HABIT 405 



heart, etc. These organs send word of their wants to the 

 central nervous system, and get back the efferent im- 

 pulse, without our knowing or willing anything about the 

 operation. Only when the organs are injured do the 

 impulses which they send get to the cerebrum, so that 

 we feel pain. This "sensation of injury " gets past the 

 lower part of the central system because it is unusual; 

 there are no associating neurons to turn it back until it 

 reaches the cerebrum. 



But it is not only our internal organs that act reflexive- 

 ly; many of our external movements are not voluntary. 

 Thus, the blinking of the eyelids, which is for the pur- 

 pose of cleaning and moistening the eyeballs, is invol- 

 untary. If it were voluntary, we might forget to do it. 

 When you put your hand against a hot stove, you jerk 

 away the hand before you feel the pain. The "jerking 

 away" is a reflex act. It is so important for your body 

 that your hand shall not be burned, that a part of the 

 afferent impulse is switched over into the efferent chan- 

 nels without going to the cerebrum. But it is necessary 

 for your mind to know that hot stoves burn your hand, so 

 the message is also sent to the cerebrum, and becomes a 

 part of your memory. 



406. Habit. Why is it that the control of so much 

 that the body does is left to involuntary, reflex action? 

 We can understand some of the reasons very readily. 

 In the first place, reflex action relieves the cerebrum and 

 the mind of a great deal of work. Then, by being freed 

 from the duty of controlling the internal organs, the 

 cerebrum has time to do a much higher kind of work for 



