GENERAL NERl/E PHYSIOLOGY 223 



for nerves do not appear susceptible to fatigue, and the supply of 

 blood to them is small. 



Neurokeratin is found in the neuroglia; fat, cholesterin, lecithin, 

 and protagon, in the medullary sheath. 



4. The effect of the conduction of the impulse. The 



nature of the result of the conduction of the impulse to the 

 end-organ does not depend upon the nature of'the stimula- 

 tion, but upon the nature of the end-organ. For example, 

 each effective stimulation of a motor nerve is followed only 

 by a muscular contraction, of a secretory nerve by secretion, 

 of a sensory nerve only by sensation. In the last case only 

 that kind of sensation is produced which is specific for the 

 sense cell of the organ acted upon. 



The nerve fibres may be classified as to the direction in 

 which they normally carry the impulse as: 



1. Centrifugal (motor, secretory), conducting the impulse 

 from the nerve cell to the peripheral organ. 



2. Centripetal (sensory, nerves acting reflexly), conduct- 

 ing from a sense organ to a nerve cell. 



3. Intercentral conduction from one nerve cell to another. 



Besides conducting the impulses the nerve fibres have an influ- 

 ence on the nutrition of the organs which they innervate. After the 

 nerves have been cut, the organs which they supply undergo dis- 

 turbances of nutrition, e.g. the dying of a muscle after section of 

 its nerves. 



3. GENERAL PHYSIOLOGY OF THE NERVE CELLS 



All the functions of the nervous system which we cannot 

 explain from the known functions of the nerve fibres we 

 ascribe to the nerve cell, for no other nerve elements are 

 known to which they can be ascribed. These functions do 

 not belong to the whole nerve cell, but to parts of the pro- 

 toplasm. The nucleus has probably only a trophic function. 



The trophic action of nerve cells is illustrated by the fact that 

 nerve fibres separated from the nerve cells degenerate. In many 

 cases this is also true for end-organs (muscles) supplied by these 

 nerve fibres. 



