EFFECTS OF PRESSURE ON VEINS. 183 



being a hindrance by the closure of the valves and the 

 anastomoses of the veins. 



The effects of such muscular pressure are well shown by 

 the acceleration of the stream of blood when, in venesec- 

 tion, the muscles of the fore-arm are put in action, and by 

 the general acceleration of the circulation during active 

 exercise; and the numerous movements which are con- 

 tinually taking place in the body while awake, though 

 their single effects may be less striking, must be an im- 

 portant auxiliary to the venous circulation. Yet they 

 are not essential ; for the venous circulation continues 

 unimpaired in parts at rest, in paralysed limbs, and in 

 parts in which the veins are not subject to any muscular 

 pressure. 



Besides the assistance thus afforded by muscular pressure 

 to the movement of blood along veins possessed of valves, 

 it has been discovered by Mr. Wharton Jones that, in the 

 web of the bat's wing, the veins are furnished with valves, 

 and possess the remarkable property of rhythmical con- 

 traction and dilatation, whereby the current of blood 

 within them is distinctly accelerated. Mr. Jones found 

 that the contraction occurred, on an average, about ten 

 times in a minute ; the existence of valves preventing 

 regurgitation, the entire effect of the contractions was 

 auxiliary to the onward current of blood. It is reasonable 

 to infer that veins in other parts may, when furnished 

 with valves, possess a like power. 



Agents concerned in the Circulation of the Blood. 



The agents concerned in the circulation of the blood, 

 which have been now described, may be thus enume- 

 rated : 



1 . The action of the heart and of the arteries. 



2. The vital capillary force exercised in the capillaries. 



3. The possible slight action of the muscular coat of 



