CHAP, iv.] THE VASCULAR MECHANISM. 345 



decrease brought about by stimulating the depressor, is observed. 

 The curve of the blood pressure, after a latent period during which 

 no changes are visible, rises steadily, reaches a maximum and 



FIG. 76. EFFECT ON BLOOD PKESSUBE CURVE OF STIMULATING SCIATIC NERVE 

 UNDER URARI (Cat). 



x marks the moment in which the current was thrown into the nerve. Artificial 

 respiration was carried on, and the usual respiratory undulations are absent. 



soon slowly falls again, the fall sometimes beginning^ to appear 

 before the stimulus has been removed. This rise of pressure, 

 since it may be observed in the absence of any increase in the 

 heart beat, such at least as could give rise to it, must be due to 

 the constriction of certain arteries ; the arteries in question being 

 those of the splanchnic area certainly, and possibly those of other 

 vascular areas as well. The effect is not confined to the sciatic ; 

 stimulation of any nerve containing afferent fibres may produce 

 the same rise of pressure, and so constant is the result that the 

 experiment has been made use of as a method for determining the 

 existence of afferent fibres in any given nerve and even the paths 

 of centripetal impulses through the spinal cord. 



If, on the other hand, the animal be under the influence 

 not of urari but of a large dose of chloral, instead of a rise of 

 blood pressure a fall, very similar to that caused by stimulating 

 the depressor, is observed when an afferent nerve is stimulated. 

 The condition of the central nervous system seems to determine 

 whether the effect of afferent impulses on the central nervous 

 system is one leading to an augmentation of vaso-constrictor 

 impulses and so to a rise, or one leading to a diminution of vaso- 

 constrictor impulses and so to a fall of blood pressure. 



176. We have used the words ' central nervous system ' in 

 speaking of the above ; we have evidence however that the part 

 of the central nervous system acted on by the afferent impulses 

 is the vaso-motor centre in the spinal bulb, and that the effects in 

 the way of diminution (depressor) or of augmentation (pressor) are 

 the results of afferent impulses inhibiting or augmenting the tonic 

 activity of this centre or of a part of this centre especially 

 connected with the splanchnic nerves. The whole brain may be 

 removed right down to the bulb, and yet the effects of stimulation 

 in the direction either of diminution or of augmentation may still 

 be brought about. If the bulb be removed, these effects are no 



