150 HANDBOOK OF PHYSIOLOGY. 



The action of the depressor nerve illustrates a somewhat unusual 

 effect of afferent impulses, as it causes an inhibition of the vaso-motor 

 centre. As a rule, the stimulation of the central end of an afferent 

 nerve produces a reverse effect, or, in other words, increases the tonic 

 influence of the centre, and by causing considerable constriction of cer- 

 tain arterioles, either locally or generally, increases the blood-pressure. 

 Thus the effect of stimulating an afferent nerve may be either to dilate 

 or to constrict the arteries. Stimulation of an afferent nerve too may 

 produce a kind of paradoxical effect, causing general vascular constric- 

 tion and so general increase of blood-pressure, but at the same time local 



FIG. 138. Tracing showing the effect on blood -pressure of stimulating the central end of the 

 Depressor nerve hi the rabbit. To be read from right to left. T, indicates the rate at which the record- 

 ing-surface was travelling, the intervals correspond to seconds; c. the moment of entrance of cur- 

 rent; 0, moment at which it was shut off. The effect is some time in developing and lasts after 

 the current has been taken off. The larger undulations are the respiratory curves ; the pulse oscil- 

 lations are very small. (M. Foster.) 



dilatation which must evidently have an immense influence in increasing 

 the flow of blood through the part. 



Not only may the vaso-motor centre be reflexly affected, but it may 

 also be affected by impulses proceeding to it from the cerebrum, as in the 

 case of blushing from mind disturbance, or of pallor from sudden fear. 

 It will be shown, too, in the chapter on Eespiration that the circulation 

 of deoxygenated blood may directly stimulate the centre itself. 



Local Tonic Centres. Although the tone of the arteries is influ- 

 enced by the centres in the cerebro-spinal axis, certain experiments prove 

 that this is not the only way in which it may be influenced. Thus the 

 dilatation which occurs after section of the cervical sympathetic in the 

 first experiment cited above, only remains for a short time, and is soon 

 followed although a portion of the nerve may have been removed en- 

 tirely by the vessels regaining their ordinary calibre; and afterwards 

 local stimulation, e. g., the application of heat or cold, will cause dilata- 

 tion or constriction. From this it is probable that there exists a distinct 

 local mechanism for each vascular area, and that the influence exerted by 



