THE WAYS OF LIFE 189 



actions involve (1) a receptor of a stimulus — a sensory 

 or perceptory nerve-cell — from which impulses pass in to 

 the central nervous system, (2) a ' motor ' aerve-cell which 

 connects the central nervous system with a muscle or 

 a gland, and (3) between these two a ' communicating ' 

 nerve-cell connecting them within the nervous system. The 

 three structural units taken together constitute a reflex arc, 

 but in actual fact reflex actions are always more complex 

 than this diagrammatic analysis suggests and cannot be 

 isolated, except in theory, from other reflexes to which they 

 are linked. 



When a single-celled organism contracts itself or draws 

 away from a stimulus — the simplest sort of response that a 

 living creature can make — it seems most convenient to use 

 the term reaction, keeping the term reflex action for multi- 

 cellular animals in which there are differentiated elements 

 forming a reflex arc. From simple reflex actions, such as 

 drawing the finger away from a hot object, there is a 

 graduated series leading on to such complicated reflex 

 actions as coughing and sneezing and sucking. 



Reflex actions require no attention, no will, no con- 

 sciousness, no brain ; they are invariable reactions of parts 

 of the body to a particular stimulus, and depend upon pre- 

 estabUshed structural arrangements and fimctional sensi- 

 biUties. It seems convenient to admit that they hardly 

 rise to the level of behaviour, for that term implies 

 that the organism as a whole is an agent and that it 

 exhibits a concatenated series of actions. In behaviour 

 there is a more or less effective succession of adjust- 

 ments of the whole creature. That the Hnks of the 

 chain may be reflexes, is a view held by many investi- 

 gators. 



