A STUDY OF THE BLOOD 119 



the end of a few moments the blood becomes viscid ; it soon 

 thickens to the consistency of jelly, and if the bottle be 

 now inverted, none of the blood will escape. An examina- 

 tion made at the end of several hours shows that the jelly- 

 like mass is gradually shrinking in size iintil finally it comes 

 to occupy about half the capacity of the bottle. This dark 

 red mass is called the blood clot, which retains the shape of 

 the bottle, and by its form and color reminds one of a jar 

 of currant jelly. 



Blood Serum. If the blood is not disturbed for several 

 days, a transparent, straw-colored liquid will be seen sur- 

 rounding the clot and filling the other part of the space in 

 the bottle. To this liquid is given the name blood serum. 

 Blood, then, when taken from the body becomes separated into 

 two nearly equal portions, the jellylike clot and the liquid serum, 

 and to this process of separation is given the name co-ag-u-la'tion 

 or blood clotting. 



Cause of Coagulation. When- one examines with a com- 

 pound microscope a drop of fresh beef -blood, red and white 

 corpuscles similar to those described in human blood (see 

 p. 25), are seen floating in the liquid plasma. In a short 

 time, however, little threadlike fibers make their appearance 

 in the serum, and extend in all directions across the drop. 

 These soon shorten. In this process the red and white cor- 

 puscles are gathered together as though caught in the threads 

 of a net. The liquid serum, meanwhile, is squeezed out of 

 the mass. This same process takes place in the bottle of 

 beef-blood. The fibers at first extend from one side of the 

 glass to the other, thus forming a more or less solid mass, 

 which holds the blood in the bottle, even when it is inverted. 

 The fibers soon shorten and lose their hold upon the glass, 

 and by their shrinkage the cylindrical clot is formed. 

 . Blood Fibrin. We have seen that coagulation is caused 

 by the fine threads that appear spontaneously as soon as 

 blood is shed. Still clearer proof that this is the fact is 

 furnished by the following experiment. Get the butcher at 



