COAGULATION. 431 



serum. When serum is added to a solution of fibrinogen clotting 

 takes place, but the process is much accelerated if at the same time 

 one adds some tissue-extract and calcium chlorid. The efficacy 

 of these two latter substances, which in themselves are incapable 

 of changing the fibrinogen to fibrin, is explained on the assumption 

 that they activate some of the remaining thrombogen in the blood- 

 serum. Delezenne has shown that if the blood of birds is with- 

 drawn carefully, by means* of a cannula inserted into an artery, 

 it clots very slowly, and if centrifugalized at once the supernatant 

 plasma when removed may remain unclotted for some days. This 

 result seems to be due to the fact that the corpuscular elements 

 in the blood of these animals disintegrate less readily. A similar 

 result holds good for the blood of terrapins, as was pointed out 

 long ago by the present author. If, however, in withdrawing the 

 blood it comes into contact with the tissues, at the wound, for 

 instance, it will clot quickly, and it would appear that a zymo- 

 plastic substance is furnished by the tissues. In the bird the 

 normal clotting of the blood to stop wounded vessels must depend 

 upon this co-operation from the outside tissues. In the mammal 

 the blood itself contains all the sources necessary for prompt coagu- 

 lation, but it is quite possible that here also the clotting in ordinary 

 hemorrhages from wounds may be normally accelerated by zymo- 

 plastic substances supplied from the vessel walls or the surrounding 

 tissues. 



After the active thrombin is formed its manner of action upon the 

 fibrinogen is also a matter concerning which we have little positive 

 knowledge. Hammarsten has supposed that the thrombin causes 

 a splitting of the fibrinogen molecule, with the formation of the 

 insoluble fibrin and a soluble globulin, fibrin globulin, 



which can be found in small quantities in the serum after coagula- 

 tion. This conclusion has not, however, been demonstrated to be 

 correct. Some observers have suggested that the enzyme causes a 

 rearrangement in the structure of the fibrinogen molecule, while 

 others have given some reasons for believing that the action of the 

 thrombin is hydrolytic, as is the case with most of the enzymes of 

 digestion. Thus, Fuld* states, from experiments upon the blood- 

 plasma of birds, that the rapidity of clotting varies, not directly 

 with the amount of enzyme (thrombin) present, but rather in pro- 

 portion to the square root of the amount, thus following the law of 

 Schiitz for hydrolytic enzymes. 



Summary. By way of summary the following statements may 



* Fuld, "Beitrage zur chem. Physiol. u. Pathol.," 2, 514. 



