EXCITATION AND INHIBITION 



405 



one require a stronger current from the induction coil than the other set do, 

 possibly due to their excitable substances having a different optimal rate of stimu- 

 lation. It is a very suggestive fact that, as was shown by Biedermann at a later 

 date (1887, 2), the fine branching nerve fibres in the muscles are always double, a 

 slender fibre accompanying a thicker one, even to their endings in the muscle 



B 



FIG. 113. ACTION OF THE VAGUS NERVE ON THE BEATS OF THE A'ENTRICLE OF THE TORTOISE. 



Upper figure On stimulation, marked by the signal, the beats cease, after a latent period, with a decrease of 

 diastolic tone (the tracing lever pulls downwards). The effect lasts considerably longer than the stimulation, 

 and is followed by an increase in the magnitude of the beats. The latent period is better shown in the lower 

 figure, where the times of stimulation are marked by white rectangles on the heart tracing itself. 



fibres. The same muscle fibre is supplied by a fine and a coarse fibre, and it is 

 difficult to avoid the conclusion that one of these is excitatory, the other inhibitory, 

 for the muscle cell. Fig. 112 is a diagram of the facts as made out by 

 Biedermann and by Mangold (1905), who also investigated the question and found 

 this plan of innervation to be a very general one in the Arthropods. It will be 

 seen presently that the fact has an important bearing on the theory of inhibition. 



The arrangement by which the bivalve molluscs open and close their shells was 

 investigated by Pavlov (1885), who found that the powerful adductor muscle is 



