36 A MANUAL OF PHYSIOLOGY 



given, on the average volume of each corpuscle. For instance, when 

 the molecular concentration, and therefore the osmotic pressure 

 (p. 360), of the plasma is reduced, as by the addition of water or the 

 abstraction of salts, water passes into the corpuscles and they swell ; 

 when the molecular concentration of the plasma is increased, by the 

 abstraction of water or the addition of salts, water passes out of the 

 corpuscles, and they shrink. 



Laking of Blood. Even in thin layers blood is opaque, owing to 

 reflection of the light by the red corpuscles. It becomes trans- 

 parent or * laky ' when by any means the pigment is brought out of 

 the corpuscles and goes into true solution. Repeated freezing and 

 thawing of the blood, the addition of water, the passage of electrical 

 currents, constant and induced,* putrefaction, heating the blood to 

 60 C., and many chemical agents (as bile-salts, ether, saponin), cause 

 this change. The blood-serum of certain animals breaks up the 

 coloured corpuscles of others, and sets free their pigment for 

 example, the serum of the dog destroys the corpuscles of the rabbit. 



Since changes begin in the blood as soon as it is shed, 

 having for their outcome clotting or coagulation, we have to 

 gather from the composition of the stable factors of clotted 

 blood, or of blood which has been artificially prevented 

 from clotting, some notion of the composition of the un- 

 altered fluid as it circulates within the vessels. The first 

 step, therefore, in the study of the chemistry of blood is the 

 study of coagulation. 



Coagulation of the Blood. When blood is shed, its viscidity 

 soon begins to increase, and after an interval, varying with 

 the kind of blood, the temperature of the air, and other 

 conditions, but in man seldom exceeding ten, or falling 

 below three, minutes, it sets into a firm jelly. This jelly 

 gradually shrinks and squeezes out a straw-coloured liquid, 

 the serum. Under the microscope the serum is seen to 

 contain few or no red corpuscles ; these are nearly all in the 

 clot, entangled in the meshes of a kind of network of fine 

 fibrils composed of fibrin. In uncoagulated blood no such 

 fibrils are present ; they have accordingly been formed by a 

 change in some constituent or constituents of the normal 

 blood. Now, it has been shown that there exists in the 

 plasma the liquid portion of unclotted blood a substance 

 from which fibrin can be derived, while no such substance 

 is present in the corpuscles. In various ways coagulation 

 can be prevented or delayed, and the plasma separated from 



* Hermann has shown that the laking action of induced currents is 

 due simply to the heating of the blood. 



