iv REFLEX ACTION 41 



favourable to its own existence. If the mouse were not 

 stimulated to run- away by the sight of the cat, it would 

 not be long for this world, and if the cat were not so 

 excited by the sight of the mouse and of other food as to 

 make the movements necessary for catching, seizing, and 

 eating, she in her turn would starve. The most primitive j 

 form in which response is adapted to requirements is that I 

 in which a simple sensory stimulus calls forth a uniform 1 

 reaction on the part of the organism. Such a response is 

 known as a Reflex action. This is an extended usage, and 

 etymologically is, it must be admitted, not altogether 

 appropriate. The term applies strictly to animals with a 

 developed nervous system. In such animals the reflex act 

 consists of two distinct movements or processes, a sensory 

 or afferent process, and a motor or efferent process. Thus 

 if one inadvertently touches a hot iron the burning of the 

 hand sends a wave of excitement along the sensory nerves 

 to some part of the central nervous system. This is the 

 sensory or afferent process. From the brain descends a 

 back wave of excitement causing the contraction of several 

 muscles whereby the hand is withdrawn. This is the 

 efferent or motor process. The two together with what- 

 ever central process is required to connect them 1 make up ' 

 a reflex action. 



But the above definition, which corresponds to the 

 extended usage of the term now common, applies very 

 clearly to the responses of many animals which have no 

 nervous system at all. If the long hair-like " flagellum " 

 of a Poteriodendron be lightly touched as it waves about 

 in the water, the effect is instantaneous. The waving 

 thread suddenly rolls itself up in a coil, while the body, 

 which consists of a mass of protoplasm standing on a 

 sort of stalk, fixed to the bottom of a miniature cup, is 

 withdrawn hastily to the bottom of the cup, where it 



1 The more modern definitions of reflex action make a different 

 partition, distinguishing a receptor organ, where the excitement starts 

 \'g.) the retina), an effector, which carries out the response (e.g., the 

 muscles of the arm), and a conductor, which includes the entire nervous 

 portion between the two. The centre is in this description only of 

 importance as a name for any point or points where different conductors 

 may meet and different impulses impinge on one another (see Sherring- 

 ton, Integrative Action of the Nervous System, p. 7, &c.). 



