VOLUNTARY ACTION 237 



them, in this case the muscles of the lid, contract and 

 the eyelid winks, thus protecting the eyeball from the 

 object in the shortest possible time. 



Voluntary action. In case the sensory impulses are 

 not strong enough to produce reflex movement, they 

 pass without overflow to the cortex of the brain. There 

 they excite certain of the higher nerve centres, and as 

 a result we are conscious of sensations. These nerve 

 centres in turn excite still other nerve centres, until 

 eventually motor nerve centres are stimulated and motor 

 impulses are transmitted to the muscles, thereby pro- 

 ducing movement. For example, as we sit at the break- 

 fast table, the waves of light reflected from a glass of 

 water pass into the eyes and there start up sensory 

 nervous impulses which travel along the optic nerves to 

 the portion of the cortex known as the visual centre. 

 This nerve centre is excited, with the result that sensa- 

 tions are aroused, that is, we see a glass of water. The 

 sensory impulses from the visual centre may then be 

 communicated through associative nerve fibres to other 

 centres, the excitation of which results in our desiring 

 the water. Thereupon the proper motor nerve centres 

 are stimulated and in consequence we reach for the 

 glass of water. This kind of action which involves the 

 higher nerve centres of the cortex, is called voluntary 

 action, in contrast with reflex action, which results 

 from the stimulation of motor neurons in the spinal cord 

 or base of the brain because of the overflow of excep- 

 tionally strong impulses. 



Habits. If a voluntary act is repeated many times, 

 it tends to become habitual. The same sensory impulses 

 excite the same nerve centres and the same movements 

 result, until finally it is difficult for them to act in any 

 other way. In other words, a habit has been formed. 



