108 CIRCULATION 



general blood pressure. If, now, the depressor is stimulated 

 little effect is produced, because the blood pressure is already 

 so low that little more fall can be Brought about. If, however, 

 the general pressure is raised by stimulation of the splanchnic 

 or by the injection of saline solution, then stimulation of the 

 depressor produces a typical fall. This nerve is normally made 

 active by stimuli arising from the endocardium of the heart 

 when that organ is overcharged with blood. The impulses 

 are conveyed to the vasomotor centre, which causes a dilata- 

 tion of the arterioles all over the body. Unlike the vagi, the 

 depressor is active only at times. 



In general it may be said that weak stimulation of any 

 sensory nerve like the sciatic produces augmentor effects, while 

 a strong stimulation produces inhibitory effects. Stimulation 

 of the central end of the abdominal sympathetic produces 

 through the vagi a reflex inhibition of the heart. Dilatation of 

 the stomach has experimentally been shown to inhibit the heart. 



The cardio-inhibitory centre is situated in the bulb at the 

 level of a mass of cells known as the nucleus of origin of the 

 tenth nerve. The centre is probably always in action, since 

 section of the vagi, which removes the influence of the centre, 

 is followed by an increase in the rate of the heart beat. The 

 continuous activity of the centre is due to a stream of impulses 

 that come from all portions of the body. After cutting off 

 most of these by dividing the spinal cord near the bulb, section 

 of the vagi no longer increases the heart rate. The augmentor 

 centre is situated somewhere in the bulb and is also continuously 

 active. This is shown by sectioning the vagi and then extirpat- 

 ing the inferior cervical and first thoracic ganglia on both sides, 

 which causes a slowing of the heart. Dividing the cord in the 

 cervical region after the vagi have been cut has the same effect. 

 Inhibition of the heart through the vagi is more easily obtained 

 when the augmentor fibers have been severed. Whenever 

 sensory nerves are stimulated, producing an accelerated heart 

 beat, it is probable that both the augmentors and the cardio- 

 inhibitory centres are stimulated, but the first more strongly, 

 so that its effects prevail. There are a few cases on record 

 where the heart centres in the medulla were apparently influ- 

 enced by voluntary impulses from the cerebral cortex, but these 

 are extremely unusual. 



