Origin of Basal Ganglia 83 



ing to and from the various sensory organs of 

 the animal's body. The mid-brain, therefore, 

 of crayfish and of insects corresponds to that 

 part of the brain of vertebrates to which we 

 shall subsequently refer as their basal ganglia. 



The hind-brain of insects gives off nerves to 

 supply the animal's alimentary canal ; it forms, 

 as it were, the head of the chain of ganglia 

 alluded to in our description of the central 

 nervous system of crayfish. 



We may now turn our attention to the sensory 

 organs of insects, which consist of structures 

 derived for the most part from the ectoderm, 

 specially modified either in the form of single 

 or groups of cells, which act as the receptors of 

 energy derived from visual, olfactory, and tac- 

 tile impressions. 1 



The visual sensory organs of insects consist 



1 "It is probable the structures of the various sensory organs 

 that first receive the external stimulus act like a sieve, arresting 

 certain qualities of the exciting energy and permitting certain other 

 qualities to pass on, so as to act on the specialised living- nervous 

 substance of the various sensory centres. These receptors of energy 

 have thus a certain power of selection or natural fitness developed 

 by means of the action of their environment or struggle for 

 existence." (Professor T. Ziehen, Introduction to Physiological 

 Psychology, pp. 40, 42.) As we have shown, after leaving a 

 sensory organ by nerve paths energy becomes subjected to further 

 selection through the action of an interrupted system of dendrons, 

 so that by the time it reaches the living matter of the correspond- 

 ing cerebral cell it has assumed a specific form. 



F 2 



