578 THE HUMAN BODY. 



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 com- 

 prehend reflex acts we must assume a manifold union of 

 sensory 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 the fibres- 

 of the anterior and posterior nerve-roots directly or indirect- 

 ly end. The continuity of this network serves to explain 

 general reflex convulsions, and the spread of an afferent 

 impulse, or its results, through the whole cord, with the 

 consequent emission of efferent 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 resistance 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 conduc- 

 tivity 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 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 



