352 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



always, in the normal and pathological exudations. It is absent from blood- 

 serum, being used up during the process of clotting. It occurs in very small 

 quantities in blood, compared with the other proteids. There is no good 

 method of determining quantitatively the amount of fibrinogen, but estimates 

 of the amount of fibrin, which cannot differ very much from the fibrinogen, 

 show that in human blood it varies from 0.22 to 0.4 per cent. In horse's 

 blood it may be more abundant 0.65 per cent. As to the origin and the 

 special physiological value of this proteid we are, if possible, more in the dark 

 than in the case of paraglobulin, with the exception that fibrinogen is known to 

 be the source of the fibrin of the blood. But clotting is an occasional phe- 

 nomenon only. What nutritive function, if any, is possessed by fibrinogen 

 under normal conditions is unknown. No satisfactory account has been given 

 of its origin. It has been suggested by different investigators that it may 

 come from the nuclei of disintegrating leucocytes (and blood-plates) or from 

 the dissolution of the extruded nuclei of newly-made red corpuscles, but here 

 again we have only speculations, which cannot be accepted until some experi- 

 mental proof is advanced to support them. 



Coagulation of Blood. One of the most striking properties of blood is 

 its power of clotting or coagulating shortly after it escapes from the blood- 

 vessels. The general changes in the blood during this process are easily fol- 

 lowed. At first shed blood is perfectly fluid, but in a few minutes it becomes 

 viscous and then sets into a soft jelly which quickly becomes firmer, so that 

 the vessel containing it can be inverted without spilling the blood. The clot 

 continues to grow more compact and gradually shrinks in volume, pressing out 

 a smaller or larger quantity of a clear, faintly yellow liquid to which the name 

 blood-serum has been given. The essential part of the clot is the fibrin. Fibrin 

 is an insoluble proteid which is absent from normal blood. In shed blood, 

 and under certain conditions in blood while still in the blood-vessels, this fibrin 

 is formed from the soluble fibrinogen. The deposition of the fibrin is peculiar. 

 It is precipitated, if the word may be used, in the form of an exceedingly fine 

 network of delicate threads which permeate the whole mass of the blood and 

 give the clot its jelly-like character. The shrinking of the threads causes 

 the subsequent contraction of the clot. If the blood has not been shaken 

 during the act of clotting, almost all the red corpuscles are caught in the fine 

 fibrin meshwork, and as the clot shrinks these corpuscles are held more firmly, 

 only the clear liquid of the blood being squeezed out, so that it is possible to 

 get specimens of serum containing few or no red corpuscles. The leucocytes, 

 on the contrary, although they are also caught at first in the forming mesh- 

 work of fibrin, may readily pass out into the serum in the later stages of clot- 

 ting, on account of their power of making amoeboid movements. If the blood 

 has been agitated during the process of clotting, the delicate network will be 

 broken in places and the serum will be more or less bloody that is, it will 

 contain numerous red corpuscles. If during the time of clotting the blood is 

 vigorously whipped with a bundle of fine rods, all the fibrin will be deposited 

 as a stringy mass upon the whip, and the remaining liquid part will consist of 



