THE STORY OF THE BRAIN AND NERVE 111 



purpose of affording information to the brain through 

 a particular channel. The impression of imminent 

 danger is conveyed along the optic nerve, or that of 

 sight, arousing in other parts of the brain conscious- 

 ness of the attack about to be made. The message 

 thus received by certain brain centres is instantly 

 " reflected " to other centres, namely, those govern- 

 ing the muscles of the neck, the result being that 

 these muscles, stimulated to action, withdraw the 

 head from the threatened blow. In this case, the 

 incoming, or afferent message is "reflected," and 

 converted into an outgoing or efferent message. It 

 may be, however, that the simple reflex action may 

 become more complicated and more extended in its 

 range. For if the threatened individual seeks safety 

 in flight, the reflex action will extend to the stimula- 

 tion of the muscles of the legs and body, carrying 

 him out of the reach of danger. A man crossing 

 the street hears the sound of a rapidly driven vehicle 

 coming up behind him. In this case the primary 

 message is received by his ear, and the same process 

 is gone through as in the case of the eyes. The 

 message is "reflected" from the brain and sent to the 

 muscles by centres governing these organs so as to 

 hasten the man's footsteps, and thus enable him to 

 seek safety on the pavement. When the mouth 

 waters at the sight of something good to eat, the 

 message conveyed from the brain is " reflected," in this 

 case, to the salivary glands of the mouth, producing 

 a copious flow of saliva. Many more illustrations 

 of reflex action might be given, but suffice it to say 

 that the principle animating all our acts, whether of 

 simple or complex nature, is the same. Occasionally 

 a reflex action starts from the outside of the body, 

 as, for example, when the finger comes in contact 



