308 NERVOUS SYSTEM. 



longitudinal section of the cord in the median line, leaving 

 only a slight communication between the two sides, we may 

 sometimes succeed, by strongly irritating the skin of one leg. 

 in producing reflex movements, not only in the same leg, 

 but in the leg of the opposite side ; and it is reasonable to 

 suppose that the irritation is propagated from one side to 

 the other through the cells of the gray matter. 1 



The conditions essential to the manifestations of reflex 

 phenomena depending upon the action of the cord are very 

 simple and easily understood. 



In the first place, it is necessary that one or more of the 

 posterior roots of the spinal nerves should be in communica- 

 tion with the cord, in order to conduct the impression to this 

 nerve-centre. If all of the posterior roots be divided, there 

 is no nervous communication between the periphery and the 

 centre, and no movements follow irritation of the surface. 

 When the excitability of the cord is exaggerated, as in poi- 

 soning by strychnine, a single posterior root is sufficient to 

 conduct an impression to the cord, which will give rise to 

 violent contractions of all the muscles. 2 This is due to a dis- 

 persion of the impression, under these conditions of increased 

 excitability, from the single point of entrance of the poste- 

 rior root, throughout the cord. In animals that have been 

 simply decapitated, a similar dispersion of impressions may 

 also take place. If a comparatively feeble single impression 

 be made upon any part of the general surface, as the rule, 

 the subjacent muscles only are the seat of contraction ; but 

 if the impression "be more powerful, or if it be prolonged, as 

 when we apply a drop of acetic acid to any part of the skin 

 of a frog, this impression may be diffused throughout the 

 cord, producing contractions of the general muscular system. 

 We have already shown, in treating of the general properties 

 of the sensory nerves, that an impression made at any point 

 in the course of a nerve is conducted to the centre. Reflex 



1 LONGET, Traiti de physiologie, Paris, 1869, tome Hi., p. 260. 

 * BERNARD, Systeme nerveux, Paris, 1858, tome i., p. 342. 



