very Small Portions of Time. 321 



never perhaps experienced anything similar. When, however, 

 we reflect on the limited accuracy of our perceptions, and that 

 we naturally cannot perceive more quickly than our nerves, which 

 are the necessary media of all our perceptions, transmit im- 

 pressions, it is easy to see that the expression of our own expe- 

 rience can give us no information here. Earlier physiological 

 theories were inclined to regard the agent which propagates itself 

 through the nerves as electricity, and hence believed it necessary 

 to ascribe to it an immense velocity. The latest admirable in- 

 vestigations of Du Bois Reymond on the electromotive action of 

 the muscles and nerves have established facts which are perfectly 

 inconceivable, except on the assumption that the propagation 

 of an impression from the end of the nerves towards the brain 

 is always accompaniedby a change in the position of theponderable 

 molecules of the nervous fibres themselves; hence this propa- 

 gation depends upon the action of one ponderable molecule upon 

 the other. It belongs, therefore, to the same class of physical 

 phenomena as that to which the propagation of sound belongs, 

 and thus an inordinate velocity, such as that supposed, is rendered 

 improbable. 



I must commence with the simplest cases of the investigation. 

 I chose the muscle of a frog connected with the nerves proceed- 

 ing from it, but severed from the body of the animal. Such a 

 muscle retains its vitality long enough to permit of two or three 

 hours' continuous experiment without any considerable change, 

 which is not at all the case with the detached muscles of warm- 

 blooded animals. When any point of the nervous thread is 

 injured by cutting, burning, or what is most effectual, when an 

 electric current is sent through a portion of the nerve, this exci- 

 tation produces the same effect as that which, in ordinary cir- 

 cumstances, is produced by the will. The muscle contracts, 

 that is, it becomes active for a moment. The contraction passes 

 so quickly, that its single stages cannot be observed. The pro- 

 blem to be decided is, whether the contraction takes place later 

 when a distant portion of the nerve is excited than when the excited 

 portion is nearer to the brain. To resolve this, we must measure 

 the time which passes between the excitation and the contraction 

 of the muscle. Experiment, however, soon showed that the 

 activity of the muscle is by no means instantaneous, but appears 

 some time after the excitation of the muscle, increases gradually 

 to a maximum and then sinks, first quickly and afterwards by 

 slow degrees ; so that the greatest part disappears in about one- 

 third of a second, but the remaining portion requires several 

 seconds afterwards. This cannot be recognized in the muscles 

 which act in obedience of the will, on account of the quickness 

 of the contraction ; but we may have observed it in the involun- 



