CIRCULATION OF BLOOD. 37 



2. With a high power : note the following points: 



a. The walls of the arteries and veins are much thicker than 



those of the capillaries, which latter are often difficult 

 to see. 



b. The white corpuscles have a marked tendency to creep 



along the sides of the vessels, while the red corpuscles 

 rush far more rapidly along the middle of the stream : 

 this is seen best in the small arteries. 



C. The variations in calibre of the small arteries and 

 capillaries : whilst under observation an artery or 

 capillary may be seen to change its size to a consider- 

 able extent, 



d. The indefinite character of the capillary circulation. 



Owing to changes of size in adjacent vessels, the 

 direction of flow of the blood in a given capillary may 

 become reversed. 



e. The elasticity of the red corpuscles : seen best when 



they are turning the corners of the capillary network. 



f. The tendency of the white corpuscles to migrate through 



the walls of the capillaries into the tissues outside. 

 This is much increased by the application of some 

 irritant substance, as a drop of weak acid, to the web. 



