NERVE STIMULI. 501 



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 

 centres 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 prep- 

 aration is commonly made from the leg of a frog, the sciatic 

 nerve being carefully prepared from the thigh, while the gastroc- 

 nemius 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 facili- 

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



