244 BIOLOGY AND HUMAN LIFE 



rally ; many people never do them at all. And they are not 

 done "on purpose" each time, for those who have the habit do 

 not stop to think each time. 



201. Inhibition. The pike and the burnt baby illustrate what 

 may be called negative habit, the habit of avoiding or repressing 

 an action. Thus, Don't spit ! signs are supposed to restrain the 

 impulse to eject something from the mouth. The process of 

 sending out a nerve impulse to stop an action started by an 

 earlier impulse is called inhibition, and inhibition is just as 

 important a part of our control as is doing. All movements can 

 be performed skillfully or accurately just in proportion as a 

 person has had practice in doing and inhibiting. 



202. How practice makes habits. Many boys try their mus- 

 cles from time to time, to see how they are coming on. This 

 growth as a result of exercise probably comes from the facts 

 (i) that the contraction of the muscles calls forth a reflex that 

 increases the flow of blood, and (2) that with increased nourish- 

 ment the muscle fibers increase in size or in number during the 

 rest between exercises. The case is not so simple when the 

 neurons are exercised. It appears that the number of neurons 

 does not increase after birth. When the nervous system de- 

 velops, the axons and the dendrites grow out. The outgrowths 

 of the nerve cell have been compared to the pseudopodia of the 

 ameba, for, like the pseudopodia, they are extensions of the 

 protoplasm. Unlike the pseudopodia, these extensions cannot 

 be withdrawn ; but, like the pseudopodia, they take part in the 

 whole life of the cell. The whole cell, including the very ends 

 of the finest branches, acts as a unit. As a nerve cell is exer- 

 cised (by receiving impressions, by sending out stimuli, or by 

 discharging its energy in some other way) these extensions of its 

 protoplasm are formed : and this is the basis for the associations 

 that modify the conduct of the animal as it gets older. 



The development of the ability to do things or to control the 

 organs generally is thus the result of use (exercise) or disuse. 

 With advancing age the neurons, like other cells, become less 

 and less capable of growth and of forming new associations. 



