CONTROL OF THE CIRCULATION. 73 



blood ; and, further, when the colored corpuscles of human 

 blood are seen flatwise they present a circular outline, while 

 the frog's colored corpuscles are elliptical. But with a 

 little study the two may be distinguished. As in the frog's 

 blood, the colorless corpuscles have ameboid movements, 

 though they are not very marked unless the blood be 

 warmed to about the temperature of the human body. 



Flexibility and Elasticity of the Corpuscles. It will 

 be well here to examine again the frog's web. (See p. 54.) 

 It will occasionally be seen that when one of the colored 

 corpuscles is pressed against an angle at the forking of 

 the blood stream, it is sometimes bent, and that as soon as 

 the pressure is discontinued the corpuscle springs back to 

 its former shape, showing that it is elastic. 



Frog's Blood. A drop of frog's blood, mounted as the human blood 

 was, will be helpful, as there is a very decided difference in the size and 

 shape of the colored and colorless corpuscles. Further, the colorless 

 corpuscles of the frog will show ameboid movements, i.e. slow changes 

 of form, if watched a while. 



The Plasma. The plasma consists chiefly of water, 

 having in solution various salts, including common salt; 

 it also contains the nourishing materials for the tissues. 

 These nourishing materials, obtained from the food by 

 digestion, consist chiefly of proteids, fats, and sugar. The 

 plasma also contains waste matters, from the working 

 tissues, on their way out of the body. How the food is pre- 

 pared for the building of tissue, and how the waste matter 

 is removed from the body, we shall study a little later. 



The Color of Blood. The difference in color of an in- 

 dividual corpuscle and the blood in the mass may be better 

 understood by comparing it with something that we see 

 more frequently. A tumbler of currant jelly has a rich, 



