VASOMOTOR NERVES 67 



traction of the muscular coats of the arteries is brought 

 about by the increased action of the nervous system. '■ 



That this is the real state of the case may be proved 

 experimentally upon rabbits. These animals may be 

 made to blush artificially. If, in a rabbit, the sym- 

 pathetic nerve (Fig. 22, C. Sy.), which sends branches 

 to the vessels of the head is cut, the ear of the rabbit, 

 which is covered by so delicate an integument that the 

 changes in its vessels can be readily perceived, at once 

 blushes. That is to say, the vessels dilate, fill with blood, 

 and the ear becomes red and hot. The i-eason of this is, 

 that when the sympathetic is cut, the nervous impulse 

 which is ordinarily sent along its branches is interrupted, 

 and the muscles of the small vessels, which were pre- 

 viously slightly contracted, become altogether relaxed. 



And now it is quite possible to produce pallor and cold 

 in the rabbit's ear. To do this it is only necessary to 

 irritate the cut end of the sympathetic which remains 

 connected with the vessels. The nerve then becomes 

 excited, so that the muscular fibres of the vessels are 

 thrown into a violent state of contraction, whicli di- 

 minishes their calibre so much that the blood can haidly 

 make its way thiough them. Consequently, the ear 

 becomes pale and cold. 



This experiment on the blood-vessels of the rabbit's ear 

 is of fundamental importance as proof of the existence of 

 nerves which control locally the muscular elements of the 

 walls of the smaller ai-teries ; and inasmuch as this con- 

 trol consists in causing movements of the walls of the 

 vessels, by means of which their calibre is regulated, the 

 nerves which exert the control receive the general name 

 of vaso-motor nerves. But from the fact that when 

 the cut end of the sympathetic nerve is irritated, or, as 

 the physiologist says, is " stimulated," the muscular walls 



1 Sudden paleness is perhaps most frequently due to a failure or 

 stoppage of the heart's beat, as in fainting. But it may also be observed 

 when there is no change in the beat of the heart. 



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