FUNCTIONS OF THE BLOOD 285 



by crushing them or tying ligatures about them, or the introduc- 

 tion of some foreign substance into the circulation. 



Methods of Hastening or Retarding Coagulation. Since the 

 process of clotting is in several steps there are a corresponding 

 number of points at which its normal course may be broken into, 

 either with the effect of hastening the result or of retarding it 

 or even preventing it altogether. Anything which quickens the 

 disintegration of the blood-plates, as the application of a hand- 

 kerchief to a wound, which acts by increasing the foreign surface 

 in contact with the blood, makes the blood clot more quickly. 

 The application of heat has this same effect; probably it acts both 

 by increasing the rate of destruction of blood-plates and by has- 

 tening the chemical reactions involved in the process as a whole. 

 Cold, as would be expected, has the converse effect. An increase 

 in the calcium content of the blood shortens the coagulation time. 

 Coagulation may be retarded, as we have seen, by cold or by de- 

 priving the blood of its calcium content. Blood drawn into a 

 strong solution of sodium or magnesium sulphate and well mixed 

 will not clot, these salts appearing to interfere in some way with 

 the formation of the thrombin; such " salted" blood will clot if 

 thrombin is added or if diluted sufficiently with water. There are 

 a number of other substances which either retard or prevent 

 coagulation; their mode of action is for the most part obscure. 

 Among them are sodium fluoride, leech extract, snake venom, and 

 commercial peptone solution. This latter is of interest because 

 it only produces its effect of destroying the coagulating power of 

 the blood when introduced into the veins of the animal. When 

 added to shed blood it has no effect on coagulation. It has been 

 shown that this property does not reside in the peptone itself, 

 but in some unknown impurity which is associated with it in the 

 process of preparation. The blood of persons suffering from fever 

 does not clot as quickly as that of healthy persons. This fact has 

 been known since the days when bloodletting was a common 

 practice. Its explanation remains obscure. 



" Bleeders." There is a pathological condition, fortunately not 

 very common, known as hemophilia, in which the blood will not 

 clot. Persons suffering from this disease are called bleeders. Such 

 persons are in danger of bleeding to death from slight wounds; a 

 nosebleed, or the bleeding which follows the extraction of a tooth, 



