26 EXPERIMENTS ON THE CIRCULATION 



After the experiments on the kidneys were per- 

 formed, it occurred to me, that the observation of 

 the effects of venous obstruction in the vessels of a 

 frog's foot might serve to illustrate the action of the 

 same cause in the former cases. 



Accordingly, the hinder extremity of a frog wad 

 so included within two ligatures that the artery and 

 bone were the only parts left free. There was some 

 slight oozing of serous fluid from the wound in the 

 ham, and the limb below the ligature speedily 

 became red and swollen. On looking at the vessels 

 in the web with a moderate magnifying power, the 

 veins were seen to be much enlarged, and, as it 

 were, prolonged into the capillaries in consequence 

 of the accumulation of blood in, and distension of, 

 the latter. 



At first sight, the blood appeared to have lost all 

 motion ; but on examining more attentively, some 

 small arteries were observed in the act of pouring 

 in fresh blood, the impulse of which propelled on- 

 wards the column in the veins at each contraction of 

 the heart. During the cessation of that action a 

 retrograde movement occurred, the two alternate 

 actions causing an oscillation of the contained blood. 



At the end of half an hour the globules had begun 

 to cohere, and formed by their union irregular 

 masses and cylindrical columns, which moved to 

 and fro synchronously with the contraction and 

 relaxation of the ventricle ; the former motion being 

 evidently produced by the momentum of the arterial 

 blood, the latter resulting from the resiliency of the 

 elastic tissues. 



