NERVES. 75 



it manifests itself outside of the nerve, either in the produc- 

 tion of motions, secretions, reflexes, or states of conscious- 

 ness. The excitability of a nerve admits of being 

 measured by ascertaining the minimum excitation by 

 which the signs of the excitatory state can be evoked. 

 For this purpose the exciting agent used must be measur- 

 able. In the case of nerves of voluntary muscles, the 

 excitability can also be judged of by measuring the degree 

 of shortening of the muscle produced. 



When the circulation ceases in a nerve or in the whole 

 body, its vital properties alter. Its excitability at first 

 increases, then gradually declines, until it is extinguished. 

 These changes take place in the same order in all nerves, 

 but occur earlier in parts nearest the centres (Ritter and 

 Valli) ; thus the intra-muscular parts of motor nerves 

 survive longest. Injury of a nerve produces increase of 

 excitability in the neighbourhood. The extinction of 

 excitability of a living nerve consequent on cessation of 

 circulation, is immediately followed by structural changes 

 affecting chiefly its medullary sheath. 



In a similar manner loss of excitability and consequent 

 change of structure, are produced by severance of a nerve 

 from the cerebro-spinal centres. It is believed that they 

 may also result from want of exercise. Nerve is more 

 excitable and more vulnerable than muscle, but is less 

 affected by want of blood supply and less readily 

 exhausted by repeated excitation. The two last facts 

 indicate that its exchange of material is much less 

 active. 



Influence of the voltaic citrrent on excitability. (i.) Under 

 the influence of a constant current flowing along a nerve, 

 its excitability is increased near the negative pole (cathode), 

 decreased near the positive pole (anode), but the nerve is, 

 as a rule, not excited so long as the intensity does not vary. 

 (2.) Every variation of intensity of a current so directed 

 excites the nerve. Other things being equal, the degree 



