THE CIRCULATION 273 



Special Characters of Blood Flow. 



(a) Arteries. The flow of blood in an artery is rhythmi- 

 cally accelerated with each ventricular systole. This is due 

 to the pulse wave. As the wave of high pressure passes 

 along the vessels, the blood tends to flow forwards and 

 backwards from it so that in front of the wave there is 

 an acceleration of the stream and behind it a retardation. 

 In a wave at sea the same thing happens, and a cork 

 floating on the surface is moved forward in front of the 

 wave and again backwards after the wave has passed. 



(b) Capillaries. In the capillaries the flow is uniform. 



(c) Veins. In most veins, too, it is uniform, but in the 

 great veins near the heart it undergoes periodic accelera- 

 tions 



1st. With each diastole of auricle and ventricle. 

 2nd. With each inspiration. 



3rd. By muscular action squeezing the blood out of the 

 small veins. 



In all vessels the blood in the centre of the stream 

 moves more rapidly than that at the periphery on account 

 of the friction between the blood and the vessels. An 

 "axial" rapid and "peripheral" slow stream are, there- 

 fore, described. This is well seen in any small vessel placed 

 under the microscope, and in such situations it will be found 

 that, while the erythrocytes are chiefly carried in the axial 

 stream, the leucocytes are more confined to the peripheral 

 stream, where they may be observed to roll along the vessel 

 wall with a tendency to adhere to it. 



When from any cause the flow through the capillaries is 

 brought to a standstill, these leucocytes creep out through 

 the vessel wall and invade the tissue spaces. This is the 

 process of diapedesis, which plays an important part in 

 inflammation. 



In at least two situations the circulation presents special 

 characters. 



1. Circulation Inside the Cranium. Here the blood circu- 

 lates in a closed cavity with rigid walls, and therefore its 



18 



