FUNCTIONS OF THE SPINAL CORD 899 



plexity of organization the nervous impulse passing up an afferent 

 fibre is in general offered, instead of a single efferent path, a choice of 

 many potential routes when it reaches the spinal cord. We have 

 previously (p. 872) described the course taken by the fibres of the 

 posterior roots on entering the cord. It is obvious that through the 

 main fibres and their collaterals an extensive connection partly direct, 

 partly by the link of intermediate neurons is established with the 

 motor cells on both sides of the cord. But the facts of physiology 

 demonstrate an even ampler connection than the mere anatomical 

 study of the distribution of the root-fibres would suggest. Indeed, the 

 phenomena of strychnine-poisoning seem to show that every afferent 

 fibre is potentially connected with the motor mechanisms of the whole 

 cord, or at least with a very large proportion of them. For in a frog under 

 the influence of this drug, stimulation of the smallest portion of the skin 

 will cause violent and general convulsions, which are unaffected by de- 

 struction of the brain, but cease at once on destruction of the cord (p. 904) . 



In an unpoisoned reflex frog that is, a frog in which interference 

 with the single spinal reflexes has been prevented by section of the bulb 

 or destruction of the brain the movements resulting from stimulation 

 of a given receptive area are by contrast surprisingly limited, localized, 

 and constant. Thus, a harmful stimulus of a certain intensity applied 

 to a toe will elicit time after time a raising of the leg in other words, 

 an excitation of muscles whose motor nerves arise from cells in the same 

 region of the cord into which the afferent fibres from the receptive skin 

 area enter. The localization of the reflex is in this case without, doubt 

 dependent upon the fact that the connections of the afferent fibres with 

 the group of efferent neurons in question are more direct and more 

 intimate than with any other group. This anatomical isolation of a 

 given reflex arc is never complete, but so far as it goes it may be 

 assumed to be constant and incapable of variation. Under normal 

 conditions the anatomical isolation is always supplemented by a 

 physiological isolation, which is susceptible of variation in the direction 

 either of increase or of diminution. 



It is therefore a question of great interest how the isolated con- 

 duction of the impulses in a given reflex arc, in so far as it depends upon 

 the physiological condition of the arc and of its connections, is normally 

 achieved. The best answer which can at present be given is that it is 

 not equally easy for a reflex excitation to pass across all the synapses 

 which, are potentially open to it, and that a lowering of the resistance 

 of the synapses in the favoured path is probably quite as important a 

 factor in the isolation as an increase of the resistance in those which 

 are to be barred. Following the path of least resistance, the excitation 

 traverses the synapse or synapses which it is easiest for it to break 

 through. What property of the synapse is associated with resistance 

 to the passage of the impulse is unknown. But it is a variable property, 

 and when a general reduction in the resistance is produced, as by strych- 

 nine or tetanus toxin, an excitation impressed upon a single afferent 

 path may force a great many synapses normally impervious to it. 



While it is convenient in a preliminary survey to speak of the resist- 

 ance to spreading of the excitation in the cord being diminished by 

 strychnine or by tetanus toxin, we shall see presently that more than 

 this is involved (p. 903). 



Principle of the Common Path. In considering the architecture 

 of the cerebro-spinal nervous system as a basis of reflex action, one 

 feature is of such importance as to deserve special mention. The 



