THE SENSORI-MOTOR SYSTEM 13 



The Sensori-Motor System. 



The organs of mobility include the skeleton, which is a system 

 of rigid and jointed bones to which the muscles are attached; the 

 sense organs, by means of which the animal comes into relation 

 with its environment; and the nervous system, which is the link 

 between the sense organs and the muscles. The animal is placed 

 in the midst of an environment which continually changes, or it 

 moves about from place to place^ and therefore the conditions 

 under which it lives are variable. It must find shelter, and the 

 nature of this will vary with the climate and seasons; it must 

 find food, and the nature and abundance of this also changes; 

 and it must avoid its natural enemies. Therefore it must become 

 aware of the events that proceed outside itself, and that is why 

 it sees, hears, smells, and " feels." The events that occur in 

 the environment affect or stimulate its sense organs; but that 

 is not enough, for the stimuli must result in actions of some kind. 

 Now we can imagine the stimulation of an organ of sense to 

 produce two main kinds of response : First, the response may be 

 mechanical, constant, and predictable, just such a response that 

 would occur in a model that we could easily construct. In an 

 artificial animal or automaton we press a spring, and the machine 

 begins to walk or to lift a limb, or rolls its eyes, but the result of 

 pressing the spring will always be the same. Tropisms that is, 

 the kind of responses to which we referred on p. 9, and which 

 we shall consider in greater detail in Chapter VIII. tend to be 

 responses of this nature, and a large number of the habitual 

 actions performed by animals belong to such a category of 

 relatively fixed, mechanical movements. Secondly, the responses 

 may be adaptive that is, the thing that happens when a sense 

 organ is stimulated depends on the circumstances of the moment 

 and on the experience of the animal. Thus, a man walking 

 along the street may pass a stranger without doing more than 

 look casually at the latter. If, however, the same person sub- 

 sequently becomes known to him he may thereafter nod, or bow, 

 or stop and speak. Yet the physical stimulus of the organ of 

 sense is the same in the two cases, and the difference of the 

 response depends upon the intervention of the cerebral nervous 

 system. 



With these preliminary remarks we may now consider the 

 physical apparatus of movement. 



