118 PHYSIOLOGY AND HEALTH. 



petual freshness of youth in their flesh; they are, there- 

 fore, lively and prompt in action. These changes take place 

 more slowly in the inactive; their atoms remain a longer 

 time; and their flesh is therefore always old, and indisposed 

 to action. It is easy to see this difference between the en- 

 ergy and sprightliness of one who has always accustomed 

 himself to action abroad, and the heavy sluggishness of 

 another, who has lived delicately, and avoided exercise. 

 Compared with his years, the one is ever young, while the 

 other is ever old. 



258. We have no means of knowing how, or by what 

 means, the final work of nutrition is done. We only know 

 the instruments with which it is accomplished, and the ma- 

 terials that are used. Anatomists have examined the blood- 

 vessels, and chemists have analyzed the blood, and have 

 taught us the shape of one and the composition of the other ; 

 and there our knowledge stops. We cannot penetrate any 

 farther into the mysteries of nature. So far as the eye of 

 man can discover, the blood-vessels are the same in structure 

 and character, in all the prgans and tissues of the body, 

 and the same blood is found in all. And yet, with a wonder- 

 ful precision, these little nutrient vessels select out of this 

 common storehouse of nutriment just those elements, and in 

 just their varied proportions, that are needed to form the 

 various kinds of flesh and substance that compose the ani- 

 mal body. 



259. In the fat, the organs of nutrition select from the 

 blood 79 parts of carbon, 11 parts of hydrogen, and 9J- 

 parts of oxygen ; and with these form fatty atoms. In the 

 hair, they take 50 parts of carbon, 6 parts of hydrogen, 17 

 parts of nitrogen, and 26 parts of oxygen and sulphur, and 

 make an atom of hair. And from the blood in the muscle, 

 they take 51 parts of carbon, 7 parts of hydrogen, 15 parts 

 of nitrogen, 21 parts of oxygen, and 4 parts of other mat- 

 ters, and form muscular pai deles. In a similar manner, the 

 vessels of the brain select the brain ; and in the skin, and 

 in all other organs, they select the very kinds and proportions 



