332 THE GROWTH OF THE BRAIN. 



ruption of a portion of those paths by which the im- 

 pulses pass from one element to another need cause the 

 complete disappearance of the elements thus discon- 

 nected, especially if the break involves only a few of 

 the terminals of the nerve fibre ; and so the slow physio- 

 logical disorganisation of the completed central system 

 is not necessarily accompanied by a corresponding 

 amount of atrophy. 



The old age of the central system is in a measure 

 independent of the degree to which it is exercised, 

 unless the exercise be so excessive as to cause continual 

 and extreme exhaustion. So far as known the lumbar 

 enlargement of the sedentary student does not grow old 

 faster than that of a professional runner, and on the 

 other hand there is no evidence to show that the best 

 exercise of the hemispheres does clearly postpone in 

 them the involutionary processes. 



It can be easily calculated how many times the heart 

 will beat, the lungs expand, and each of the many bodily 

 functions repeat itself in the course of an ordinary life. 

 Among these functions are the regular changes in the 

 nerve cells. Without doubt this rhythm (recuperation 

 and fatigue) repeats itself each day, and thus all persons 

 undergo alterations of the nervous system as regular as 

 those which occur in stature, but the intensity of this 

 change is an individual matter, and perhaps not funda- 

 mentally important as a physiological phenomenon. 

 Viewed from this side, therefore, much truth is contained 

 in the observation that, though the differences between 

 men are slight, yet such as they are, they amount to 

 a great deal. 



All through life, one after another, the various capa- 

 bilities drop away. The power of visualisation is lost, 

 pleasure in music disappears, memory becomes weak, 

 save in narrow lines ; a new language, a new science, 



