THE SENSORI-MOTOR SYSTEM 25 



distances away from the body. The olfactory, gustatory, and 

 touch organs are near receptors, for they can only be affected 

 by substances that actually come, into contact with the body. 

 Our sensation of temperature may be a near affect (as when a 

 cold or hot substance touches the skin), but it may also be a dis- 

 tance affect (as when we are warmed by the radiation from the 

 sun). Now there are receptor organs which are stimulated, or 

 affected, by changes that occur within the body itself; thus when 

 we lift anything we have a feeling of effort (the muscular sense) ; 

 when we walk we have also sensation arising from pressure set 

 up in the joints; when the body is in different postures there is 

 sense of equilibrium, which is something quite apart from seeing 

 or hearing (for it can arise when a man is blindfolded and has 

 his ears plugged); and there is also general sensation from the 

 viscera, for a man may experience stomach-ache. In these 

 cases receptor organs in the muscles, tendons, joints, ears, and 

 viscera are stimulated, producing what we call proprio-sensation. 

 Emotion (that which moves us), blushing, the pallor of fear, 

 trembling, rigidity, even the " Rabelaisian effect of fear on the 

 bowels " (" affective tone or feeling," as it has been called), are 

 the results of the stimulation of the proprioceptors. 



Here, in spite of the warning given above, we are speaking 

 of sensation as it results in " feeling," or consciousness, because 

 it is very difficult to avoid doing so ; but, nevertheless, the activity 

 of the receptor organs, in so far as we are about to relate it to 

 the initiation of movement, need not be accompanied by any 

 affection of the mind. 



The Nervous Links. 



Certain mechanisms intervene between the receptor organs 

 on the one hand and the motor organs on the other; these are 

 the peripheral and central nervous systems. Proceeding from 

 a receptor organ there is an afferent nerve (" afferent " because 

 it conveys something to the central nervous system), and pro- 

 ceeding from the motor organ there is an efferent nerve (" efferent " 

 because it conveys something from the central nervous system). 

 The reader must understand clearly that a motor organ is (in 

 general) not directly stimulated to act by something that occurs 

 outside the body ; it is stimulated via the central nervous system. 

 Receptors and afferent nerves are therefore " the way into " the 

 brain and spinal cord, while efferent nerves and motor organs 



