THE REFLEX CIRCUITS 67 



We have just described the mechanisms of certain reflexes. 

 The question at once arises, In what sense do we know the 

 mechanism of a nervous reaction? Certainly not in the sense 

 that we understand all of the factors involved in nervous conduc- 

 tion and correlation. But we do have a practical knowledge of 

 the combinations of neurons necessary to effect certain definite 

 results, much as the practical electrician may be able to wind a 

 dynamo or build a telephone, even though his knowledge of the 

 theory of electricity be very small. 



Summary. The reflex arcs or reflex circuits rather than 

 the neurons of which these circuits are composed are, from 

 the physiological standpoint, the most important units of the 

 nervous system. Reflex acts are to be distinguished, on the 

 one hand, from the simpler non-nervous reactions known as 

 tropisms and taxes, and, on the other hand, from voluntary 

 acts and acquired automatisms. Many instincts are chain 

 reflexes of very complex sorts, the completion of one reaction 

 serving as the stimulus for the next, and so on in series. The 

 simplest true reflex requires a receptor, a center or adjuster, 

 an effector, and the afferent and efferent conductors which put 

 the center into physiological relation with the receptor and the 

 effector respectively. Five types of reflex circuits were distin- 

 guished (see Fig. 18) and illustrations of them given. All of the 

 reflex centers are interconnected by systems of fibers, either in 

 the form of definite tracts or else by more diffuse connections 

 in the neuropil. Localization of cerebral function is, therefore, 

 only approximate, with the possibility of all sorts of intercon- 

 nection of different reflex systems as occasion may require. 

 This is the neurological basis of the greater plasticity of 

 behavior of higher vertebratesas contrasted with invertebrates 

 and lower vertebrates. 



LITERATURE 



DEWEY, J. 1893. The Reflex Arc Concept in Psychology, Psychol. 

 Review, vol. iii, p. 357. 



HERRICK, C. JUDSON. 1913. Some Reflections on the Origin and Sig- 

 nificance of the Cerebral Cortex, Jour, of Animal Behavior, vol. iii, pp. 222- 

 236. 



HERRICK, C. JUDSON and COGHILL, G. E. 1915. The Development of 

 Reflex Mechanisms in Amblystoma, Jour. Comp. Neur., vol. xxv, pp. 65-85. 



