114 THE VERTEBRATE ANIMAL: IRRITABILITY 



seriation of the reflexes involved. Learning is the putting together 

 of a series of reflexes which, when they are finally established, 

 become a habit. The retention of habits involves memory of 

 some sort. 



In any detailed discussion of behavior that takes account of the 

 distinctions between emotions and instincts, the formation and 

 retention of habits, and the inter-relations that exist between these 

 divisions, the student must follow the investigations and argu- 

 ments of the psychologists in order to form his opinions. It can 

 be said, however, with an increasing degree of certainty, that 

 analysis of nervous coordination in all its complications depends 

 on knowledge and understanding of the reflex arc. 



Principle of the Common Path. — The examples of reflex action 

 that have been given are representative. It can be seen that, begin- 

 ning with the simplest type of reflex arc, in which one specific 

 response occurs as the result of a stimulus, the series becomes 

 increasingly complex as the result of the interpolation of one or 

 manyadjustor neurones between the afferent and efferent neurones. 

 Both simple and complex reflexes may enter into allied and antago- 

 nistic combinations or form chains for purposes of coordination. 

 Patterns and orders of compound reflexes are transmitted from gen- 

 eration to generation and give rise to instinctive correlations. The 

 mechanism of the reflex arc obviously makes possible the highest 

 degree of coordination. Sherrington has generalized the facts of 

 nervous coordination in his '' principle of the common path." 

 Each afferent neurone is a special pathway by which impulses 

 from its particular receptor enter the central nervous system. 

 Within the central nervous system the impulse may travel over 

 varied paths, formed by synapses between adjustor neurones, and, 

 theoretically, may produce a reaction in any of the effectors. The 

 efferent neurones, over which impulses travel from the central 

 nervous system to the effectors, differ from the afferent neurones 

 in that they are not private paths for particular impulses. It is a 

 commonplace that many different kinds of stimuli can produce the 

 same reaction or effect. Consider, for example, the many and varied 

 stimuli to which man responds by walking. The efferent neurone 

 is, therefore, a " common path " over which impulses from 

 receptive regions all over the body can be discharged at a 

 particular effector (Fig. 62 E). By means of the adjustor neu- 

 rones of the central nervous system, connections are made pos- 



