228 THE BLOOD. 



opening of the vein runs freely and coagulates slowly ; while that 

 which is drawn toward the end of .the operation, when the tension 

 of the veins has been relieved and the blood trickles slowly from 

 the wound, coagulates quickly. Secondly, the shape of the vessel 

 into which the blood is received and the condition of its internal 

 surface. The greater, the extent of surface over which the blood 

 comes in contact with the vessel, the more is its coagulation 

 hastened. Thus, if the blood be allowed to flow into a tall, narrow, 

 cylindrical vessel, or into a shallow plate, it coagulates more rapidly 

 than if it be received into a hemispherical bowl, in which the ex- 

 tent of surface is less, in proportion to the entire quantity of blood 

 which it contains. For the same reason, coagulation takes place 

 more rapidly in a vessel with a roughened internal surface, than in 

 one which is smooth and polished. The blood coagulates most 

 rapidly when spread out in thin layers, and entangled among the 

 fibres of cloth or sponges. For the same reason, also, hemorrhage 

 continues longer from an incised wound than from a lacerated one ; 

 because the blood, in flowing over the ragged edges of the lace- 

 rated bloodvessels and tissues, solidifies upon them readily, and thus 

 blocks up the wound. 



In all these cases, there is an inverse relation between the rapidity 

 of coagulation and the firmness of the clot. When coagulation 

 takes place slowly, the clot afterward becomes small and dense, and 

 the serum is abundant. When coagulation is rapid, there is but 

 little contraction of the coagulum, an imperfect separation of the 

 serum, and the clot remains large, soft, and gelatinous. 



It is well known to practical physicians that a similar relation 

 exists when the coagulation of the blood is hastened or retarded by 

 disease. In cases of lingering and exhausting illness, or in diseases 

 of a typhoid or exanthernatous character, with much depression of 

 the vital powers, the blood coagulates rapidly and the clot remains 

 soft. In cases of active inflammatory disease, as pleurisy or pneu- 

 monia, occurring in previously healthy subjects, the blood coagulates 

 slowly, and the clot becomes very firm. In every instance, the 

 blood which has coagulated liquefies again at the commencement of 

 putrefaction. 



The coagulation of the fibrin is not a commencement of organization. 

 The filaments already described, which show themselves in the clot 

 (Fig. 62), are not, properly speaking, organized fibres, and are en- 

 tirely different in their character from the fibres of areolar tissue, or 

 any other normal anatomical elements. They are simply the ulti- 



