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 horns 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- 
