466 



VETERINARY PHYSIOLOGY 



Friction has also a certain effect. A river runs much 

 faster in mid-stream than along the margins, because near 

 the banks the flow is delayed by friction, and, the more 

 broken up and subdivided is the channel, the greater is the 

 friction and the more is the stream slowed. 



In the capillary system considerable resistance is offered 

 by friction in the innumerable small channels, and this is 

 markedly influenced by the viscosity of the blood (p. 475). 



Measurement of Velocity. — The velocity of flow in the 

 arteries and veins may be measured by various methods, of 



Fig. 195. — Diagram of the Sectional Area of the Vascuhir System, upon 

 which the velocity of the flow depends. A.R., arteries; (7., 

 capillaries ; V. , veins. 



which one of the best is that by means of the stromuhr, an 

 instrument by which the volume of blood passing a given 

 point in an artery or vein in a given time may be deter- 

 mined {Practical Physiology). 



The velocity of the flow in the capillaries may be 

 measured in transparent structures by means of a microscope 

 with an eye-piece micrometer. 



The velocity of the blood varies greatly but is roughly as 

 follows : — 



Carotid of the dosf about . 

 Capillaries about 

 Veiti (jugular) about 



300 mm. per sec. 

 0'5 to 1 m. ,, 

 150 mm. 



