THE CENTRAL NERVOUS SYSTEM 



797 



nerve-endings in the skin, or of special sense-endings, as in the retina 

 or internal ear. The conducting mechanism or conductor is made 

 up of at least two neurons, one the afferent portion of the reflex 

 arc connected with the receptor, the other the efferent portion of 

 the arc, connected with the organ, sometimes termed the effector 

 organ a muscle, e.g., or a gland which accomplishes the end-effect. 

 The transference of the excitation from the afferent to the efferent 

 neuron takes place across the intervening synapse. The simple 

 isolated reflex arc, as thus described, although a convenient abstrac- 

 tion, corresponds but little to anything which actually exists in one 

 of the higher animals. With increasing complexity 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. 769) 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 inter- 

 mediate 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 poten- 

 tially 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 arrows indicate the direction of the 

 the influence of this drug, afferent and efferent impulses, 

 stimulation of the smallest 



portion of the skin will cause violent and general convulsions, which 

 are unaffected by destruction of the brain, but cease at once on 

 destruction of the cord (p. 886). 



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

 duction of the impulses in a given reflex arc 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. Following the path of least re- 

 sistance, 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 strychnine or tetanus toxin, an excita- 

 tion 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 

 resistance 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. 801). 



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 



FIG. 337- DIAGRAM OF A SIMPLE 

 REFLEX ARC. 



