THE NERVOUS SYSTEM 



407 



and these, together with observations made on persons who had 

 lost the power of movement of certain parts of the hodv, and who, 

 after death, were found to have had diseases localized in certain' 

 parts of the cerebrum, have given to us our knowledge on this sub- 

 ject. 



Reflex Actions, their Meaning. — If through disease or for other 

 reasons the cerebrum does not function, no will power is exerted, 

 no intelligent acts per- 

 formed, and no in- ^al^^B^^^ 

 coming impulses are 

 received as sensations. 

 All acts performed in 

 such a state are known 

 as rejiex actions. An 

 example of a reflex 

 may be obtained by 

 crossing the legs and 

 hitting the knee a sharp 

 blow. The leg, below 

 the knee, will fly up as 

 a result of reflex stimu- 

 lation. The involun- 

 tary brushing of a fly 

 from the face or the 

 attempt to move away 

 from the source of an- 

 noyance w^hen tickled 

 with a feather, are other examples. In a reflex act, a person does 

 not think before acting. The nervous impulse comes from the 

 outside to cells that are not in the cerebrum. The message is 

 short-circuited back to the surface by motor nerves, without ever 

 having reached the thinking centers. The nerve cells which take 

 charge of such acts are located in the cere]-)ellum or spinal conl. 

 But some reflex acts may be controlled in part by the will. Wo 

 need not, for instance, brush away a fly, although we cannot stop 

 the churning of the stomach or the process of breathing, yet sud- 

 den fright may cause us to take a certain sharp or long breath or 



Regions of the head and action of the different partx 

 of the brain. 



