ON THE MAMMALIAN NERVOUS SYSTEM. 499 



EFFECT of an Interval of Rest on the Simple Reflex Discharge. 



Giilvanomctric deflections. 



, , After rest of 



Before rest, ^ m ; nut(JS- 



Excitation of the central end of the left 7th lumbar posterior 



root gave in the left sciatic nerve j 16 



Excitation of 1st sacral as above 20 



42 

 39 



EFFECT of Want of Rest on the Simple Reflex Discharge. 



f 18 Coil 2000 



Cat (203) Excitations following at two minutes' interval of right j 15 4000 

 7th lumbar posterior root gave in the right sciatir ^ After 15 rnin. rest 



nerve I 12 2000 



17 4000 



26 Coil 2000 

 Cat (378) Excitations following each other at 1 minute's interval < , 



L 8 ',' 8000 



Character of the Electromotive Change which is Produced by the Discharge of a 

 Nerve Centre as Contrasted with that Artificially Induced in a Nerve Channel, i.e., 

 Fibres. 



The gal vano metric method appears to us to suggest differences between the passage 

 of the nerve excitatory condition (so called " nerve impulses ") along fibres according 

 to whether it is a centre which is the source, or an artificial stimulus in the actual 

 course of the fibres. Thus, if a peripheral nerve be excited directly by any form of 

 excitation, i.e., electrical or mechanical, the effect in the nerve (i.e., the excitatory 

 condition) shows itself in the galvanometer as a movement which begins sharply and 

 proceeds moderately rapidly, but steadily, so long as the excitation lasts, stopping 

 when the stimulus is left off ; upon which return of the needle to zero begins. 



It is quite otherwise when the effect in the nerve is due to the passage of the 

 excitatory condition (the " discharge ") from a nerve centre. When this latter is 

 excited and the electrical change in the efferent path observed, it shows itself as a 

 slowly developed deflection which gains speed as it moves, does not stop immediately 

 when the stimulation is left off, and only slowly returns towards zero, which it 

 frequently does not completely reach. This peculiar character of the deflection move- 

 ment is special and easily recognised. If the activity of the centre be impaired, 

 eg., by cooling, drying, &c., the movement is still more sluggish, and is perhaps of 

 more interest ; it is late in development, i.e., does not commence for one or two seconds 

 after the excitation has begun. 



3 8 2 



