CIRCULATION OF BLOOD 39 



b. The fine mesliwork of bloodvessels along which the 

 blood can be seen flowing. Those bloodvessels are of 

 three kinds. 



i. The arteries, carrying blood to the web, are dis- 

 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 capillaries, which latter are often 

 difficult to see. 



b. TJie 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 variation in calibre of the small arteries and 

 capillaries : whilst under observation an artery or 

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

 siderable 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. 



t 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. 



