134 THYSIOLOGY OF MUSCLES AND NERVES. 



ductor passes to 'the nerve, and returns from there 

 to the slide aS', which slips along the wire A B, The 

 branch-current traversing the nerve is strengthened or 

 weakened accprding as this slide is placed further from 

 or nearer to A. 



By means of a rheochord of this sort there is no 

 difficulty in making the currents within the nerve so 

 weak that they exercise no influence at all. If their 

 strength is then gradually increased, a pulsation is 

 always first seen to occur in the fresh nerve when the 

 current is closed, whatever the direction of the current 

 within the nerve. In order to be able to indicate the 

 direction, it has become customary to speak of such a 

 current, when it passes within the nerve from a central 

 to the more peripheric parts, as descending, and when 

 it passes in the opposite direction, as ccscending. 



Ascending and descending currents, therefore, when 

 they are weak, afford pulsations only on the closing 

 of the current. If the strength of the current is in- 

 creased, pulsations gradually begin to occur also on 

 the opening of the current, at first usually with the 

 descending current, though, when the strength is in- 

 creased yet more, they occur in connection with the 

 ascending current also. Finally, the pulsations in all 

 four cases are of equal strength. If, however, the 

 strength of the current is yet further increased, two 

 of these four pulsations again become weaker — the 

 closing pulsation with the ascending current, and the 

 opening pulsation with the descending current. A 

 strenofth of current is at last reached at which these 

 two pulsations entirely cease, so that pulsations occur 

 only on the closing of the descending, and on the 

 opening of the ascending currents. These phenomena, 



