LETTERS TO BROTHER JOHN. 279 



may be ridden to death ; and, therefore, bodily 

 exertion may be carried too far. But there is no 

 danger of a man undergoing too much exertion 

 voluntarily, and for his health's sake. Pain and 

 fatigue will always operate as sufficient, nay, even 

 irresistible, restraints. 



I have said, that persons of sedentary habits 

 become frequently sensible of a feeling of want 

 a sinking at the stomach, as they express it ; which 

 they seek to relieve by eating or drinking. I have 

 said, too, that although these persons require the 

 excitement of a stimulus, yet food or wine does not 

 furnish the stimulus required, but, on the contrary, 

 only adds to the evil. 



You know I have all along mentioned four things 

 as necessary to life ; one of which, you cannot have 

 forgotten, is STIMULI. But I shall disuse the word 

 " stimuli " ; because, being used in the plural, it is 

 awkward to introduce it correctly without periphra- 

 sis ; and I will use the word " excitement " instead. 



The exciting properties of arterial blood I have 

 just been describing to you, and shewing you how 

 rapid exercise produces its exhilarating effects ; viz. 

 by increasing the quantity of arterial blood, and 

 driving it, in rapid currents, through all the count- 

 less avenues of the brain and body. It is to the 



