206 VASO-MOTOR NERVES. [BOOK i. 



In the instances just quoted, the effect of the stimulation of 

 the afferent nerve may be spoken of as a general one ; it is the 

 general blood -pressure which is diminished or increased ; though 

 in the case of the depressor at all events it is chiefly in the 

 splanchnic area that the constriction or dilation takes place. 



There are however some remarkable cases where a local effect 

 can be readily distinguished from the general effect, because the 

 two are in opposite directions. Thus if in a rabbit under urari, the 

 central stump of the auricularis magnus nerve or of the auricularis 

 posterior be stimulated, the rise of general pressure which is 

 caused by the stimulation of this as of any other afferent nerve, is 

 accompanied by a dilation of the artery of the ear. That is to say, 

 the afferent impulses passing along the auricular nerve while 

 affecting the central nervous system in an ordinary way, so as to 

 cause constriction of many of the arteries of the body (but chiefly 

 probably the splanchnic vessels), at the same time so affect some 

 particular part of the central nervous system, more especially 

 connected with the vaso-motor fibres governing the artery of the 

 ear, as to lead to the dilation of that vessel. 



So also in the same animal stimulation of branches of the 

 tibial nerve causes dilation of the saphena artery, together with 

 constriction of other arteries, as shewn by the concomitant rise of 

 pressure. And there are probably innumerable instances of the 

 same kind of action going on in the body during life, for it is 

 evident that the object of the local dilation, viz. the increased flow 

 of blood to the organ, must be assisted if a general constriction 

 is at the same time taking place in other regions. 



The general effect may not always be obvious, may perhaps be 

 absent, so that the local dilation or constriction, as the case may 

 be, is the only obvious result of the vaso-motor action. When the 

 ear of the rabbit is gently tickled, the effect that is seen is a 

 blushing of the ear, and though this may be in part due, as we 

 shall see, to the action of a local mechanism, the case we have just 

 cited shews that the central nervous system must be largely 

 engaged. When the right hand is dipped in cold water, the 

 temperature of the left hand falls, on account of a reflex con- 

 striction of the vessels of the skin of that hand caused by the 

 stimulus applied to the other. Many more instances might be 

 quoted, and we shall again and again come upon examples. The 

 numerous pathological phenomena classed under sympathetic 

 action, such as the affection of one eye by disease in the other, 

 are probably in part at least the results of reflex vaso-motor 

 action. 



We have said enough to shew that the calibre of the small 

 arteries, which by determining the peripheral resistance forms one 

 important factor regulating the flow of blood, is subject to in- 

 fluences proceeding from all parts of the body, and that these 

 influences reach the arteries in a reflex manner by means of the 



