56 THE HUMAN BODY. 



primarily determines the quantity of fibrin formed : the more 

 fibrinogen the more fibrin, though never quite so much as the 

 fibrinogen which disappears. The ferment acting on fibrin- 

 ogen in the presence of a salt of calcium, in some way causes 

 it to become fibrin, but does not itself enter into the fibrin ; it 

 is not used up in the process, and the amount of fibrin ulti- 

 mately formed is the same whether much or little ferment 

 be present; but the more ferment the quicker the clotting. 

 The presence in small quantity of many neutral salts seems to 

 favor coagulation, but none except the lime-salts are essential. 

 The part they play is obscure; and when present in large pro- 

 portions they prevent coagulation of blood or plasma, prob- 

 ably by hindering the formation of ferment. If fresh blood 

 be mixed with an equal bulk of a saturated solution of mag- 

 nesium sulphate (Epsom salts) 'or of common salt, it will not 

 clot; but if this mixture be largely diluted with water, then 

 some ferment is formed and clotting takes place. 



The Proximate Causes of Normal Blood Coagulation. 

 As all the phenomena of clotting, with the formation of fibrin 

 agreeing in all respects with that formed during the natural 

 coagulation of drawn blood, can be obtained in artificial solu- 

 tions of fibrinogen, it is obvious that the process is not, as was 

 once supposed, a so-called vital but a purely chemical one : 

 but we still are far from a satisfactory explanation why the 

 fibrinogen of the plasma does not clot in normal circulating 

 blood contained in healthy blood-vessels. It is, in fact, much 

 easier to point out what are not the proximate causes of the 

 coagulation of drawn blood than what are. 



Blood when removed from the Body and received in a 

 vessel comes to rest, cools, and is exposed to the air, from 

 which it may receive or to which it may give off gaseous 

 bodies. But it is easy to prove that none of these three 

 things is the cause of coagulation. Stirring the drawn blood 

 and so keeping it in movement does not prevent but hastens 

 its coagulation: and blood carefully imprisoned in a living 

 blood-vessel, and so kept at rest, will not clot for a long time; 

 not until the inner coat of the vessel begins to change from 

 the want of fresh blood. Secondly, keeping the blood at the 

 temperature of the Body hastens coagulation, and cooling re- 

 tards it; blood received into an ice-cold vessel and kept sur- 

 rounded with ice will clot more slowly than blood drawn and 

 left exposed to ordinary temperatures. Finally, if the blood 



