64 CIRCULATORY SYSTEM. 



shape on being swept back into the stream. Capillaries at 

 times change their calibre and become too narrow for the 

 passage of the red cells. The leucocytes are swept along by 

 the general blood stream ; they tend, however, to attach 

 themselves to the wall of the vessel, and can be seen accumu- 

 lating upon the surface whenever the current slows sufficiently. 

 In the veins where the flow is less rapid than in the arteries 

 the leucocytes monopolise a peripheral space in which they may 

 be seen to be rolled along by the current as they cling to 

 the wall of the vessel. 



