BLOOD AND LYMPH 203 



specific gravity of a sodium sulphate solution in which a drop 

 of blood neither sinks nor floats. (Chemical Physiology.) 



Taste and Smell are characteristic, and must be experienced. 

 Reaction. Blood is alkaline, and the degree of alkalinity is very 

 constant in health. It is more alkaline in herbivorous than 

 in carnivorous animals. It is increased during digestion and 

 diminished after muscular exercise. (Chemical Physiology.) 



The cells of the blood constitute about 33 per cent, of its 

 weight, and the total solids of the blood are about 20 per cent. 



Clotting OP Coagulation. In the course of about three 

 minutes the blood of the dog when shed becomes a solid jelly. 

 In the blood of the horse the process is much slower and the 

 blood cells may sink before clotting occurs, thus leaving the 

 upper part of the clot colourless. The process starts from the 

 sides of the vessel, and spreads throughout the blood until, when 

 clotting is complete, the vessel may be inverted without the 

 blood falling out. In a short time, drops of clear fluid appear 

 upon the surface of the clot, and in a few hours these have 

 accumulated to a considerable extent, while the clot has con- 

 tracted and drawn away from the sides of the vessel, until it 

 finally floats in the clear fluid the Serum. If clotting occurs 

 slowly, as it does in the horse, the erythrocytes subside, leaving 

 a layer of clear plasma above, which, when coagulation takes 

 place, forms a " buffy layer " in the upper part of the clot. 

 Clotting is due to changes in the plasma, since this fluid will 

 coagulate in the absence of corpuscles. 



The change may be represented thus : 

 Blood 



Plasma Corpuscles 



Serum Clot 



The change consists in the formation of a series of fine 

 elastic threads of fibrin throughout the plasma, and if red 

 corpuscles are present they are entangled in the meshes of 

 the network and give the clot its red colour. 



These threads may be readily collected in mass upon a 

 stick with which the blood is whipped as it is shed. The 



