ORGANS OF THE SENSORI-MOTOR SYSTEM 107 



specialized sense-organs (eyes, auditory organs, taste and smell 

 organs) tend to be situated in that part, or head, which precedes 

 the other parts in motion. And, since in moving forward by 

 paired locomotory organs, the animal experiences fields of force 

 to right and left, there is a corresponding bi-lateral symmetry 

 in the great distance-receptors. 



39^. The Conduction of Stimuli. The receptor organ is 

 essentially the termination of a nerve-fibre (Section 12^). This 

 termination, of many different forms, or structures, is different 

 from the rest of the nerve-fibre. It is the termination that is 

 stimulated by, or reacts with the external physical agency. 

 But being affected by, or changed by interaction with the external 

 agency the nerve-termination originates a physical disturbance 

 called the nervous-impulse and this is propagated along the 

 remainder of the nerve-fibre. 



Thus when a sense-organ is stimulated an impulse is set up 

 in the nerve " attached to " it and this impulse travels up into 

 the central ganglion in which the nerve ends. Everywhere in 

 the body of the higher animal, but mainly in the head and in 

 the skin in general, there are receptor organs that are susceptible 

 of being stimulated by physical events occurring in the cavities 

 of the body, in the bodily tissues, on the surface of the skin, 

 outside the body and at the remotest parts of the universe. In 

 the higher animal, and particularly in man, it is these latter 

 distance-receptors that have become of increasing significance. 

 In civilized man those natural and artificial receptors that are 

 stimulated by radiant energy have now become the chief means 

 whereby he comes to act upon and know his environment, while 

 the near-receptors have become of less significance. Smell and 

 taste now count far less than hearing, and hearing is of less 

 significance than is the reception of electro-magnetic radiation. 



Whatever they may be, the stimuli that originate in the 

 receptor organs are conducted to the central ganglia via the 

 afferent nerves. 



40. OAT NERVOUS CONDUCTION 

 Events occurring inside or outside the animal body stimulate 

 receptor organs, which are essentially the peripheral ends of 

 afferent nerves. 



