CHAP, iv.] THE VASCULAR MECHANISM. 325 



blood-pressure &c., the nerve is known by the name of the de- 

 pressor nerve. As we shall point out later on, by means of 

 this afferent nerve from the heart the peripheral resistance is, 

 in the living body, lowered to suit the weakened powers of a 

 labouring heart. 



This gradual lowering of blood-pressure by diminution of 

 peripheral resistance affords a marked contrast to the sudden 

 lowering of blood-pressure by cardiac inhibition ; compare Fig. 60 

 with Fig. 56. 



175. But the general blood-pressure may be modified by 

 afferent impulses passing along other nerves than the depressor, 

 the modification taking on, according to circumstances, the form 

 either of decrease or of increase. 



Thus, if in an animal placed under the influence of urari 

 (some anesthetic other than chloral &c. being used) the central 

 stump of the divided sciatic nerve be stimulated, an increase 

 of blood-pressure (Fig. 61) almost exactly the reverse of the 



FIG. 61. EFFECT ON BLOOD-PRESSURE 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. 



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 without any corresponding 

 change in the heart's beat, reaches a maximum and after a while 

 slowly falls again, the fall sometimes beginning to appear before 

 the stimulus has been removed. There can be no doubt that the 

 rise of pressure is due to the constriction of certain arteries ; the 

 arteries in question being those of the abdominal splanchnic area 

 certainly, and possibly those of other vascular areas as well. The 

 effect is not confined to the sciatic ; stimulation of any nerve con- 

 taining 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, quite similar to that caused by stimulating 

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



