204 



VASO-MOTOR NERVES. 



[BOOK i. 



must be due to the diminution of peripheral resistance occasioned 

 by the dilation of some arteries. And there is evidence that the 



./UUUUL/UUULAJl^^ 



FIG. 46. 



TBACINO, SHEWING THE EFFECT ON BLOOD-PRESSURE OF STIMULATING THE 

 CENTRAL END OF THE DEPRESSOR NERVE IN THE KABBIT. 



(To be read from right to left. ) 



T indicates the rate at which the recording surface was travelling ; the intervals 

 marked corresponds to seconds. G the moment at which the current was thrown 

 into the nerve ; the moment at which it was shut off. The effect is some 

 time in developing and lasts after the current has been taken off. The larger 

 undulations are the respiratory curves ; the pulse- oscillations are very small. 



arteries thus dilated are chiefly if not exclusively those arteries of 

 the abdominal viscera which are governed by the splanchnic 

 nerve. For if both the splanchnic nerves are divided previous to 

 the experiment, the fall of pressure when the depressor is stimulated 

 is very small, in fact almost insignificant. The inference from this 

 is clear ; the afferent impulses passing along the depressor 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 smaller vascular areas. 



The condition of the splanchnic or other vascular areas may 

 moreover be changed, and thus the general blood-pressure modi- 

 fied, 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 (dog) placed under the influence of urari 

 the central stump of the divided sciatic nerve be stimulated, an 

 increase of blood-pressure, almost exactly the reverse of the de- 

 crease brought about by stimulating the depressor, is observed. 



