FROG : NERVOUS SYSTEM AND SENSE ORGANS 77 



pulses are carried, not directly from organ to organ, but 

 from each organ to the central nervous system, whence, if 

 action is to take place, fresh impulses are directed to other 

 organs. It is by the intervention of the central nervous 

 system that there take place in an orderly manner the very 

 complex responses which the simplest stimuli evoke in the 

 body of one of the higher animals. Even such slight and 

 passing actions, for instance, as a leap from danger or the 

 snapping up of an insect involve the harmonious action of 

 numerous muscles in a manner which would be impossible 

 without some co-ordinating mechanism. 



Fig. 40. — A diagram of the "reflex arc.'' 



af.n., Afferent nerve ; c.n.s., central nervous system ; ef.n., efferent nerve ; 

 mus., muscle ; sen. , sensory surface. 



The actions which are excited through the nervous system 

 are of two kinds, reflex and voluntary. A reflex action is one in 

 which stimulation of an afferent nerve brings about through 

 an efferent nerve the production of activity in some tissue 

 in an involuntary manner. Thus touching the eye brings 

 about contraction of the muscle of the eyelid so that blinking 

 takes place, but this happens without any effort of the will of 

 the animal, which exercises no choice as to whether- it shall 

 take place or not. In a reflex action the same Stimulus is 

 always followed by the same response. In blinking the action 

 is conscious, but many reflex actions are purely unconscious, 

 as when the passage of food over the opening of the bile-duct 

 causes through the central nervous system a discharge of bile 



