138 



Life and Health 



At first the clot includes the whole mass of blood and 

 is of a uniform color. But in a short time a pale yellowish 



fluid begins to ooze 

 out and to collect 

 on the surface. 

 The clot gradually 

 shrinks, until at the 

 end of a few hours it 

 is much firmer and 

 floats in the yellowish 

 fluid. 



This remarkable 

 process is known as 

 coagulation, or the clot- 

 ting of blood ; and the 

 liquid which separates 



FIG. 67. Human Red and White Blood Cor- from the clot is called 

 puscles, as seen under the Microscope. serum. 



(Magnified 1000 diameters. Two white corpuscles The prOCCSS of the 

 are seen, with their nuclei stained to show black.) clotting of blood is not 



yet fully understood, 



but is claimed "to be due in part to the action of a fer- 

 ment in the white corpuscles which produces a substance 

 called fibrin. 



Experiment 59. To show the blood corpuscles. A moderately 

 powerful microscope is necessary to examine blood corpuscles. Let 

 a small drop of blood (easily obtained by pricking the finger with a 

 needle) be placed upon a clean slip of glass and covered with thin 

 glass such as is ordinarily used for microscopic purposes. 



The blood is thus spread out into a film and may be readily 

 examined. At first the red corpuscles will be seen as pale, disk-like 

 bodies floating in the clear fluid. Soon they will be observed to stick 

 to each other by their flattened faces, so as to form rows. The white 

 corpuscles are to be seen, but are much less numerous. 



