30-1 NERVOUS ACTION. 



parts in which the peripheral ends of the nerve are situated. 

 It is on this account that, after the amputation of a limb, 

 pains are often endured, which are referred by the patient 

 to the excised member. If, on the other hand, one of these 

 stimuli be applied to the trunk of a motor nerve, a con- 

 traction takes place in the muscles to which it is distri- 

 buted, notwithstanding that the nerve may have been 

 divided in the interval between the part irritated and the 

 nervous centre. It is thus seen that the functional activity 

 of a nerve may be aroused by a simple modification in the 

 condition of its fibres, and the effect is the same as if it 

 had originated in the nervous centre or peripheral extremity 

 as the case may be, and had been conveyed along the whole 

 length of the constituent fibres. 



Nerve-fibres can only act in one direction. Thus, if a 

 sensory nerve be divided, and -the peripheral extremity 

 subjected to irritation, no sensation is experienced. Irrita- 

 tion of the portion still in connection with the brain, however, 

 gives rise to the most lively pain, referable, as above stated, 

 to the part from which the nerve conducted. In the same 

 manner, if a motor nerve be divided and an irritant applied 

 to the end still in connection with the brain, no effect either 

 sensory or motor is produced ; but when that in connection 

 with the muscles is treated in a similar manner, a contrac- 

 tion in the latter immediately follows. These experiments 

 can only be satisfactorily made by cutting down upon and 

 irritating the inferior and superior roots of the spinal 

 nerves, the former being exclusively motor, the latter ex- 

 clusively sensory. At any other point of their course, the 

 nerves contain fibres of both varieties, and no relative 

 experiment can be made. 



When a section has been made of a motor nerve, and its 

 detached end stimulated so as to produce a powerful mus- 



