150 UNCONSCIOUS NERVOUS OPERATIONS 



stand for a few minutes, it becomes a firm mass of jelly. 

 After an hour or more, a yellow fluid, called serum, begins 

 to ooze from the clot, which shrinks in size. The clotting 

 is caused by the formation, in the liquid blood, of a close 

 network of fine fibrils, called fibrin, in which the corpus- 

 cles of both kinds are entangled, while serum is the plasma 

 of the blood, minus an element in its composition called 

 fibrinogen, which changes into the solid fibrin in the coag- 

 ulation. It is thought that when the blood leaves the 

 blood vessel, or in some way comes in contact with foreign 

 matter, a portion of the white corpuscles are broken up, 

 and thus is set free a peculiar substance called fibrin fer- 

 ment. It is this which acts upon the fibrinogen, and 

 causes it to become fibrin. The fibrin may be gotten out 

 from a quantity of freshly drawn blood by quickly stir- 

 ring or whipping it with a bunch of twigs. The tiny 

 white threads cling to the sticks, and by washing in water 

 may be freed from the few entangled corpuscles which 

 remain, leaving the fibrin pure. 



This power which the blood has to clot is of great 

 value, since by its means small breakages in or injuries to 

 the innumerable tubes conveying the blood throughout 

 the system are quickly stopped, and the serious hemor- 

 rhage which would otherwise result is quickly checked, 

 while the ruptured wall of the blood vessel is given time 

 to heal. 



209. Lymph. It is by the blood that nutriment is car- 

 ried to every part of the body; but the blood is always 

 inclosed within the walls of the tubes called blood vessels, 

 and, as blood, does not come in contact with the cells of 

 the tissues. 



In the capillaries, which are the finest ramifications of 

 the blood vessels, some of the plasma passes from the 



