30 Additional Notes. 



stimulus, which though it be employed in quantity beyond its usual 

 state, seems to increase the production of sensorial power bevoncl the 

 expenditure of it (unless its excess is great indeed) and thence to 

 give permanent strength and energy to the system; I mean that of 

 volition. This appears not only from the temporary strength of 

 angry or insane people, but because insanity even cures some diseases 

 of debility, as I have seen in dropsy, and in some fevers; but it is 

 also observable, that many who have exerted much voluntary effort 

 during their whole lives, have continued active to great age. This 

 however may be conceived to arise from these great exertions being 

 performed principally by the organs of sense, that is by exciting and 

 comparing ideas; as in those who have invented sciences, or have 

 governed nations, and which did not therefore exhaust the sensorial 

 power of those organs which are necessary to life, but perhaps rather 

 prevented them from being sooner impaired, their sensorial power not 

 having been so frequently exhausted by great activity, for very 

 violent exercise of the body, long continued, forwards old age; as is 

 seen in post-horses that are cruelly treated, and in many of the poor, 

 who with difficulty support their families by incessant labour. 



III. Theory of the Approach of Age. 



The critical reader is perhaps by this time become so far interested 

 in this subject as to excuse a more prolix elucidation of it. 



In early life the repetition of animal actions occasions them to be 

 performed with greater facility, whether those repetitions are pro- 

 duced by volition, sensation, or irritation; because they soon become 

 associated together, if as much sensorial power is produced between 

 every reiteration of action, as is expended bv it. 



But if a stimulus be repeated at uniform intervals of time, the 

 action, whether of our muscles or organs of sense, is performed with 

 still greater facility and energy; because the sensorial power of asso- 

 ciation mentioned above, is combined with the sensorial power of 

 irritation, and forms part of the diurnal chain of animal motions; that 



