NERVE STIMULI. 499 



THE ACTIVE STATE OF NERVE-FIBRES. 



Nerves pass into a state of activity in response to a variety of 

 stimuli ; their changed condition, however, cannot be readily 

 recognized, because the only change we can detect in the nerve 

 is that in its electric state. We soon know, however, when a 

 nerve is conducting an impulse, if it be connected with its termi- 

 nals. In the case of a nerve bearing impressions from the skin 

 to the nerve centres, we get evidence of a sensation being felt, and 

 when the nerve is motor that is, bearing impulses from the cen- 

 tres to the muscles, we judge of the state of activity of the nerve by 

 means of the muscle contraction which it .brings about. For all 

 ordinary experimental purposes we use the nerve of a frog with the 

 muscle it supplies intact. This nerve-muscle preparation is com- 

 monly made from the leg of a frog, the sciatic nerve being care- 

 fully prepared from the thigh, while the gastrocnemius is cut away 

 from all its attachments except that to the femur, which is retained 

 as a point of fixation. In fact, the same method as is used for 

 the indirect stimulation of muscle is employed for the study of the 

 excitability of nerve-fibres. 



NERVE STIMULI. 



Besides the normal physiological impulse which comes from 

 the cells in connection with the nerve-fibres, they may be made to 

 pass into the active state by a variety. of stimuli, differing little 

 from those which are found to affect muscle, when applied directly 

 to that tissue. They may be enumerated as follows : 



1. Mechanical stimulation. Almost any mechanical impulse, 

 applied to any part of a nerve, causes its excitation. The stim- 

 ulus must -have a certain degree of intensity, and have a definite, 

 though it may be a very short, duration. If mechanical stimuli 

 be frequently applied to a nerve in the same place, the irritability 

 of the. part is soon destroyed ; but if fresh parts of the nerve be 

 stimulated, at each blow the nerve passes into a state of tetanus? 

 as shown by the contraction of the muscle to which it is supplied. 



2.- Chemical stimulation. First, must be named drying of the 

 fibre, whether this be caused by ordinary evaporation, or facil- 

 itated with blotting paper, exposure over sulphuric acid, or the 



