OF THE BLOOD. 45 



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 dition the blood is always fluid: withdrawn from the vessels 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 separates 

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

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

 clot. 



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 experiment 

 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. 



The fibrin of the blood also plays an important part, for 

 M. Magendie has shown, that when blood deprived of its 

 fibrin is injected into the veins of a dog, the animal dies with 

 symptoms resembling those of putrid fevers. 



88. The influence of the blood over nutrition may 

 also be readily demonstrated. Withdraw the blood more 

 or less from any organ, and it gradually wastes away 

 in proportion to the quantity withdrawn ; on the contrary, 

 the greater size of the muscles in those who employ them 



