104 ELECTRO-PHYSIOLOGY CHAP. 



with given intensity and ascending direction of the single 

 induction currents, a visible acceleration of cardiac activity, while 

 reversal of the current was sufficient to produce inhibition. This 

 is no doubt due to the fact that, as will be seen later, the 

 excitation in the two cases actually occurs at different points of 

 the nerve (in the first instance nearer the cross-section), where 

 the different degrees of excitability thus give rise to different 

 excitatory effects. 



As the vagus-nerve of the frog contains distinct accelerating 

 and inhibitory fibres, there are no less certainly differences of 

 excitability in the same. It may further be concluded that the 

 accelerating (augmentor) fibres are less excitable than the de- 

 pressors, but more resistant to all processes that threaten to 

 destroy the excitability of both kinds of fibres. This recalls the 

 analogous relations of excitability in the vaso-constrictor fibres of 

 a nerve-trunk that conducts both vaso-constrictor and vaso-dilator 

 impulses. 



At the same time it is not the functionally analogous fibres 

 that agree in regard to excitability and resistance, since the cardiac 

 inhibitory fibres agree with the vaso- constrictors, the cardiac 

 accelerators with the vaso -dilators respectively. This contrast 

 finds its most pronounced expression in the relations of excitability 

 between the motor and inhibitory fibres of the antagonist muscles 

 of the crayfish claw. From the above evidence we are forced 

 to conclude that each of these muscles is supplied by two 

 kinds of fibres (motor and inhibitory) which are functionally 

 distinct, and differ in regard to excitability, not merely quantitatively, 

 since the one effect invariably occurs before the other, in regular 

 proportion with the strength of excitation, but also qualitatively, 

 seeing that the inhibitory fibres of the one muscle correspond in 

 their excitatory conditions with the motor fibres of the antagonist 

 muscle. 



Similar differences of excitability to those described above for 

 centrifugal nerve-fibres seem to obtain in centripetal fibres also, 

 as shown by the varying effects of excitation of the central end 

 of the vagus, in proportion with the strength of stimulation. 

 Two kinds of fibres have been, determined in the vagus, which 

 have an opposite action upon the respiratory centre ; the one 

 favouring inspiration, the other expiration, when stimulated. On 

 exciting with induction currents, the intensity of which has been 



