154 LETTERS TO BROTHER JOHN. 



which they are produced are supplied. And the 

 quantity of blood with which these organs are 

 furnished must depend upon the vigour and activity 

 of the heart and arteries, whose office it is to convey 

 it. Thus, then, it becomes clearly manifest, that a 

 vigorous circulation is absolutely necessary to the 

 assimilation (vulgarly called digestion) of our food. 

 Whatever causes and habits of life, therefore, are 

 calculated to give strength and activity to th e circu- 

 lation as, for instance, exercise is clearly of the 

 first importance to the nutrition, and, therefore, to 

 the health and strength of the body ; and whatever 

 causes and habits have a tendency to depress the 

 energy of the circulation to allow the blood to 

 creep languidly through the body, instead of dancing 

 along its channels cheerily and energetically as, 

 for instance, cushioned laziness, which rides when it 

 should walk must, of necessity, have the direct 

 effect of impairing assimilation, and therefore of 

 enfeebling the strength and sapping the very foun- 

 dations of health. 



But the energy of the circulation must exclusively 

 depend upon the energy of the heart and arteries ; 

 and the energy of these, as has been already shewn, 

 depends necessarily upon the energy of their con- 

 tractility ; and energetic contractility depends on 



