THE SPINAL CORD. 403 



system at large. Decapitation in the frog is often followed, for a few 

 moments, by an interval of nervous paralysis, in which no phenomena 

 of reaction can be obtained. Even injuries in which the nervous cen- 

 tres are not directly interested, such as opening the abdomen and 

 removing the abdominal organs, may produce a similar effect. In some 

 instances this period of depression is very short, so as to be almost 

 imperceptible ; in others it lasts for several minutes. After it has 

 passed off, the reflex irritability of the cord returns, and, if the cord 

 itself have been wounded or divided, may even be perceptibly increased 

 in intensity. 



It is for this reason that reflex action often seems more vigorous and 

 prompt in the frog after removal of the head, or after transverse division 

 of the cord at its upper part. The wound induces an increased excita- 

 bility, in consequence of which sensitive impressions produce a more 

 energetic reaction. This is shown by the observations of Tiirck, Ber- 

 nard, and Yulpian, in which, after section of one lateral half of the cord, 

 the hind leg on that side is withdrawn more rapidly from an acidulated 

 solution than the other; and in which reflex action, in decapitated 

 animals, becomes more marked, in consequence of successive transverse 

 sections, in the cervical and dorsal regions. 



The reflex action of the cord may be increased by poisonous sub- 

 stances. Strychnine is the most efficient in this respect, and produces 

 an exalted irritability of the spinal cord, in consequence of which a 

 slight irritation of the skin is followed by excessive muscular reac- 

 tion. In a decapitated frog, under ordinary conditions, the reflex action 

 of the cord is distinct but moderate in degree. Slight irritations have 

 but little effect, and the pinching of one hind foot usually causes retrac- 

 tion of that limb only. But if a solution of strychnine be injected 

 beneath the skin, at the end of ten or fifteen minutes, when absorption 

 has taken place, the reflex irritability of the cord is exaggerated in a 

 marked degree. The animal still remains motionless if undisturbed ; 

 but the least irritation applied to the skin, such as the contact of a hair 

 or a feather, or the jar produced by a blow upon the table near by, 

 w^ill often cause violent convulsions, in which all the limbs take part. 

 As these effects are produced in the decapitated animal, they are inde- 

 pendent of the action of the brain. Strychnine, accordingly, acts upon 

 the spinal cord by increasing its excitability, thus causing convulsive 

 movement from slight external irritation. 



Similar results may follow, as a secondary consequence, from wounds 

 or injuries either of the spinal cord or of peripheral nerves. Brown- 

 Sequard * has shown that in guinea-pigs a section of one lateral half 

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

 of the nervous centres that epileptiform convulsions, of very intense 

 character, may be excited by pinching the skin of the face and neck, on 

 the corresponding side. The phenomena of tetanus in man, following 



* Researches on Epilepsy. Boston, 1857. 



