CIRCULATION OF THE BLOOD. 155 



heart cannot, with sufficient ease, propel blood into the already too full or 

 too tense arteries. 



The action of the depressor nerve illustrates the effect of afferent im- 

 pulses in causing an inhibition of the vaso-motor centre as regards its 

 action upon certain arteries. There exist other nerves, however, the 

 stimulation of the central end of which causes a reverse action of the 

 centre, or, in other words, increases its tonic influence, and by causing 



FIG. 139. Tracing showing the effect on blood pressure of stimulating the central end of the De- 

 pressor nerve in the rabbit. To be read from right to left. T, indicates the rate at which the re- 

 cording-surface was traveling, the intervals correspond to seconds; C, the moment of entrance of 

 current; O, 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 nerves; the pulse oscilla- 

 tions are very small. (M. Foster.) 



considerable constriction of certain arterioles, either locally or generally, 

 increases the blood-pressure. Moreover, the effect of stimulating an 

 afferent nerve may be to dilate or constrict the arteries either generally 

 or in the part supplied by the afferent nerve; and it is said that stimula- 

 tion of an afferent nerve may produce a kind of paradoxical effect, causing 

 general vascular constriction and so general increase of blood-pressure but 

 at the same time local dilatation. This must evidently have an immense 

 influence in increasing the flow of blood through a 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 point 

 out that this is not the only way in w r hich it may be affected. 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 

 entirely by the vessels regaining their ordinary calibre; and afterward 



