THE BLOOD. 227 



vascular walls is an element in the exciting cause of coagulation. 

 It is of course impossible to withdraw blood from the system without 

 inflicting such an injury ; and we know that in cases of phlebitis, coagula- 

 tion often takes place within the affected veins, when the only condition 

 present to explain it is the inflammatory alteration of the vascular walls. 



The coagulation of fibrine is not a commencement of organization. 

 It is simply the passage of one of the ingredients of the blood from 

 its normal condition to a state of solidity. The coagulable matter, 

 when solidified, has lost its natural properties as a constituent of the 

 plasma, and they cannot afterward be restored. The clot, therefore, once 

 formed, even within the body, as in cases of ligature, apoplexy, or 

 extravasation, becomes a foreign substance, and is absorbed by the 

 neighboring parts during convalescence. At first it is comparatively 

 voluminous, soft, and red. Its more fluid parts are then taken up, and 

 it becomes smaller and denser. As absorption goes on, its coloring 

 matter diminishes, and finally disappears. The time required for com- 

 plete reabsorption of a clot varies, according to its size and situation, 

 from a few days to several months. 



Nature of Coagulation. The coagulation of blood has been the 

 subject of much laborious investigation. The difficulty of understand- 

 ing its nature depends on the fact that the blood, which continues fluid 

 under normal conditions while circulating in the vessels, solidifies 

 promptly and inevitably on its withdrawal. It is evident that the 

 solid fibriue which we obtain after coagulation is not the material which 

 was present beforehand in the blood ; but that it has been produced, 

 by some alteration, from a preexisting fluid substance. Any theory of 

 the process, to be satisfactory, must explain not only the coagulable 

 property of the fibrine-producing ingredient, but also the fluidity of 

 the blood in its natural condition, notwithstanding that it contains a 

 material so ready to assume the solid form. It is unnecessary to con- 

 sider the former theories of coagulation, which have now been aban- 

 doned as inconsistent with known facts. It is not due to the cooling 

 of the blood, to the contact of air, nor to the escape of a gaseous sol- 

 vent ; since it will occur in the absence of all these conditions. Of 

 late years, the only views on this subject which have attracted general 

 attention are those of Denis, in which coagulation is explained by the 

 decomposition of a previously existing substance, and those of Schmidt, 

 which attribute it to the union of two substances previously distinct. 



According to Denis, the blood contains an albuminous matter, 

 termed " plasmine," in the proportion of 25 parts per thousand. When 

 withdrawn from the circulation, it separates into two new substances ; 

 namely, fibrine (3 parts per thousand) which coagulates, and paraglob- 

 uline (22 parts per thousand) which remains fluid. The basis for this 

 theory is that if fresh blood be drawn into a concentrated solution of 

 sodium sulphate, to prevent its coagulation, and sodium chloride be 

 added to the mixture in the proportion of ten per cent., it throws 

 down a white, pasty substance, which represents 25 parts per thousand 



