THE HEART 57 



medulla, but the probability of its existence is considerable. On 

 the other hand, it is not a physiological necessity ; acceleration 

 might be brought about by inhibition of the cardio-inhibitory 

 centre, and experiment has shown the possibility of the effect 

 being produced through this channel. 



The Nature of Inhibition. — The discovery of the inhibitory 

 action of the vagus was a great addition to physiological know- 

 ledge ; it settled the important point that such an effect could 

 be produced through the nervous system, and has been the 

 means of adding considerably to the physiological knowledge 

 of other organs besides the heart. The actual means by which 

 inhibition in the heart is brought about is still a matter of 

 speculation ; the view most generally accepted is that pro- 

 pounded by Gaskell. He regards the vagus as the protecting 

 nerve of the heart, and reasoning from the observed fact that 

 after its stimulation and consequent inhibition there is on recovery 

 an improvement in the rate or force of the heart-beats, he con- 

 cludes that during inhibition there is a building up — anabolism — 

 of the muscular tissue which results in the improved condition 

 of the heart. Such changes are of an opposite character to 

 those occurring during contraction, which are of a katabolic or 

 tissue-destroying nature, by which complex substances are 

 converted into simpler ones, with the production of heat and 

 energy. The latter changes are regarded as brought about by 

 the sympathetic system. During inhibition the heart is being 

 repaired, during contraction its substance is being used up, and 

 Gaskell believes that all muscular tissue is similarly provided 

 with anabolic and katabolic nerves. 



The Depressor Nerve. — The nervous mechanisms considered up 

 to this point are concerned in bringing about some modified 

 action of the heart, under the guiding influence of a nerve centre 

 in the medulla. We have now to consider the case where a 

 nerve running from the heart to the medulla is engaged in a 

 regulative action which, unlike that of the vagus or sympathetic, 

 is not a direct action on the heart itself, but is brought to bear 

 indirectly on the heart through the instrumentality of the 

 vascular (arterial) system. This nerve is the depressor. It is a 

 branch of the vagus distributed to the heart — some say to the 

 walls of the aorta — and from there runs up the neck as a separate 

 branch in the horse, cat, and rabbit, but in other animals is 

 contained in the trunk of the vagus. It joins the superior 

 laryngeal nerve, and finally reaches a centre in the medulla 

 which regulates the movements of the bloodvessels of the body, 

 known as the vasomotor centre. The impulses which pass along 

 it are afferent — viz., they pass to the central nervous system 

 and not out of it. The heart in this way is placed directly in 



