974 PHYSIOLOGY 



of the action of the vagus on the heart, much work has been expended with 

 a view to determining the intimate nature of the inhibitory process. In the 

 former neurogenic theory it was supposed that the vagus altered the activity, 

 perhaps by a process of ' interference,' of the ganglion- cells responsible for 

 the origination of the rhythm. Many facts, however, point to the in- 

 hibitory impulses as being continued to the heart-muscle itself. Thus 

 tetanisation of any portion of the frog's ventricle, especially if it be filled 

 with blood, causes an evident relaxation of the part between the electrodes. 

 Application of nicotine to the heart prevents stimulation of the trunk of the 

 vagus from having any influence on the heart, presumably from paralysis 

 of the cells of Remak's ganglion, which lie at the termination of the vagus 

 fibres, or of the synapses between the vagus fibres and the ganglion- cells. It 

 is still possible to inhibit the heart by direct stimulation either of the fibres 

 leaving this ganglion in the sino-auricular junction, or of the nerve-trunks 

 which run in the inter- auricular septum. We must conclude therefore that 

 the inhibition of the heart-muscle is peripheral and depends on the direct 

 action of the nerve fibres on the muscle-cells themselves. These nerve 

 fibres are paralysed by atropine, after administration of which no inhibitory 

 effects can be produced by stimulation of nerve or muscle or any part of the 

 heart. On the other hand, muscarine apparently stimulates the inhibitory 

 nerve- endings, and when applied to the isolated auricle or ventricle causes 

 weakening of the beat and finally complete inhibition, an effect which can 

 be removed by its antagonist atropine. 



Two views have been held as to the essential nature of the inhibitory 

 process. According to that put forward by Claude Bernard, the natural 

 tendency of any tissue during rest is towards anabolism. Activity in- 

 volves disintegration or breaking down of the living material, and this 

 disintegration must be succeeded by a process of building up or anabolism, 

 which restores the tissue to its previous functional condition. On this view 

 the state of inhibition would merely prolong the period of rest intervening 

 between two periods of activity, so allowing a greater time for restitution to 

 take place, with a corresponding improvement in the functional capacity of 

 the tissue. According to Hering and Gaskell a state of anabolism can be 

 induced in a tissue comparable to the state of sudden disintegration which 

 is associated with activity. Excitation of the vagus nerve does not merely 

 allow the normal process of building up, which goes on during rest, to take 

 place, but actually hastens this process, just as the excitation of a motor 

 nerve to a skeletal muscle induces an active breakdown of the contractile 

 tissue, or the excitation of the augmentor nerve to the heart induces an 

 increased rate of beat and therefore increased functional activity. 



If stimulation of an inhibitory nerve induces the opposite chemical 

 change to that occurring during activity, one would expect to find that, just 

 as an active part of a tissue is negative to an inactive part, so a part of the 

 tissue which is under the influence of an inhibitory stimulus should be 

 electro-positive to any part which is not being so stimulated. According to 

 Gaskell this condition is realised in the heart of the tortoise. The auricles 



