VASOMOTOK ACTIONS. 225 



abdominal viscera in a state of moderate tonic constriction, fail altogether, 

 and those arteries in consequence dilate just as they do when the abdominal 



FIG. 79. 



Tracing showing 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. 



splanchnic nerves are divided, the effect being possibly increased by the 

 similar dilatation of other vascular areas. Since stimulation of the nerve 

 of which we are speaking always produces a fall, never a rise, of blood-pres- 

 sure the amount of fall, of course, being dependent on circumstances, such 

 as the condition of the nervous system, state of blood-pressure, etc. 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 resist- 

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

 heart. 



This gradual lowering of blood-pressure by diminution of peripheral 

 resistance affords a marked contrast to the sudden lowering of blood-pres- 

 sure by cardiac inhibition. (Compare Fig. 79 with Fig. 75.) 



161. But the general blood-pressure may be modified by afferent im- 

 pulses 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 anaes- 

 thetic other than chloral, etc., being used), the central stump of the divided 

 sciatic nerve be stimulated, an increase of blood-pressure (Fig. 80) almost 



FIG. 80. 



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 car- 

 ried on, and the usual respiratory undulations are absent. 



exactly the reverse of the decrease brought about by stimulating the de- 

 pressor, is observed. The curve of the blood-pressure, after a latent period 

 during which no changes are visible, rises, steadily without any corre- 

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



15 



