CHAP, iv.] 



THE VASCULAR MECHANISM. 



205 



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 splanchnic area certainly, 

 and possibly of other vascular areas as well. The effect is not 

 confined to the sciatic; stimulation of any nerve containing af- 

 ferent 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 not urari but 

 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. The condition of the central nervous 

 system seems to determine whether the reflex effect on the vaso- 

 motor fibres is in the direction of constriction leading to a rise, or 

 of dilation leading to a fall of blood-pressure. 



FIG. 47. KISE OF BLOOD-PRESSURE FROM STIMULATION OF NOSTPIL WITH SMOKE. 



The respiration and cardiac rhythm are at the same time rendered more slow. 

 The mark x indicates the time of stimulation. 



Stimulation of a sentient surface in many cases causes a similar 

 rise in blood-pressure as shewn in Fig, 47, where a rise of blood- 

 pressure follows irritation of the nostrils. In this case however the 

 rise in blood-pressure is accompanied by changes in respiration and 

 in the cardiac rhythm. 



