THE CIRCULATION OF THE BLOOD AND LYMPH 145 



we know but little. It may be that the chemical changes 

 in the nerve-cells of the inhibitory centre lead of themselves 

 to the discharge of impulses along the inhibitory nerves. 

 But there is some evidence that, in the complete absence of 

 stimulation from without, the activity of the centre would 

 languish, and perhaps be ultimately extinguished. For 

 when the greater number of the afferent impulses have been 

 cut off from the medulla oblongata by a transverse section 

 carried through its lower border, division of the vagi pro- 

 duces little effect on the rate of the heart. Be this as it 

 may, we know that the activity of the inhibitory centre is 

 profoundly influenced and that both in the direction of an 

 increase and of a diminution by impulses that fall into it 

 through afferent nerves and by stimuli directly applied to 

 it. And we may assume that the same is true of the 

 augmentor centre. When, for instance, the central end of 

 one vagus is stimulated, the other being intact, the usual 

 result is a slowing or weakening of the heart, which, how- 

 ever, is generally less marked than when the stimulation is 

 applied to the peripheral end of the nerve. But sometimes 

 the heart is accelerated without any preceding inhibition. 



The depressor nerve, a branch of the vagus, which is 

 easily found in the rabbit as a slender nerve running close 

 to the sympathetic in the neck, and a little to its inner side, 

 falls into the same category with the vagus itself as regards 

 its reflex action on the heart, to which it bears an important 

 relation. Stimulation of its peripheral end has no effect, for 

 the cardiac fibres which it carries are afferent, not efferent. 

 But excitation of its central end causes a marked fall of 

 blood-pressure (p. 161), accompanied by, but not essentially 

 due to, a distinct slowing of the heart. If the animal is not 

 anaesthetized, there may be signs of pain, and for this reason 

 the depressor has sometimes been spoken of, somewhat 

 loosely, as the sensory nerve of the heart. Stimulation of 

 the cephalic end of the cervical sympathetic in the dog often 

 causes reflex slowing of the heart and general dilatation of 

 the vessels, although sometimes the opposite effects. The 

 abdominal sympathetic (of the frog) also contains afferent 

 fibres, through which reflex inhibition of the heart can be 



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