OF THE BLOOD. 49 



frog, for example, there are 88 parts of water in 100 of the 

 blood. 



Analogous differences are observed in comparing the relative 

 qualities of the serum and globules of the blood in different 

 animals; while as we shall subsequently see there exists a 

 remarkable relation between the amount of the globules and 

 the animal heat. Birds, of all animals, have the blood richest 

 in red "globules, and in them the animal heat is greatest. 

 Mammals, less warm than birds, have from 7 to 12 per cent., 

 whilst in reptiles and fishes, the proportion does not exceed 

 5 or 6 per cent, of the whole weight of the blood. 



85. Coagulation of the Blood. In its ordinary con- 

 dition the blood is always fluid: withdrawn from the vessels 

 of the living animal, and left for a time to itself, it sepa- 

 rates into two portions, a semi-solid mass and a liquid 

 portion, in which the mass floats ; the solid part is called the 

 dot. 



This phenomenon (the formation of the clot) is due to the 

 presence of fibrin in the blood ; it is held in solution in the 

 serum during life, but when this loses its influence over it, 

 it solidifies, inclosing with it the red globules, and thus forming 

 the red gelatinous mass called the clot. The simple experi- 

 ment of beating up the blood with little rods as it flows from 

 the veins, and thus removing the fibrin, which adheres to 

 the rods, proves that the coagulability of the blood depends 

 on the presence of this substance. 



Another experiment equally simple shows that the fibrin is 

 contained in the serum, and not in the red globules, as -was 

 long supposed. Throw on a filter the blood of a frog ; all 

 the serum may be made to pass, and the globules retained ; 

 in the serum thus separated from the globules a clot is 

 formed, which, however, is colourless. 



86. Use of the Blood. The blood is the special agent 

 of nutrition, and the general restorer of what is lost. 



87. But in addition, it is proved, by the simple experi- 

 ments of bloodletting and of transfusion, to form an essential 

 stimulus for the performance of the functions of life. By 

 severe bloodletting or loss of blood we become enfeebled and 

 seemingly dead ; but if, before this happens, the blood of 

 another animal be transfused into the veins of the suffering 

 individual, the vitality is restored. The importance of the 

 globules is also proved by the same experiment, for if simple 

 serum be so transfused, death takes place. 



