CIECULATION OF BLOOD 39 



tinguished by the fact that when they divide, 

 the direction of flow of the blood is from the 

 larger trunk to its branches. 



■ ii. The capillaries form a close network of very small 

 and thin-walled vessels, along which the blood 

 flows from the arteries to the veins. 



iii. The veins, carrying the blood away from the web 

 back towards the heart, are distinguished from 

 the arteries by the fact that the blood in them 

 flows from smaller to larger vessels. 



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



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



than those of the capUlaries, 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 



capiQaries : 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. 



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

 they are turning the corners of the capillary net- 

 work. 



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, such as a drop of weak acid, to the 

 web. 



