THE ANALYSIS OF BEHAVIOUR IN ANIMALS 155 



side and then the stimulated organ itself. Stronger stimuli 

 lead first to the withdrawal of both antennules,then the antenna, 

 and finally the eyes of both sides. 



The unit of structure in the synaptic system is the reflex 

 arc. But it should be remembered that the barrier between 

 one reflex arc and another is physiological rather than structural ; 

 the injection of strychnine results in a condition in which 

 stimulation of any receptive area will elicit convulsive move- 

 ments of all the muscles of the body. The simplest reflex 



lanql.'oD 



Fig. 38.— Diagram of simple reflex arc {stellate ganglion of Cephalopod) 



path is one in which the receptor is represented by the 

 peripheral arborisations of an afferent neurone whose dendrites 

 terminate distally in connexion with the cell-body of a motor 

 neurone. Such an ideally simple reflex arc is probably 

 realised in the reflex paths which have their synapses in the 

 stellate ganglion of the Cephalopod (Fig. 38). The stellate 

 ganglion is connected by the pallial commissures with the 

 brain and by the stellar nerves with the musculature of the 

 mantle. Frohlich (19 10) showed that local stimulation of its 

 surface evokes generalised contraction of the musculature of 

 the mantle so long as the stellar nerves are intact. If the 



