VASO-MOTOR NERVES AND NERVE-CENTRES. 277 



tion of the cardio-inhibitory nerves. The latent period is 

 also noticeably long and the effect of the stimulus outlasts 

 considerably the time of its application. 



Blushing. The depressor nerves control a great part of the 

 vaso-constrictor centre (especially that portion of it connected 

 with the splanchnic nerves) and so can bring about dilatation 

 of a large number of arteries their influence is accordingly 

 called into play when general arterial pressure is to be lowered, 

 but is useless for controlling local blood-supply. This is 

 managed in part by other afferent nerves, each of which 

 inhibits a small part only of the vaso-constrictor centre, gov- 

 erning the arteries of a limited tract of the Body; the dilata- 



FIG. 103. Tracing of pressure from femoral artery of a rabbit showing the influ- 

 ence of stimulation of the central end of the depressor nerve; to he read from right 

 to left: a b c d e, tracing of arterial pressure, the small variations indicating heart- 

 beats; op, tracing of seconds pen; s, moment of commencement of stimulation; , 

 cessation of stimulation; xg, line of no pressure. 



tion of these increases the amount of blood flowing through the 

 particular region to which they are .distributed, but does not 

 affect the total resistance to the blood-flow sufficiently to 

 influence noticeably the general pressure in the arterial system. 

 In blushing, for example under the influence of an emotion, 

 that part of the vase-motor centre which supplies constrictor 

 nerves to the arteries of the skin of the neck and face, is 

 inhibited by nerve-fibres proceeding from the cerebrum to the 

 medulla oblongata, and the. face and neck consequently be- 

 come full of blood and flush up. Quite similar phenomena 

 occur under other conditions in many parts of the Body, 

 although when not visible on the surface we do not usually 

 call them blushes. The mucous membrane lining the empty 

 stomach is pallid and its arteries contracted, but as soon as 

 food enters the organ it becomes red and full of blood ; the 

 food stimulating afferent nerve-fibres there, which inhibit 



