276 LEISURE-TIME STUDIES. 



latter. This second impulse, which may simply consist of 

 the first or sensory one directed or " reflected " into a new 

 channel and modified by the brain, we term a "motor" 

 impulse, because, as we have seen, its office is that of pro- 

 ducing motion in muscles. If, now, we take a wide survey 

 of the field of animal life, we shall find that "reflex" nerve- 

 action forms the apparently universal rule wherever bodily 

 action follows upon the outward stimulation of the world. 

 It is immaterial whether the original impulse comes from 

 the nervous system or from the world ; in any case it is 

 "reflected" from the great nerve-centre to muscles, to a 

 sense-organ, or to some other part or tissue of the body. 

 When we " will " to perform any bodily action, such as that 



FIG. 46. Diagrams of nerve-action. 



of touching a table, the thought or idea generated in the 

 brain (B) passes outwards on its "motor" journey, and puts 

 the muscles of the finger (F) in movement or in action ; and 

 we are made aware that the act has been accomplished only 

 through a second or "sensory" impression which has been 

 transmitted or reflected to the brain. Similarly when the 

 "mouth waters," that is, when saliva is secreted rapidly 

 and enters the mouth, at the sight of some " dainty dish,' r 

 the sensory impulse flashes from the eye (E) to the brain (B), 

 and is thence " reflected " as a motor or " secretory " impulse 

 to the salivary glands (s), with the effect of producing the 

 familiar result. When we touch the tip of a snail's tentacle 

 or feeler, the feeler itself is rapidly withdrawn, and the 

 animal itself retreats within its shell.. Reflex nerve-action 

 evidently holds sway here, just as in man. For the sensory 



