686 NERVOUS SYSTEM. 



patients suffering from paraplegia, there is no evidence of true sensation or volition in the 

 spinal cord ; and, in man and the higher animals, we must regard all muscular movements 

 which depend solely upon the action of the cord as a nerve-centre as automatic and 

 entirely independent of consciousness and of the will. 



It is easy to determine, by experiments to which we have already incidentally alluded, 

 that the muscular movements dependent upon nervous action, occurring in decapitated 

 animals, are due to the action of the spinal cord as a nerve-centre. In an animal in which 

 the reflex phenomena are very marked, as they are after decapitation, especially if the 

 animal be poisoned with strychnine or opium, all movements immediately cease when 

 the cord is destroyed. That the gray matter of the cord is the part concerned as a centre 

 in the production of these phenomena, is probable, in view of what we know with regard 

 to the general functions and properties of this substance ; and experiments have shown 

 that this is the fact. If, in a decapitated frog, we make an incomplete longitudinal sec- 

 tion 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. 



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 communication 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 commu- 

 nication between the periphery and the centre, and no movements follow irritation of the 

 surface. When the excitability of the cord is exaggerated, as in poisoning 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. This is due to a dispersion of the impres- 

 sion, under these conditions of increased excitability, from the single point of entrance of 

 the posterior root, throughout the cord. In animals that have been simply decapitated, 

 a similar diffusion 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 mus- 

 cular 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 movements may, consequently, be produced by stimulating the 

 sensory nerves in their course or by irritating the posterior roots of the spinal nerves. 



We have already stated that the cord must retain its anatomical integrity, in order to 

 receive an impression made upon the centripetal nerves and transform it, as it were, into 

 a stimulus, which is reflected back by the motor nerves and produces muscular contrac- 

 tion. It is also evident that the motor nerves must retain their connection with the cord 

 and be in a condition to conduct the stimulus reflected by the cord to the muscles. 



The reflex excitability of the spinal cord is increased to a marked degree by separating 

 this portion of the cerebro-spinal axis from the encephalon, and the same is true for the 

 lower portion of the cord, when a section is made in the dorsal or lumbar region. It 

 is difficult to find an entirely satisfactory explanation of this fact ; and the phenomena 

 observed under these conditions are, in this regard, like the exaggerated sensibility of 

 portions of the general surface after section of certain columns of the cord. 



In experiments upon the lower animals, the reflex phenomena are greatly exaggerated in 

 intensity in the tetanic condition observed in poisoning by opium or strychnine. Take, for 

 example, a frog decapitated and poisoned with strychnine. No reflex movements 

 occur unless an impression be made upon the sensory nerves ; but the slightest irrita- 



