348 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



take ten million to cover a square inch; but on account 

 of their flatness it would take more than twelve thou- 

 sand to make a pile an inch high. Thus a cubic inch of 

 blood globules would contain one hundred and twenty 

 thousand million globules. Now it has been estimated 

 that the actual number in a cubic inch of blood is about 

 seventy thousand millions. Therefore considerably more 

 than half of the blood consists of these globules. 



Behavior of Blood. — If we watch fresh-drawn blood 

 with a microscope under a glass cover we observe that 

 the disks run together and pile on one another like a 



pile of coins thrown down 

 (Fig. 235). This is the re- 

 sult of coagulation. If a 

 little water on the finger is 

 touched to the side of the 

 cover, so as to be drawn 

 in and mingled with the 

 blood, then the globules 

 become round like mar- 

 bles, and roll around freely 

 in the current. They have 

 imbibed the water and be- 

 come swollen. If the fluid 

 in which they float should 

 dry away ever so little, 

 they shrivel and become 

 irregular in shape (Fig. 235 /-'). The same change takes 

 place if a strong solution of salt be added. The salt 

 draws water out of the globules and shrivels them. But 

 if a iveak solution of salt be added it prevents coagu- 

 lation, and at the same time does not alter the shape. 

 The disklike shape may be thus examined at leisure. 



Structure. — The red globules are not cells, but solid 

 masses, a little softer in the center. 



Fig. 235. — Blood globules : rr r, red 

 globules ; I /, white globules or 

 leucocytes. 



