406 THE SPINAL CORD. 



There are certain important facts which still remain to be noticed 

 regarding the mode of action of the spinal cord and its nerves. 

 They are as follows : 



1. An irritation applied to a spinal nerve at the middle of its course 

 produces the same effect as if it traversed its entire length. Thus, if the 

 sciatic or median nerve be irritated at any part of its course, con- 

 traction is produced in the muscles to which these nerves are dis- 

 tributed, just as if the impulse had originated as usual from the 

 brain. This fact depends upon the character of the nervous fila- 

 ments, as simple conductors. Wherever the impulse may originate, 

 the final effect is manifested only at the termination of the nerve. 

 As the impulse in the motor nerves travels always in an outward 

 direction, the effect is always produced at the muscular termination 

 of the filaments, no matter how small or how large a portion of 

 their length may have been engaged in transmitting the stimulus. 



If the irritation, again, be applied to a sensitive nerve in the 

 middle of its course, the painful sensation is felt, not at the point 

 of the nerve directly irritated, but in that portion of the integument 

 to which its filaments are distributed. Thus, if the ulnar nerve be 

 accidentally struck at the point where it lies behind the inner con- 

 dyle of the humerus, a sensation of tingling and numbness is pro- 

 duced in the last two fingers of the corresponding hand. It is 

 common to hear patients who have suffered amputation complain of 

 painful sensations in the amputated limb for weeks or months, and 

 sometimes even for years after the operation. They assert that 

 they can feel the separated parts as distinctly as if they were still 

 attached to the body. This sensation, which is a real one and not 

 fictitious, is owing to some irritation operating upon the divided 

 extremities of the nerves in the cicatrized wound. Such an irrita- 

 tion, conveyed to the brain by the sensitive fibres, will produce 

 precisely the same sensation as if the amputated parts were still 

 present, and the irritation actually applied to them. 



It is on this account also that division of the trifacial nerve is 

 not always effectual for the cure of tic douloureux. If the cause of 

 the difficulty be seated upon the trunk of the nerve, between its 

 point of emergence from the bones and its origin in the brain, it is 

 evident that division of the nerve upon the face will be of no 

 avail ; since the cause of irritation will still exist behind the point 

 of section, and the same painful sensations will still be produced in 

 the brain. 



2. The irritability of the motor filaments disappears from within out- 



