276 LETTERS TO BROTHER JOHN. 



But the true reason why a few can commit ha- 

 bitual intemperance with comparative impunity is, 

 because, in these persons, the imjjression-receiving 

 power (sensibility) is naturally exceedingly dull 

 while the impression-resisting power (viz. the cir- 

 culation) is naturally extremely vigorous. Their 

 blood, propelled by a large and powerful heart, 

 and rapidly and thoroughly oxidized by capacious 

 lungs, is driven, with energetic force, to every point 

 of the body ; thus not only enabling it to resist the 

 impressions of the deleterious matters introduced 

 into the stomach, but also rapidly to repair what- 

 ever slight injuries are really inflicted. 



There is yet another reason why bodily exercise 

 is indispensable to health. The blood is wholly 

 incapable of fulfilling any of its multifarious and 

 all-important offices (except the secretion of bile) 

 until it has been oxidized in the lungs. The more 

 rapidly, then, that it is driven through the lungs, 

 the larger will be the proportion of it which is 

 oxidized, and so rendered fit to fulfil its function 

 of nourishing the body the greater will be the 

 proportion contained in the arteries (where alone 

 it is of use)-, and the smaller will be the quantity 

 of black blood left in the veins (where it is of no 

 use, except as before excepted). 



