ACTION AS A NERVOUS CENTRE. 461 



or the transverse division of the cord itself at its upper part. The 

 wound of the nervous substance induces an increased excitability of its 

 gray matter, in consequence of which sensitive impressions of a moderate 

 character produce a more energetic muscular reaction. This is shown 

 by the observations of Tiirck, Bernard, and Yulpian, in which, after a 

 section of one lateral half of the cord, the posterior leg on that side is 

 withdrawn more rapidly from an acidulated solution than the other ; and 

 in which the reflex action of the cord, in decapitated animals, becomes 

 more and more marked, for the posterior limbs, in consequence of suc- 

 cessive transverse sections made from before backward, in the cervical 

 and lumbar regions. 



The reflex action of the cord may also be increased by poisonous 

 substances. Strychnine is the most efficient in this respect, and pro- 

 duces very rapidly an exalted condition of irritability in the spinal cord, 

 in consequence of which a slight irritation of the skin is followed by 

 excessive muscular reaction. If a frog be simply decapitated and left 

 in repose for a short time, the reflex action of the cord manifests itself, 

 as usual, in a distinct but moderate degree. Slight irritations have no 

 perceptible effect, and the pinching of the skin in one hind foot usually 

 causes retraction of that limb only. But if a solution of strychnine 

 be injected underneath the skin, at the end of ten or fifteen minutes, 

 when absorption has taken place, the reflex irritability of the cord is 

 found to be exaggerated in a very marked degree. The animal still 

 remains motionless if undisturbed ; but the slightest irritation applied 

 to the skin, the contact of a hair or feather, or the jar produced by 

 striking the table upon which it is placed, will often be sufficient to 

 throw it into violent convulsive action, in which all the limbs take part. 

 As these effects are produced in the decapitated animal, no influence can 

 be attributed to the action of the brain. Strychnine, accordingly, is a 

 poison which acts directly upon the spinal cord by increasing its excita- 

 bility, and by thus causing convulsive movements in consequence of 

 slight external irritation. 



Similar results are known to follow from wounds or injuries either of 

 the cord itself or of peripheral parts of the nervous system. Brown- 

 S^quard has found 1 that in Guinea-pigs a section of one lateral half of 

 the spinal cord sometimes produces, after a few weeks, such a condition 

 of the nervous centres that the animal becomes epileptic, and that epi- 

 leptiform convulsions of a very intense character may be excited by 

 pinching the skin of part of the face and neck, on the side corresponding 

 with the lateral section of the cord. The phenomena of tetanus in man, 

 following wounds of the peripheral nerves, are also of a reflex convulsive 

 character. The tetanic spasm is often, if not always, excited by an 

 external cause ; but this cause is so slight that in the healthy condition 

 it would have no perceptible effect. The accidental movement of the 

 bedclothes, the shutting of a door, the passing of a carriage in the 



1 Researches on Epilepsy. Boston, 1857. 



