144 ANIMAL BEHAVIOUR 



associated with changes in the intensity of hght penetrating into 

 the sea — day and night, dull and bright sunlight and the phases 

 of moonlight. Doubtless they are examples of phototactic 

 behaviour, but we know so little of the physiology of the organisms 

 concerned that an explanation, purely on this basis, is apt to be 

 somewhat artificial and formal. 



53. ON REFLEX ACTIONS 



In a case of simple, organic response, as when the isolated 

 frog's muscle is artificially stimulated, either the effector organ, 

 or the nerve going to the latter is directly stimulated. In a 

 reflex act the stimulus is applied to a receptor and then the 

 latter transmits an impulse to its afferent nerve, which impulse 

 stimulates a nerve-centre. An impulse then issues from the 

 centre along an efferent nerve and this, going to an effector 

 organ, stimulates the latter to action. Thus the original stimulus 

 was first thought about as being reflected out from a nervous 

 centre. It will be seen that, in the main, a reflex act is based 

 upon a morphological conception. 



Examples of reflexes, in the ordinary sense, are (i) a sneeze, 

 when the stimulus is an irritation of the nasal mucous membrane ; 

 (2) blinking, when the stimulus may be some visual one, suggest- 

 ing damage to the eyes ; (3) the kicking movements of a hind 

 leg which can be elicited from a dog, lying on his side, when 

 the skin of the flank is lightly tickled ; and (4) the familiar 

 " knee-jerk," which is a kind of reflex. These are trivial examples 

 and do not indicate the importance of the conception — all 

 ordinary behaviour has a basis of reflex actions, which are com- 

 bined, " concatenated," inhibited, controlled, '' conditioned," 

 etc., with the results that we see in animal activity. 



The central point for consideration is the role of the nerve- 

 centres in reflex activity. These centres, in the higher verte- 

 brate animals (which are those that we know well enough to 

 theorize about), are the following : 



(i) The grey matter of the central columns of the spinal cord. 

 (The peripheral columns are tracts of nerve-fibres.) This grey 

 matter is primitively segmentally arranged and is still function- 

 ally segmental. For each pair of spinal nerves in connection 

 with the cord there is a ganglionic region. (But these overlap.) 



