262 



PHYSIOLOGY 



CHAP. 



division of the nerves to the limb (paralytic vaso-dilatation) the 

 diminution of resistance due to relaxation of vascular tone may 

 cause current velocity to rise to more than double the normal. 



Tetanic excitation of the nerve, on the contrary, increases 

 resistance by increase of vascular tone (vaso-constriction) and com- 

 pression of the vessels by muscular contraction, with consequent 

 diminution of velocity, as shown in the following table : 



The results of experiments to determine the effect of temporary 

 anaemia of different vascular regions due to compression of the 

 arteries are also interesting. 



The following are examples : 



From this table we see (a) the marked compensatory rise of 

 velocity on constriction of an adjacent area ; (V) the marked rise of 

 velocity after temporary anaemia of the vascular area on the same 

 side, owing to the diminution of peripheral resistance due to the 

 resulting vaso-dilatation. 



As regards the mean velocity of the blood-flow in the veins, we 

 have already stated that it must be less than that in the corre- 

 sponding arterial region, in proportion as the total area of the 

 venous system is greater. 



The velocity- of blood-flow in the axes of the capillaries can be 

 determined without difficulty under the microscope by measuring 

 the time taken by a red corpuscle to traverse a certain distance, 



