206 LETTERS TO BROTHER JOHN. 



of a watch would be injured by any cause which 

 weakened the elasticity of its main-spring. The 

 watch would lose time its movements would be too 

 feeble it would go too slow. So with the human 

 machine : its actions are enfeebled its nutrition 

 is languidly and imperfectly performed it goes 

 too slow. The indication of time is the final cause 

 of the movement of the watch ; and the nutrition 

 of the body is the final cause of the nutritive actions. 

 Both these final causes depend upon the healthy 

 existence of the first cause the elasticity of the 

 main-spring in the watch, and contractility and sen- 

 sibility in the body. It is manifest, therefore, that 

 whatever weakens the energy of the first cause 

 must have a direct tendency to interfere with, and 

 prevent the fulfilment of, the final cause ; that is, in 

 the human machine, the nutrition of the body. It 

 is this destructive influence of venous blood on the 

 life of our organs which has caused me purposely 

 to speak of it with so much bitterness, whenever 

 I have had occasion to mention it. I did so in 

 order that your mind might be thoroughly im- 

 pressed with a sense of the great and necessary 

 distinction to be drawn between venous and arte- 

 rial blood. It is difficult to make this impression 

 on the mind of a person not conversant with 



