TRANSMISSION OF STIMULI 103 



negative this assumption. Thus dilute glycerine excites contractions when 

 it comes into contact with the nerve or nerve-ending of a nerve-muscle 

 preparation, but not if the latter is curarized, whereas if applied to a 

 Nitella or Chara cell, which has recovered from the first shock-stoppage 

 due to the action of a dilute solution of curare, dilute glycerine usually 

 accelerates streaming, while dilute salt solution always retards it, although 

 a crystal of salt readily excites the nerve of a fresh nerve-muscle prepara- 

 tion and produces a muscular contraction. Similarly an electrical stimulus 

 produces the same shock-stoppage in a curarized cell as in the absence 

 of curare, the only difference being that the latent period of recovery is 

 slightly prolonged. On the other hand, in a curarized nerve-muscle 

 preparation the motor end-plates are paralysed, and the muscle responds 

 to direct excitation, but not to stimuli applied to the nerve. 



Similarly veratrin, muscarin, atropin, &c., are powerful neuro-muscular 

 poisons, the first two acting almost entirely on muscle alone, but they 

 exercise a relatively feeble action on streaming plant-cells. Apparently, 

 therefore, the latter do not possess anything corresponding even approxi- 

 mately to the neuro-muscular mechanism of Coelomate animals, and if 

 there is any differentiation into better-conducting fibrils and feebly con- 

 ducting ground substance, it must necessarily be of very rudimentary 

 character, and is probably for the most part only a temporary development. 



SECTION 47. Transmission of Stimuli and rate of Propagation 

 in Cells and in Tissues. 



Stimuli are probably transmitted both longitudinally and transversely 

 through the ectoplasm and endoplasm, just as they are in a muscle-fibre. 

 In a striated muscle-fibre the stimulus is rapidly propagated (i to 13 mm. 

 per second), whereas in cells of Nitella and of Chara the rate of propagation 

 at room-temperature appears to lie between I and 8 mm. per second, 

 which is less rapid than that in the muscle of the heart (10 to 15 mm. 

 per second). This rate of propagation is, however, sufficient to show that 

 the streaming plasma (i to 3 mm. per minute), is not responsible for the 

 transmission of stimuli. More exact determinations than the above are 

 difficult, even when very long cells of Chara or Nitella are used. This 

 is chiefly owing to the variable duration of the latent period, which 

 necessitates rapid successive observations of the time of stimulation and 

 of stoppage at the stimulated and unstimulated ends of the cell. The 

 difference between the first two times gives the latent period of response, 

 and on subtracting this from the interval of time between the^ application 

 of the stimulus and the stoppage at the unstimulated end, we get the 

 time of propagation through a distance equal to the greater part of the 

 length of the cell. Single induction shocks were applied to an end of 

 the cell insulated by vaseline, and the times determined by the aid of 



