THE BLOOD AND THE LYMPH 261 



Soon after, threads of fibrin shot outward from all these little masses 

 and soon the contained erythrocytes and leukocytes were firmly en- 

 meshed in this net, made of the finest of fine threads. These immediately 

 began to contract upon the corpuscles, so making firmer the clot. Of 

 this latter, however, not more than 0.2 per cent, is fibrin. Cray-fish 

 blood contains particles which are like large thrombocytes and on the 

 blood's shedding these break up with extreme quickness, with the result 

 that cray-fish blood coagulates much more promptly than that of most 

 animals. 



In human blood coagulation begins as an increasing viscosity immedi- 

 ately and by the end of from three to ten minutes (depending on external 

 conditions partly), the blood has formed into a jelly from which on over- 

 turning no liquid escapes. This mass, the clot, is gradually concentrated 

 by the contraction of the fibrin, and lessening in size for a day or two 

 leaves around it a yellowish limpid liquid called serum. Various agents 

 vary the rapidity of coagulation. The more foreign matter there is 

 scattered through and around the blood and the more the latter is agitated 

 the quicker it clots. A few degrees of heat above the body-temperature, 

 and injury to the endotheliurii of the containing blood-vessels act in 

 a similar direction. So does the addition of calcium salts (for example, 

 the lactate), to the circulation of the animal previously, or of the xanthins, 

 uric acid, etc., to the shed blood. Acids and alkalies, glycerin, oxalates, 

 magnesium sulphate, peptone, egg-albumin, and considerable cold are 

 some of the agents which inhibit coagulation. It has long been a mystery 

 why the blood fails to coagulate in the uninjured blood-vessels, even 

 when they are excised and hung up, but it is now likely that the normal 

 endothelium secretes some enzyme or other form of substance which 

 prevents the clotting. When blood does clot in the vessels the process 

 begins in the center and not about the walls where the inhibiting influence 

 would be the stronger. Biirker found that the coagulation-time was 

 constant for each individual. He also learned that the shortest time is 

 late in the afternoon, and that independently of the outside temperature 

 at the hour. Although cold is in our list of inhibiting influences, it is 

 often used in medicine to check hemorrhage because sometimes, as in 

 the vagina, its contracting-power over the bleeding blood-vessels more 

 than counteracts its influence over the coagulating process. In hemo- 

 philia thyroid extract is said to be effective, but in what way is unknown ; 

 adrenalin acts by vaso-constriction. 



The chemistry of blood-coagulation has not been as yet made out with 

 certainty, but many things imply that the general reactions are somewhat 

 as follows : Under the negative influence of the removal of the inhibiting 

 substance secreted by the endothelium, or under the stimulus of some 

 sort coming from contact with foreign bodies, or both, the thrombocytes 

 of the shed blood (or leukocyte- or erythrocyte-nuclei) exude a nucleo- 

 proteid which unites w r ith the calcium salts of the plasma to form a 

 coagulating ferment called thrombin. This enzyme acts on the fibrinogen 

 (perhaps a globulin) of the plasma and converts it into two substances. 



