132 HUMAN LONGEVITY. 



The simple but certain fact is what we have already 

 stated, viz. that the healthiness of the organs ministering to 

 the several functions of life constitutes -the health of the 

 man ; and in the same proportion tends to prolong his age. 

 Though physiologists hold some verbal dispute 011 this point, 

 yet can we hardly define Vitality otherwise than as a force or 

 power, acting through the intervention of the nervous system 

 upon organic structures, and depending upon the integrity of 

 all these parts for its own amount and completeness. While 

 admitting that the power is one which controls, and often 

 seemingly contradicts, the physical laws most familiar to us, 

 still we see that it is a power generated within the body ; that 

 matter and organisation are necessary to its action ; and that 

 by these it is variously and unceasingly altered through 

 every part of individual existence. We are enabled, and 

 indeed almost compelled, to speak of it as a quantity ; 

 varying in different individuals by original organisation ; 

 in each fluctuating continually during life ; and reduced 

 to its minimum where life is prolonged into old age. What 

 poetry has described as the blood in ' languid eddies loitering 

 into phlegm,' may not be physiologically true ; yet it is in 

 some sort sanctioned by the doctrine of a great physiologist 

 as to the especial vitality of this wonderful fluid ; of which, 

 even now, we do but partially know the physical properties 

 and the changes it undergoes in health and in disease. 



Carrying this view into the practical question how vitality 

 may best be maintained and prolonged into old age, we 

 must look mainly to four general conditions, which may be 

 said to include all that is most essential to the fulfillment 

 of the object. These are, air, as belonging to the function 

 of respiration; aliment; exercise of the body; and 

 exercise of the mental functions. 



The first of these topics that of the air we breathe - 

 presents more difficulties than might be supposed ; owing in 



