138 HANDBOOK OF PHYSIOLOGY. 



forming thrombin, which in turn causes a splitting of the fibrinogen 

 molecule the calcium remaining with one portion of the molecule to 

 form insoluble fibrin, the other portion passing into solution as fibrin- 

 globulin. According to Lilienfeld, the reaction first occurs between the 

 fibriuogen molecule and the nucleo-albnmin, resulting in a splitting of 

 the former. A portion of the fibrinogen molecule then unites with the 

 calcium salts to form fibrin. 



Sources of the Fibrin Ferment. Fibrin ferment cannot be obtained 

 in any appreciable amount from blood which is allowed to flow direct 

 from the living vessel into absolute alcohol. It is almost certainly a 

 result of the more or less complete disintegration of the colorless cor- 

 puscles after blood is shed, or of the third corpuscles which will be de- 

 scribed later on under the name of Hood platelets. The proofs of this 

 may be briefly summarized as follows: (1) That all strongly coagulable 

 fluids contain these corpuscles almost in direct proportion to their coag- 

 ulability; (2) That clots formed on foreign bodies, such as needles pro- 

 jecting into the interior or lumen of living blood-vessels, are preceded 

 by an aggregation of colorless corpuscles; (3) That plasma in which 

 these corpuscles happen to be scanty clots feebly; (4) That if horse's 

 blood be kept in the cold, so that the corpuscles subside, it will be 

 found that the lowest stratum, containing chiefly colored corpuscles, 

 will, if removed, clot feebly, as it contains little of the fibrin factors; 

 whereas the colorless plasma, especially the lower layers of it in which 

 the colorless corpus les are most numerous, will clot well, but if filtered 

 in the cold will not clot so well, indicating that when filtered nearly 

 free from colorless corpuscles even the plasma does not contain sufficient 

 of all the fibrin factors to produce thorough coagulation; (5) In a drop 

 of coagulating blood observed under the microscope the fibrin fibrils are 

 seen to start from the colorless corpuscles. 



Conditions affecting Coagulation. The coagulation of the blood 

 is hastened by the following means : 



1. Moderate warmth, from about 37.8-49 C. (100 to 120 F.). 



2. Rest is favorable to the coagulation of blood. Blood, of which, 

 the whole mass is kept in uniform motion, as when a closed vessel com- 

 pletely filled with it is constantly moved, coagulates very slowly and 

 imperfectly. 



3. Contact with foreign matter, and especially multiplication of the 

 points of contact. Thus, as before mentioned, fibrin may be quickly 

 obtained from liquid blood by stirring it with a bundle of small twigs; 

 and even in the living body the blood will coagulate upon rough bodies 

 projecting into the vessels. 



4. Injury to the watts of the Wood-vessels. 



5. The addition of less than twice the bulk of water. 



