THE NERVOUS SYSTEM. 79 



body, as by pinching, or by heat, or by electrifying it, there is, under 

 ordinary circumstances, one of two effects, either there is pain, or there 

 is twitching of one or more muscles to which the nerve distributes its 

 fibres. From various considerations it is certain that pain is always the 

 result of a change in the nerve-cells of the brain. Therefore, in such 

 an experiment as that referred to, the irritation of the nerve-fibre seems 

 to the experimenter to be conducted in one of two directions, i.e., either 

 to the brain (central termination of the fibre), when there is pain, or to 

 a muscle (peripheral termination) when there is movement. 



Fio. 314. An isolated sympathetic ganglion cell of man, showing sheath with nucleated-cell lin- 

 ing, B. A. Ganglion-cell, with nucleus and neucleolus. C. Branched process. D. Unbranched pro- 

 cess. Copied from Key and Retzius. x 750. (Klein and Noble Smith.) 



The effect of this simple experiment is a type of what always occurs 

 when nerve-fibres are engaged in the performance of their functions. 

 The result of stimulating them, which roughly imitates what happens 

 naturally in the body, is found to occur at one or other of their ex- 

 tremities, central or peripheral, never at both; and in accordance with 

 this fact, and because, for any given nerve-fibre, the result is always the 

 same, nerves are commonly classed as sensory or motor. 



It may be well to state, in order to avoid confusion, that the apparent 

 conduction in both directions, which seems to occur when a nerve, say 

 the ulnar or median, is irritated, depends on the fact that both motor and 

 sensory fibres are bound up together in the same nerve-trunks an arrange- 



