34 
THE VASCULAR SYSTEM OF THE FROG. 
those of the capillaries, which latter are often difficult 
to see. 
b. The tendency of the white corpuscles to creep along the 
sides of the vessels, while the red corpuscles rush far 
more rapidly along the middle of the stream: 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. 
