298 THE BLOOD. 



states that acetic or any other weak acid will effect the same 

 change in fibrinogen. This theory has also its weak points, and 

 there is great doubt whether the clot is fibrin in any true sense of 

 the term. Perhaps we can in no better way present to our readers 

 the present status of this subject, which is at best unsatisfactory, 

 than by giving the views of Schafer in his Text-book of Physi- 

 ology. The evidence seems fairly conclusive that three factors are 

 essential to bring about coagulation of the blood. These are 

 fibrinogen, the nucleoproteid prothrombin, and soluble lime salts, 

 the two latter acting in combination, and forming Schmidt's fibrin- 

 ferment or thrombin. In the normal blood-vessels the prothrom- 

 bin and lime have not entered into the necessary combination or 

 interaction which enables them to act as a ferment upon the 

 fibrinogen. This nucleoproteid prothrombin cannot by itself act as 

 a ferment, but must be exposed to the acting soluble lime salts ; 

 it does not follow, however, that the thrombin is a compound of 

 the nucleoproteid and lime ; nor is there any certainty as to just 

 what the interaction is. The prothrombin doubtless comes from 

 the leukocytes and plaques ; but it is not necessary to suppose 

 that they always undergo disintegration to produce it. It is also 

 probable that the red corpuscles may contribute to this production 

 of nucleoproteids, for they contain them, and the same is also 

 true of the epithelial cells of the blood-vessels, which are doubt- 

 less composed of living protoplasm. Schafer, in his text-book, 

 sums up the evidence as to coagulation as follows : 



1. That the coagulation of blood i. e., the transformation of 

 fibrinogen into fibrin requires for its consummation the inter- 

 action of a nucleoproteid (prothrombin) and soluble lime salts, 

 and the consequent production of a ferment (thrombin). 



2. That either nucleoproteid is not present in appreciable 

 amount in the plasma of circulating blood, or that the interaction 

 in question is prevented from occurring within the blood-vessels 

 by some means at present not understood. 



3. That the nucleoproteid (prothrombin) appears and the inter- 

 action occurs as soon as the blood is drawn and is allowed to come 

 into contact with a foreign surface, the source of the nucleoproteid 

 being in all probability mainly the leukocytes (and blood-platelets). 



4. That under certain circumstances and conditions either the 

 nucleoproteid does not appear in the plasma of drawn blood or it 

 appears, but the interaction between it and lime salts is prevented 

 or delayed. 



5. That the nucleoproteid (prothrombin) appears in the plasma 

 of circulating blood under certain conditions, being in all probabil- 

 ity shed out from the white corpuscles and blood-platelets, or in 

 some cases even from the red corpuscles ; and that when shed out 

 under these conditions from the corpuscles, or when artificially 

 injected into the vessels,, it tends at once to interact with the lime 



