246 ELEMENTARY BIOLOGY 



own lives. It is the foundation of all our learning, whether 

 learning to know or learning to do or learning to like. It is 

 also the foundation of our learning to avoid doing, or to stop 

 impulses to action. 



295. How neurons develop. We know from experience that 

 the muscles grow with exercise. The size of a muscle grows 

 measurably in a few weeks or months, and many boys try 

 their muscles from time to time, to see how they are coming 

 on. The explanation of this growth lies probably in the fact 

 that the exercise (contraction) calls forth a reflex that increases 

 the flow of blood, and that with increased nourishment the 

 muscle fibers increase in size or in number during the resting 

 periods between exercises. Now this sort of thing does not 

 take place so simply when the neurons are exercised. It ap- 

 pears that the body does not increase the number of neurons 

 after birth. But the axons and the dendrites may grow out, 

 and this is what happens when our nervous system develops. 

 The outgrowths of the nerve cell have been compared to the 

 pseudopodia of the ameba, as they are protoplasmic extensions 

 beyond the general outline of the cell, and depend upon the 

 cell's activities. Unlike the pseudopodia, they cannot be with- 

 drawn ; but, like the pseudopodia, they partake of the life of 

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

 branchings, acts as a unit. As a nerve cell is exercised, by 

 receiving impressions, by sending out stimuli, or by discharg- 

 ing its energy in some other way, these extensions of its 

 protoplasm are formed. 



296. Learning in youth. It is by means of the expanding 

 outgrowths of the cells that new connections are formed, and 

 this is the basis for the associations that modify the conduct 

 of an animal as it gets older. Like other cells of the body, 

 the neurons are less and less capable of growth with advancing 

 age. Any associations that are to be formed must be made 

 before the neurons are too old. The development of the 

 ability to do things or to control the organs generally is thus 



