vin CONDUCTIVITY AND EXCITABILITY OF NERVE 105 



graduated as carefully as possible, the first effect observed (in the 

 rabbit) will in the majority of cases be the appearance of longer 

 or shorter expiratory pauses, or arrest of respiration in the 

 .expiratory position, while stronger excitation always has an 

 inspiratory effect. The marked expiratory action of chemical 

 stimuli may depend upon their lower intensity. According to 

 Meltzer (I.e. p. 385), there is ground for assuming in the vagus- 

 trunk, as in the claw-nerve of the crayfish, four different kinds 

 of fibres : a, inspiratory ; &, inhibitory of inspiration ; c, expiratory ; 

 d, inhibitory of expiration. Here, again, it is remarkable that 

 there seems to be a gradation of excitability, resembling that in 

 the claw-muscles, since the fibres inhibitory of expiration are 

 most excited at a strength of stimulus which simultaneously ex- 

 cites the inspiratory fibres. At approximately the same strength 

 of stimulus the inspiratory fibres are excited and their antagonists 

 inhibited. 



In all these cases (where with different intensities of stimulus 

 there are obviously different effects in the peripheral and central 

 organs supplied by the same nerve-trunk) it is a question whether 

 the differences of reaction observed are due to corresponding 

 differences in the excitability of the correlated nerve-fibres, or 

 in that of the end-organ itself, or in both together. 



In the cases observed by Kollett, he inclines to the view (since 

 no such difference appears with direct excitation of the muscle) 

 that the cause of the Eitter-Eollett phenomenon lies solely in the 

 properties of the nerve, leaving undecided whether it derives only 

 from the different excitability of the fibres destined for different 

 muscles. Griitzner (59), on the other hand, regards it as probable 

 that these phenomena arise from actual physiological differences 

 in the groups of flexor and extensor muscles, as well as in the 

 .entire nerve-muscle apparatus. He bases this upon a series of 

 observations already referred to, which show a real physiological 

 disparity between these groups of muscles, and further prove that 

 the Eitter-Eollett phenomenon comes off after the exclusion of the 

 nerves and nerve-endings by curare (I.e. p. 231). 



The preceding discussion shows that there are great and almost 

 insuperable obstacles to the comparison of the specific excitability 

 of different nerves, since we are thrown back solely upon the 

 reactions from the peripheral or central end-organs, which differ 

 intrinsically in regard to excitability. And if this is apparent 



