410 THE SPINAL CORD. 



tion. This action is known, accordingly, as the reflex action of the 

 spinal cord. 



It will be remembered that this reflex action of the cord is not 

 accompanied by volition, nor even by any conscious sensation. 



The function of the spinal cord as a nervous centre is simply to 

 convert an impression, received from the skin, into a motor, impulse 

 which is sent out again to the muscles. There is absolutely no 

 farther action than this ; no exercise of will, consciousness, or judg- 

 ment. This action will therefore take place perfectly well after 

 the brain has been removed, and after the entire sympathetic sys- 

 tem has also been taken away, provided only that the spinal cord 

 and its nerves remain in a state of integrity. 



The existence of this reflex action after death is accordingly an 

 evidence of the continued activity of the spinal cord, just as con- 

 tractility is an evidence of the activity of the muscles, and irrita- 

 bility of that of the nerves. Like the two last-mentioned properties, 

 also, it continues for a longer time after death in cold-blooded than 

 in warm-blooded animals. It is for this reason that frogs and other 

 reptiles are the most useful subjects for the study of these pheno- 

 mena, as for that of most others belonging to the nervous system. 



The irritability of the spinal cord, as manifested by its reflex 

 action, may be very much exaggerated by certain diseases, and by 

 the operation of poisonous substances. Tetanus and poisoning by 

 strychnine both act in this way, by heightening the irritability of 

 the spinal cord, and causing it to produce convulsive movements 

 on the application of external stimulus. It has been observed that 

 the convulsions in tetanus are rarely, if ever, spontaneous, but that 

 they always require to be excited by some external cause, such as 

 the accidental movement of the bedclothes, the shutting of a door, 

 or the sudden passage of a current of air. Such slight causes of 

 irritation, which would be entirely inadequate to excite involuntary 

 movements in the healthy condition, act upon the spinal cord, when 

 its irritability is heightened by disease, in such a manner as to pro- 

 duce violent convulsions. 



Similar appearances are to be seen in animals poisoned by strych- 

 nine. This substance acts upon the spinal cord and increases its 

 irritability, without materially affecting the functions of the brain. 

 Its effects will show themselves, consequently, without essential 

 modification, after the head has been removed. If a decapitated 

 frog be poisoned with a moderate dose of strychnine, the body and 

 limbs will remain quiescent so long as there is no external source 



