Mammalian Nerve-Muscle and Reflex Preparations. 349 



second stage of the law of polar excitation is attained by doubling the 

 intensity of the current and is still elicited within a considerable range. By 

 further intensifying the stimulation, the reflex effect produced by closing the 

 descending, and by opening the ascending current, increases progressively to 

 a maximum (fig. 3). 



It is often difficult to make out the relation between the intensity of the 

 current and the extent of the reflex effects produced at break of the descend- 

 ing, and also, though in a less degree, at make of the ascending current, since 

 many factors are involved in the reaction. Under favourable conditions of 

 experiment, as in the preparation where any reflex effect is only inhibitory, 

 these reflex effects sooner or later attain a maximum at a certain optimal 

 strength of current, and then gradually diminish in extent with further 

 increase of strength of the current, so that the third stage of Pfliiger's law 

 holds good in the extensor reflex preparations. 



(2) The Belation between Reflex Excitation and Inhibition. 



In preparations of good decerebrate rigidity, excitation is usually produced 

 by a weak current, both at make and at break, in either direction. The 

 myogram line rises abruptly and either drops down again more or less quickly 

 to the initial line or remains at the new position. When this shortening 

 reaction following an active reflex contraction is well pronounced, and the 

 stimuli at both make and break take effect, the myogram exhibits two steps. 

 The excitatory reflex effect increases with the strength of current within 

 somewhat narrow limits. As the current is increased, the reflex contractive 

 effects produced at make of the descending and at break of the ascending 

 current, first of all diminish in extent, and finally pass over into inhibitory 

 relaxation which becomes progressively greater with further strengthening of 

 the stimulus. 



Similarly, at make of the ascending and at break of the descending current 

 reflex contraction gradually changes into inhibitory relaxation when the 

 strength of the current is step by step increased. The relaxation in this 

 case is always limited. In some preparations where the decerebrate 

 rigidity is well marked, no relaxation of the muscle is induced at all, and 

 only excitatory effects are obtained, whatever the strength of the current 

 employed. This result may be explained by assuming that, as the stimuli 

 at make of the ascending and at break of the descending current do not 

 exceed a certain intensity, such relatively weak stimuli in such preparations 

 cause only excitation. 



