted with the weight of the 

 animal, previously known. 



The result of many experiments shows that the quantity of blood in 

 various animals averages -^ to - f of the total body-weight. 



An estimate of the quantity in man which corresponded nearly with 

 this proportion, has been more than once made from the following data. 

 A criminal was weighed before and after decapitation; the difference in 

 the weight representing the quantity of blood which escaped. The 

 blood-vessels of the head and trunk were then washed out by the injec- 

 tion of water, until the fluid which escaped had only a pale red or straw 

 color. This fluid was then also weighed; and the amount of blood 

 which it represented was calculated by comparing the proportion of 

 solid matter contained in it with that of the first blood which escaped 

 on decapitation. Two experiments of this kind gave precisely similar 

 results. (Weber and Lehmann.) 



It should be remembered, in connection with these estimations, that 

 the quantity of the blood must vary very considerably, even in the same 

 animal, with the amount of both the ingesta and egesta of the period 

 immediately preceding the experiment; it has been found, for example, 

 that the quantity of blood obtainable from the body of a fasting animal 

 rarely exceeds a half of that which is present soon after a full meal. 



Coagulation of the Blood. 



One of the most characteristic properties which the blood possesses 

 is that of clotting or coagulating. This phenomenon may be observed 

 under the most favorable conditions in blood which has been drawn 

 into an open vessel. In about two or three minutes, at the ordinary 

 temperature of the air, the surface of the fluid is seen to become semi- 

 solid or jelly-like, and this change takes place, in a minute or two after- 

 ward at the sides of the vessel in which it is contained, and then extends 

 throughout the entire mass. The time which is occupied in these 

 changes is about eight or nine minutes. The solid mass is of exactly 



