30 PHYSIOLOGY FOR BEGINNERS CHAP. 



be obtained. Fibrin is a colourless, soft, stringy, elastic 

 substance. . The blood which is left after being thus whipped 

 will not clot again, however long it be allowed to stand, 

 because all the fibrin it could make has been removed. It is 

 still red in colour, and examined with the microscope is seen 

 to contain red and white corpuscles like ordinary blood. 



If a strong solution of common salt or of Epsom salts be 

 added to fresh blood, and especially if the mixture be then 

 kept cold by ice, the blood will not clot for several days, or not 

 at all. As it stands, the corpuscles will gradually settle and 

 leave a little clear plasma at the top. jLlf some of this plasma 

 is carefully drawn off the top by a glass tube and taken out 

 of the cold, and especially if a little water is added to it to 

 dilute the salt put in, it will go into a jelly, and this will lead 

 in a short time to a colourless clot and a little serum. The 

 serum formed by the clotting of plasma is exactly the same 

 as the serum formed by the clotting of the whole blood ; but 

 the clot consists only of fibrin, and has no red or colourless 

 corpuscles entangled in the fibrin. This experiment shows 

 that the clotting of blood may take place without the corpuscles. 

 It is the plasma which forms the fibrin. 



Composition of Serum. Serum is a yellow alkaline 

 fluid consisting of water holding proteids, salts, and other 

 substances in solution. There are two proteids present in 

 serum called respectively albumin and globulin. 



The white of egg consists chiefly of albumin, and the 

 albumin of the serum is very like, although not quite identical 

 with, the albumin of the egg. When the egg is boiled the 

 albumin " sets " and becomes solid, white, and opaque, and 

 in the same way if some serum is heated it becomes white, 

 opaque, and nearly solid, due to the " setting," or coagulation 

 as it is called, of the albumin in the serum. The proteid 

 globulin also coagulates in the same way when the serum is 

 heated, so that this also contributes to the result. Globulin 

 differs from albumin in that while albumin is soluble in mere 

 water, distilled water, globulin is only soluble in water contain- 

 ing certain salts ; these salts are present in serum, hence 

 globulin is present in serum in solution. After they have been 

 coagulated by heat, they are both like boiled white of egg, not 

 soluble at all. 



