COAGULATION. 415 



consists of serum plus the blood corpuscles. Blood that has been 

 whipped in this way is known as " defibrinated blood." It resembles 

 normal blood in appearance, but is different in its composition; it 

 can not clot again. The way in which the fibrin is normally de- 

 posited may be demonstrated very easily 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 focusing 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 in- 

 dividual 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 blood of the horse, 

 terrapin, and birds, 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 blood- 

 letting 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 prevented 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 phenomenon! 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 have now simply an historical 

 interest. In recent years much experimental work has been done 

 upon the subject, the result of which has been to increase greatly 

 our knowledge of the process ; but no complete explanation has yet 

 been reached. It is generally admitted that the essential constit- 

 uent of the clot namely, the fibrin is formed from the fibrinogen 

 normally present in the plasma, and that without this fibrin- 

 ogen clotting is impossible. If, for instance, blood is heated to 



