112 PRINCIPLES OF PHYSIOLOGY 



nated by various excreting organs. Thus 

 the blood does not vary much either in quantity 

 or quality, a physiological condition of great 

 importance. 



57. As already mentioned, the blood is a 

 highly complex fluid. It contains three 

 kinds of corpuscles, (1) red corpuscles, or 

 erythrocytes, (2) white or colourless corpuscles, 

 (leucocytes), of which there are several varie- 

 ties ; and (3) minute particles called 

 blood plates. The red corpuscles, chiefly 

 concerned in respiratory exchanges, exist 

 in enormous numbers, in human blood 

 amounting to as much as five million 

 in a drop of blood about one-twenty- 

 fifth of an inch in diameter. The white 

 cells probably perform several functions, 

 They may imbibe certain matters from 

 the fluid of the blood and elaborate these 

 into other substances. By their power of 

 spontaneous amoeboid movement they may 

 seize upon and digest worn out effete red cells, 

 or micro-organisms that in many diseases find 

 their way into the blood. 



58. Recent observations on the blood, 



