114 ANIMAL BEHAVIOUR 



those that extend it : in all cases when a muscle contracts an 

 antagonistic muscle relaxes, and vice versa, thus two efferent, 

 motor tracts are involved and two sets of synapses in the ganglion 

 part of the spinal cord in this case. Only one receptor (in the 

 skin) is shown, but there are very many, for the reflex act may 

 be elicited by tickling a considerable region of skin. But the 

 reflex (in general) may also be elicited from other parts of the 

 body and so another aff"erent path is also indicated. Further, 

 many analogous reflexes may be obtained by stimulating the 

 side of the body opposite to that on which the eifectors are 

 placed and such paths are not shown. When a muscle contracts 

 its proprio-ceptors are stimulated and so afferent impulses from 

 the contracting muscle itself are sent into the ganglion — these 

 are also indicated in the diagram. Lastly, the reflex may not 

 occur if the dog notices that he is being teased (or he may do 

 something else — growl or bite) and so there must be efferent 

 paths from the brain (the higher ganglion) to the region of spinal 

 cord concerned (the lower ganglion) and impulses descending 

 these latter paths may arrest, or inhibit an incipient, or potential 

 reflex act : these latter paths are indicated. 



Thus any reflex scheme must be simplified to an extraordinary 

 degree before it can be represented diagrammatically. 



All elements of behaviouristic activity are effected in some 

 such way (remembering the limitations of description suggested 

 above). Chains and combinations of reflex acts make up complex 

 bodily activities. In a limited way such mechanisms as are 

 indicated here may completely describe behaviour. In all cases 

 they are the means of behaviour, though what we shall call 

 *' experience " profoundly modifies the workings of these nerve- 

 muscle mechanisms. Presently we shall return to this subject 

 of reflex activity. 



II. SENSATION AND PERCEPTION 



When a receptor organ is stimulated a change in the state of 

 consciousness of the animal may be experienced : if, for instance, 

 there is a lightning flash, followed by violent vibrations of the 

 atmosphere, w^e may " have " the sensations, or sense-impressions, 

 or changes of consciousness which we call light and sound. In 

 this case the " having " implies both sensation and perception. 



