BLOOD. 353 



serum plus the blood-corpuscles. Blood which has been whipped in this way 

 is known as " defibrinated blood." It resembles normal blood in appearance, 

 but is different in its composition : it caunot clot again. The way in which 

 the fibrin is normally deposited may be demonstrated most beautifully under 

 the microscope by placing a good-sized drop of blood on a slide, covering it 

 with a cover-slip, and allowing it to stand for several minutes until coagu- 

 lation is completed. If the drop is now examined, it is possible by careful 

 focussing to discover in the spaces between the masses of corpuscles many 

 examples of the delicate fibrin network. The physiological value of clotting 

 is that it stops hemorrhages by closing the openings of the wounded blood- 

 vessels. 



Time of Clotting. The time necessary for the clot to form varies slightly 

 in different individuals, or in the blood of the same individual varies with the 

 conditions. It may be said in general that under normal conditions the blood 

 passes into the jelly stage in from three to ten minutes. The separation of 

 clot and serum takes place gradually, but is usually completed in from ten to 

 forty-eight hours. The time of clotting shows marked variations in different 

 animals ; the process is especially slow in the horse and the terrapin, so that 

 coagulation of shed blood is more easily prevented in these animals. In the 

 human being also the time of clotting may be much prolonged under certain 

 conditions in fevers, for example. This fact was noticed in the days when 

 bloodletting was a common practice. The slow clotting of the blood permitted 

 the red corpuscles to sink somewhat, so that the upper part of the clot in such 

 cases was of a lighter color, forming what was called the " buffy coat." The 

 time of clotting may be shortened or be prolonged, or the clotting may be pre- 

 vented altogether, in various ways, and much use has been made of this fact 

 in studying the composition and the coagulation of blood as well as in con- 

 trolling hemorrhages. It will be advantageous to postpone an account of these 

 methods for hastening or retarding coagulation until the theories of coagulation 

 have been considered. 



Theories of Coagulation. The clotting of blood is such a prominent phe- 

 nomenon that it has attracted attention at all times, and as a result numerous 

 theories to account for it have been advanced. Most of these theories possess 

 simply an historical interest, and need not be discussed in a work of this charac- 

 ter, but some reference to older views is unavoidable for a proper presentation 

 of the subject. To prevent misunderstanding it may be stated explicitly in 

 the beginning that there is at present no perfectly satisfactory theory. Indeed, 

 the subject is a difficult one, as it is intimately connected with the chemistry 

 of the proteids of the blood, and it may be said that a complete understanding 

 of clotting waits upon a better knowledge of the nature of these proteids. It 

 happens that at the present time a great deal of attention is being paid to this 

 subject by experimenters, and it is possible that at any moment new facts may 

 be discovered which will alter present ideas of the nature of the process. In 

 considering the different theories that have been proposed there are two general 

 facts which should always be kept in mind : first, that the main phenomenon 



