324 DEPRESSOR NERVE. [BOOK i. 



heart, is stimulated no marked results follow. The beginnings of 

 the nerve in the heart are therefore quite different from the 

 endings of the inhibitory fibres of the vagus, or of the augmentor 

 fibres of the splanchnic (sympathetic) system ; the nerve has 

 nothing to do with the nervous regulation of the heart treated 

 of in Sec. 5. If now while the pressure in an artery such as 

 the carotid is being registered, the central end of the nerve 

 (i.e. the one connected with the brain) be stimulated with the 

 interrupted current, a gradual but marked fall of pressure (Fig. 60) 



FIG. 60. TRACING, SHEWING THE EFFECT ON BLOOD-PRESSURE OF STIMULATING THE 



CENTRAL END OF THE DEPRESSOR NERVE IN THE BABBIT. 



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

 current was thrown into the nerve. 



in the carotid is observed, lasting, when the period of stimula- 

 tion is short, some time after the removal of the stimulus. Since 

 the beat of the heart is not markedly changed, the fall of pressure 

 must be due to the diminution of peripheral resistance occasioned by 

 the dilation of some arteries. And it is probable that the arteries 

 thus dilated are chiefly if not exclusively those arteries of the 

 abdominal viscera which are governed by the abdominal splanchnic 

 nerves ; for if these nerves are divided on both sides previous to 

 the experiment, 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 in questio'n have so affected some part of the central nervous 

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

 passing along the abdominal 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 abdominal 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-pres- 

 sure, the amount of fall of course being dependent on circum- 

 stances, such as the condition of the nervous system, state of 



