PHENOMENA. 5 



times associated with widening, sometimes with narrowing, of 

 the arteries. 



When the mesentery of the frog is spread out and examined 

 microscopically, the first change which is observed in the circu- 

 lation, as a result of the exposure to air, consists in dilatation 

 and increase in the lengtli — manifested by more or less contor- 

 tion — of the arteries. The dilatation begins immediately, and is 

 preceded by no contraction. It is, however, progressive ; the 

 diameter of the artery gradually increases for ten or twelve 

 hours, at the end of which period it is often twice as great as 

 it was before : having thus attained its maximum, its size re- 

 mains unaltered for many hours. This dilatation of the arteries 

 is followed by a similar change in the veins ; but inasmuch as 

 there is a considerable interval between the two events, a time 

 occurs at which the arteries, instead of being sensibly smaller than 

 the veins which correspond to them, far exceed them in diameter. 



Along with these changes, the rate of movement of the blood 

 is also altered. At the beginning of the process the circulation 

 is quicker than natural. Although the two changes go on at 

 the same time, the acceleration cannot be regarded as a result 

 of the increase of calibre ; for the inevitable consequence of 

 dilatation would be diminution, not increase, of the rate of 

 movement, supposing the activity of the heart and the resistance 

 opposed by the capillaries of distribution to be the same. The 

 absence of any relation between the two is still more clearly 

 shown by what is observed at a later period ; for whereas, 

 on the one hand, as has been already stated, the dilatation lasts 

 fur many hours, the acceleration is confined to the first stage of 

 the process. The rate of movement soon becomes normal, and 

 this is shortly followed by a change in the opposite direction ; 

 so that by the time the arteries are fully dilated, the circulation 

 is much slower than it was originally. 



Many experiments have been made with the view of account- 

 ing for the dilatation and contraction of the arteries and capillary 

 blood-vessels, and the most important results of these are — that 

 division of a vasa motor nerve produces congestion of all the 

 tissues to which it is distributed ; that excitation by the inter- 

 rupted electrical current, or by mechanical means, produces con- 

 striction of the minute arteries presided over by the irritated 

 nerve, and consequent anaemia ; that excitation of a sensory nerve 



