THE SPINAL CORD AND REFLEX ACTIONS. 603 



:animal be given a dose of the latter drug, a stimulus, such as 

 normally would excite only limited orderly reflexes, will excite 

 the whole cord, and lead to discharges along all the efferent 

 fibres so that general convulsions result. It has been clearly 

 proved that, in such cases, not the skin, or afferent or efferent 

 nerves, or the muscles, but the spinal cord itself is affected by 

 the poison (at least primarily), unless unnecessarily large doses 

 have been given. 



The Least-Resistance Hypothesis. In order to compre- 

 hend reflex acts we mast assume a manifold union of afferent 

 with efferent nerve-fibres; this is anatomically afforded by the 

 minute plexus of the gray network, which is continuous through 

 the whole cord, and in which many fibres of the anterior and 

 posterior nerve-roots directly or indirectly end. The contin- 

 uity of this network serves to explain general reflex convul- 

 sions, and the spread of an afferent impulse, or its results, 

 through the whole cord, with the consequent emission of effe- 

 rent impulses through many or all the anterior roots ; but, on 

 the other hand, it renders it difficult to understand limited 

 and orderly reflexes, in which only a few efferent fibres are 

 stimulated. To explain them we have to assume a great re- 

 sistance to conduction in the gray network, so that a nerve 

 impulse entering it is soon blocked and transmuted into some 

 other form of energy; hence it only reaches efferent fibres 

 originating near the point at which it enters, or fibres placed 

 in specially easy communication with that. When the frog's 

 flank is tickled, only muscles innervated from anterior roots 

 on the same side of the body, and springing from the same 

 level of the cord, are made to contract ; when the stimulus is 

 more powerful, the stronger afferent impulse radiates farther, 

 but mainly in directions determined by lines of conductivity 

 in the cord ; e.g. , to the origin of the efferent fibres which 

 cause lifting of the hind leg to the irritated spot. These 

 paths of easiest conduction, or of least resistance, in some 

 cases lie in the gray matter itself, in others in the inter-central 

 or commissural fibres of the highly conductive medullated 

 kind, which, passing out of the gray substance at one level, 

 run in the white columns to it at another, where the efferent 

 fibres of the muscles called into play originate. A still stronger 

 afferent impulse radiates wider still, and, liberating energy 

 from all the nerve-cells in the gray matter, produces a useless 

 .general convulsion. Under the influence of strychnine and 



