CONTRACTION OF ARTERIES. 



149 



vessels, the more rapidly, on the contrary, do we see the 

 contraction succeeded by a dilatation, which, however, 

 is not in its turn followed by a contraction, as it would 

 have to be to constitute a pulsation, but persists for a 

 longer or shorter time. This dilatation is not of an 

 active, but of a passive nature, and results from the 

 pressure of the blood upon the wall of the vessel which 

 has become fatigued and opposes less resistance. 



If we now proceed to examine the phenomena which 



Fig. 40. 



Fig. 49. Irregular contraction of small vessels from the web of a frog's foot after 

 the application of stimuli. Copied from Wharton Jones. 



