Chap, iv.] THE VASCULAR MECHANISM. 281 



ment, the fall of pressure when the nerve is stimulated is very- 

 small, in fact almost insignificant. The inference we draw is as 

 follows. The afferent impulses passing upwards along the nerve 



^ r ^r^^W rv ^ 



Fig. 74. Tracing, shewing the Effect on Blood Pressure of stimulating 

 the central end of the depressor nerve in the rabbit. 



On the time marker below the intervals correspond to seconds. At x an interrupted 



current was thrown into the nerve. 



in question have so affected some part of the central nervous 

 system that the influences which, in a normal condition of things, 

 passing along the splanchnic nerves keep the minute arteries of 

 the abdominal viscera in a state of moderate tonic constriction, 

 fail altogether, and those arteries in consequence dilate just as 

 they do when the splanchnic nerves are divided, the effect being 

 possibly increased by the similar dilation of other vascular areas. 

 Since stimulation of the nerve of which we are speaking always 

 produces a fall, never a rise of blood pressure, the amount of fall 

 of course being dependent on circumstances, such as the condition 

 of the nervous system, state of blood pressure and the like, the 

 nerve is known by the name of the depressor 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. 74 

 with Fig. 70. 



§ 153. 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. 75) almost exactly the reverse of the 



