AFTER- A CTION OF THE NER VES. 2 1 9 



the difference of effect is in the same direction as in the case of the 

 inhibitory fibres. Thus we see in all animals the rhythmical fibres of 

 the large veins and sinus are stirred up to increased rate of beat, and the 

 fibres of the auricles are made to contract more vigorously, while in the 

 amphibians the ventricular muscle is also most markedly influenced by 

 the augmentor fibres which supply it. On the other hand, I am not at all 

 clear whether any augmentor effect can be proved on the ventricle of the 

 crocodile or tortoise ; certainly, as far as my curves show, the effect is 

 insignificant in comparison with the increase in the force of the auricular 

 contractions, and I have grave doubts whether there is any true effect at 

 all produced, whether in fact the ventricle of Chelonia and Reptilia is not as 

 indifferent to stimulation of augmentor fibres as to stimulation of inhibitory 

 fibres. In the case of these nerves, too, the mammalian heart behaves like 

 the amphibian, according to the observations of Roy and Adami, 1 and of 

 Bayliss and Starling. 2 The comparative physiology of the vertebrate heart 

 requires further investigation, and would, I believe, yield results of very 

 great value to the morphologist as well as to the physiologist. 



3. The after-action of the inhibitory and augmentor nerves. — 

 From what has been said, we may conclude that the muscular tissue of 

 the heart is supplied by two nerves of opposite action, the one of 

 which, speaking broadly, causes a process of relaxation, or an 

 atonic condition, and the other a tonic condition in the tissue. In 

 order to try to understand the nature of these processes, we must in- 

 vestigate, not only the primary effects of stimulation of the nerves, but 

 also their after-effects. Thus we consider that the action of an ordinary 

 motor nerve upon muscle can be classed as katabolic, i.e. the meta- 

 bolism caused is destructive in its character, not only because of the 

 evidence given during the contraction, but also because of the sub- 

 sequent exhaustion which follows a series of contractions. 



What, then, are the after-effects of stimulation of the inhibitory 

 and augmentor nerves respectively, and how far do they throw light on 

 the meaning of inhibition } . 



o 



In endeavouring to formulate any view of cardiac inhibition, there are, it 

 seems to me, three possible methods of surveying the problem. 



The first is the view which has been and is still so largely held, 

 namely, that the inhibitory process takes place outside the muscular tissue, and 

 can be explained either by Rosenthal's resistance theory, or some theory of 

 interference of vibrations, similar to what is well known in the case of light. 

 This theory is absolutely untenable, in view of the fact that the nerve 

 acts directly on the muscle and diminishes its contractions. Of the other two 

 possibilities, we see that such substances as lactic acid and carbonic acid cause 

 a weakening of the contractions, and final stoppage in diastole, so that it is not 

 impossible to imagine that changes detrimental in character to the activity 

 of the tissue might produce the symptoms of inhibition. On the other hand, 

 it is conceivable that a sudden increase in the anabolic process would 

 manifest itself for the time being as a diminution of the katabolic process, and 

 so bring about the diminished contraction and other inhibitory effects, 

 although at the same time the net result would be an increase in that 

 process. 



According to the first view, inhibition, as far as the muscle is concerned, 

 means cessation of external stimuli. According to the second, it means a 

 process of exhaustion ; according to the third, a beneficial process, the after- 

 effects of which must be the opposite of exhaustion. 



1 <>p. cit., p. 244. - Op. cit., p. 413. 



