viii CONDUCTIVITY AND EXCITABILITY OF XERVE 103 



without analogy. Pawlow (57) long ago communicated observa- 

 tions on the adductor muscles of the shell of Anodonta, from 

 which it appeared that these are supplied by two kinds of nerve- 

 fibres, one motor, subserving shortening of the muscle, the other 

 inhibitory, running parallel with the first, and counteracting the 

 shortened state of the muscle, so as to induce relaxation. By 

 appropriate excitation, first one and then the other effect can be 

 brought prominently forward. The crayfish claw is, however, 

 better suited for this demonstration, since its muscles are striated, 

 and the reaction follows incomparably quicker than in the smooth 

 and sluggish molluscan muscles, in which, moreover, ganglion -cells 

 are interpolated between the nerve and muscle, and are not easily 

 excluded from interference. 



Cardiac muscle is also subserved by nerve-fibres of opposite 

 function and antagonistic action, which in many vertebrates run 

 in a common trunk, in others again are separate, and bring out 

 differences of excitability comparable in many respects with the 

 above. Heidenhain (58), and later on Lowit, showed that 

 minimal excitation of the vagus always resulted in inhibition, 

 never (under these conditions) in acceleration, of cardiac activity. 

 The latter only appears with greater strength of current, so that, 

 under the presumption of two kinds of fibres, it may be said that 

 the inhibitory nerves are more excitable than the accelerators. 

 (Similar relations exist in warm-blooded animals vagus, accelerans 

 cf. Meltzer, 58.) On the other hand, Lowit finds, I.e., that the 

 former are more easily injured by certain chemical reagents 

 than the latter. If the vagus-trunk in the frog is treated with 

 KN0 3 (J %), there is a stage at which excitation of the nerve 

 produces acceleration of cardiac activity only, while the same 

 excitation below the part treated with potasli is inhibitory. On 

 washing the nerve with 0*6 / Q NaCl the potash -effect in- 

 variably disappears again, and this transformation of the vagus 

 from accelerator to depressor may be repeated several times on 

 the same preparation. Other substances (I.e. p. 493) have a 

 similar action to KN0 3 , as also vigorous cooling (ice). Alterations 

 further seem to occur in the immediate vicinity of an artificial 

 section, as expressed by a different ratio of loss of excitability 

 in the antagonistic kinds of fibres. When Lowit placed the 

 electrodes close together (1 mm.) upon the divided vagus, so that 

 one was applied directly to the cross-section, there was invariably, 



