The Nervous System. 269 



impulse along the sensory nerve is not simply reflected 

 into a motor channel, as the name Avould imply, and there- 

 fore the term ' reflex,' though retained by custom, is not a 

 correct explanation of what really occurs. 



Other reflex acts are still more complicated, as in walk- 

 ino - , trotting, etc. Here the animal is completely uncon- 

 scious of the various groups of muscles to be brought into 

 play ; the whole process is carried out independently of 

 consciousness — in fact, if the horse had to think of each 

 step to be taken, it would soon be worn out and certainly 

 blunder. 



So far the illustrations we have given of reflex action are 

 those resulting in movement; but a reflex act is not essen- 

 tial a motor act, it may be a secretory or a nutritive one, 

 depending on the character of nerve sending out the 

 efferent impulse. In order, therefore, that a reflex act may 

 be accomplished certain conditions must be present : (1) an 

 afferent nerve, to convey the impression to the nerve 

 centre ; (2) a nerve centre, in which the outgoing impulses 

 are generated ; (3) an efferent nerve, to convey these impulses 

 outwards. 



It must not be considered that reflex actions can only 

 take place along nerves belonging to the same system ; the 

 afferent or efferent nerves may be both cerebro-spinal, or 

 one or the other may belong to the sympathetic system, or 

 both may belong to the sympathetic system. 



It is by the regular and apparently intelligent carrying 

 out of reflex actions that the normal condition of the body 

 is maintained. Swallowing, secretion of saliva, gastric 

 juice and other fluids, respiration, intestinal movements, 

 etc., are all examples of this condition. 



There are certain reflex acts of which the animal can 

 take cognizance, though it can take no part in them. 

 These are termed ' sensori-motor '; those which it is unable 

 to perceive are spoken of as ' excito-motor.' 



There are certain reflex movements in man which are 

 acquired only as the result of education — for example, 

 walking. In animals these so-called co-ordinated reflex 



