THE BLOOD 219 



for which it is reasonable to assume agents. It is the 

 same with the substances supposed to confer immunity 

 against this and that disease; we do not recognize them 

 directly, but feel that they must be there to account for 

 observed facts. 



Red Corpuscles. The great majority of the formed 

 elements in the blood are red corpuscles. Sometimes 

 they are called erythrocytes. The deep red color and the 

 opacity of blood depend on their presence, but these are 

 not evident in the single corpuscle; 

 they result from the superposition 

 of many layers. These corpuscles 

 are individually minute discs with 

 slightly hollowed surfaces. When 

 they are driven on by currents, as FIG. 45. Red blood- 

 in the blood-vessels, they are apt corpuscles. Several are 



to be cup-shaped, the bulging of the * '^S^ 

 centers showing very clearly the elas- ters are evident. In one 



tic nature of the body. case two corpuscles are 



mi Ji i i overlapped to snow that 



The size of the red corpuscles in they are transparent. 

 any given animal species is remark- They tend to run into 

 ably constant. Most cells vary p . n f as A shown ** th ^ 



. , , , , . right. A saucer-shaped 



considerably but these are as uni- f orm i s represented, 

 form as coins stamped by the same 



die. The red corpuscle of man measures Jli200 i ncn 

 across its face. Its thickness is about one-fifth as 

 great. When blood is viewed under the microscope 

 the enormous number of the corpuscles is at once sug- 

 gested. It can be determined quite accurately by 

 careful dilution and counting the elements in a small, 

 measured volume. The conclusion reached is that in the 

 original blood there are about 5,000,000 corpuscles in a 

 cubic millimeter (the space occupied by a coarse grain of 

 sugar). The whole number in the body must be some- 

 thing like 4,000,000 times this large number (20 trillion). 

 When we compare the microscopic appearance of blood 

 from different animals we find that the size of the cor- 

 puscles does not correspond at all with the size of the 



