THE BLOOD. 



37 



would have to be measured separately, as we have already 

 seen, and then an average taken of them all, before even a 

 guarded opinion could be given as to the source of the blood. 

 Still other difficulties, however, are apt to beset the microscop- 

 ist. The blood is usually dried and in small quantity. The 

 disks are then shrunken. If we endeavor to restore them to 

 their original shape, as by soaking in blood-serum, we are 

 never sure of having accomplished the object, or that we have 

 not overdone it. This statement will be better understood by 

 experiments that will be detailed at another point in this 

 chapter. Where blood-corpuscles are elliptical, as in birds, 

 there is much less opportunity for error. 



Measurements of red Blood-corpuscles. 



The number of the red globules. It has commonly been held 

 that the blood of an adult man contains 5,000,000 red corpus- 

 cles in each cubic millimetre. In anaemic conditions this num- 

 ber may be reduced below 3,000,000, while in fair physical 



