NUTRITION .^ 115 



that which was there is taken away, and its place is sup- 

 plied with white material. 



250. During the middle periods of life, these two sets of 

 vessels are equally active, and usually perform a similar 

 amount of work. The arteries carry and deposit as many 

 atoms in the flesh as the veins and absorbents carry away. 

 The one builds up as fast as the other pulls down ; so that, 

 though some pounds are added daily to the body, it does not 

 gain in weight ; neither does it lose, though as much is car- 

 ried away. But this is not the case at all the periods of life, 

 nor in every condition of health. 



251. During the period of youth, and the time of in- 

 creasing flesh, nutrition predominates ; more atoms are then 

 brought in the arteries, and deposited, than are taken away 

 by the absorbents. When the body is wasting, absorption 

 predominates, and carries off more than the arteries deposit. 

 When we grow fat, nutrition is the more active; but when 

 we grow lean, absorption prevails. The whole of the move- 

 ments of life are more rapid in the earlier years, and 

 slower in old age, than in the middle periods of life. The 

 heart beats quicker, the flow of blood is more abundant, and 

 both nutrition and absorption are more rapid in the former, 

 and slower in the latter period. 



252. Liebig supposes that every action of any of the 

 parts of the body is attended with change of particles ; that 

 when a finger moves, some of the atoms in the muscle that 

 produces the motion die and leave their places, and are re- 

 placed by others. When we move the arm, the legs, or use 

 the muscles of any other part of the body, the same changes 

 take place in the muscular atoms. All exercise increases 

 the activity of both the nutrition and absorption. In order 

 to meet the increased demands for new flesh to supply this 

 waste during exercise, the heart beats quicker more blood 

 is carried to the moving parts ; and thus more new flesh is 

 formed, as long as more is absorbed. Every one knows that 

 the heart beats rapidly, and sometimes almost palpitates, 

 while we run or otherwise exercise violently. Those who 



