154 APPLIED PHYSIOLOGY 



mental eye of anyone who has ever looked at the web of 

 a frog's foot through a microscope that it calls for no 

 description, but some of the conditions on which the 

 character of the flow depends must now be considered. 



The velocity of the capillary flow varies greatly 

 0*5 to 25 millimetres per second being given as the 

 extremes by some observers and is constantly changing 

 from time to time. It depends upon several factors, 

 amongst which the energy of contraction of the heart 

 and the pressure in the veins are of great importance. 

 If the heart be contracting feebly, or if the back pressure 

 in the veins be increased (and in most cases of circula- 

 tory failure these two factors co-operate), the blood flows 

 so slowly through the capillaries that it becomes sur- 

 charged with carbonic acid, and the patient is cyanosed. 

 Dilatation of the arterioles, which supply any part of 

 the body, quickens the flow through the corresponding 

 capillary area. This happens at the outset of inflamma- 

 tion. On the other hand, there is no constant relation 

 between general blood-pressure and the rate of the flow. 

 The viscosity of the blood is another factor of great 

 importance, but of which little is yet known. In the 

 tarry blood of cholera the viscosity is often so great that 

 it will not flow through the capillaries at all- In poly- 

 cythaemia, again, which raises the viscosity considerably, 

 the capillary flow is so much impeded that the ex- 

 tremities of the patients are usually cold or dusky. 

 Whether changes in the capillary wall may retard the 

 flow is not certainly known, but retardation from such a 

 cause has been postulated to explain the high blood- 

 pressures sometimes met with in pathology. 



