1 88 HUMAN PHYSIOLOGY 



rendered transparent, the blood as a whole becomes trans- 

 parent and " laked." The Specific Gravity is about 1055. It 

 may be estimated by finding the specific gravity of a sodium 

 sulphate solution in which a drop of blood neither sinks nor 

 floats. 



Taste and Smell are characteristic, and must be experi- 

 enced. Reaction. Blood is alkaline, arid the degree of 

 alkalinity is very constant in health. It is equivalent to 

 about 0*3 per cent, of Na 2 C0 3 . It is increased during 

 digestion, and diminished after muscular exercise. 



Clotting or Coagulation. In the course of about three 

 minutes the blood when shed becomes a solid jelly. 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 con- 

 siderable extent, while the clot has contracted and drawn 

 away from the sides of the vessel, until it finally floats in the 

 clear fluid the Serum. 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 



i i 



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 

 red fluid blood which is left, consisting of blood cells and 

 serum, is said to be defibrinated. 



