122 TEXT-BOOK OF PHYSIOLOGY. 



negative variation is about 28 meters a second; that it is at first 

 feeble, soon rises to a maximum, and then declines; that it requires 

 0.0006 to 0.0008 of a second to pass a given point. From these data 

 it is evident that the negative variation or action current has a space 

 value of about 18 mm. Transferring these statements to the nerve 

 impulse, it may be said that it is a molecular disturbance, traveling 

 at the rate of about 28 meters a second, is wave-like in character, 18 

 millimeters in length, and occupies 0.0006 to 0.0008 of a second in 

 passing any given point. 



Absence of Diphasic Action Currents. When any two points on 

 the longitudinal surface which do not exhibit a current are connected 

 with the galvanometer and a single wave of excitation passes beneath 

 the electrodes, it might be expected that, as in the case of the muscle, 

 a diphasic action current would be observed, from the fact that the 

 portions of the nerve beneath the electrodes become alternately neg- 

 ative with reference to all the rest of the nerve. This, however, is 

 not the case, the absence of the two opposing phases of the action 

 current being explained on the supposition that the negativity of the 

 two led-off points is of equal amount, and that, owing to the great 

 rapidity with which the excitation wave travels, the two phases fall 

 together too closely in time to alternately influence the galvanometer 

 needle. During stimulation of the nerve, when two currentless or iso- 

 electric points are connected, there is also an absence of the action 

 current, as was observed first by du Bois-Reymond, and which is to 

 be explained on similar grounds. It is true that an apparent action 

 current is sometimes seen when the stimulating current is very power- 

 ful or the seat of stimulation too near the diverting electrodes. This, 

 however, must be attributed to an electrotonic state of the nerve. 



The Effects of a Galvanic Current on a Nerve. When a con- 

 stant galvanic current of medium strength is made to pass through a 

 portion of a nerve, several distinct effects are produced : 



1. The development of a nerve impulse at the moment the current 

 enters and at the moment the current leaves the nerve, i. e., at the 

 moment the circuit is made and at the moment it is broken. The 

 development of the nerve impulse is made evident by the contraction 

 of the muscle if the nerve-muscle preparation be used. If the current 

 be either very weak, or very strong, the muscle contraction may not 

 always take place. 



2. The development oj electric currents on each side of the positive 

 pole or anode, and the negative pole or cathode (see Fig. 51), which 

 can be led off by means of wires into a galvanometer circuit from 

 either the artificial transverse and longitudinal surfaces, or from any 

 two points on the longitudinal surface as shown by the deflection 

 of the galvanometer needle. The direction of these electric cur- 

 rents in the nerve coincides with that of the galvanic or "polarizing 



