MEMOIR OF BARON HALLER. 35 



skin, cellular membrane, fat, dura mater, &c. Some 

 other structures again appear to possess the property, 

 but only to a limited extent, such as the veins, 

 arteries, and other vessels. This point, though at 

 first sight apparently very simple, is not free from 

 difficulties. Haller remarks, that the principal 

 artery of the silk-worm performs the office of a 

 heart ; and that in many animals, after the heart is 

 removed, the motion of the fluids is continued, for a 

 time, apparently solely by the arteries. " Upon 

 examining," he says, " with the microscope, the 

 blood in a fish and a frog, after they were deprived 

 of their heart, it continued to move for some time 

 in the vessels ; and I have seen it pass up and down 

 the vessels of smaller fish, which had no motion 

 either in their heart or gills, and which did not 

 show the least sign of sensibility. But still this 

 does not quite prove the point." Haller could never 

 witness contraction in the aorta or other great vessels 

 of any of the larger animals ; in living frogs, too, he 

 had frequently irritated the arteries with a variety 

 of stimuli, and could never discover any contrac- 

 tion occasioned thereby: and concerning the cir- 

 culation of animals generally, he states, that upon 

 examination with the microscope, he could never 

 perceive any contraction in the blood-vessels. " I 

 have viewed for hours the circulation in fishes 

 and frogs, and during the whole time, the sides of 

 the vessel remained as quiescent as those of the 

 tubes with which I examined them. If the beat of 

 the artery had occasioned any motion in the neigh- 



