NUTRITION. 45 



CHAPTER VI. 



NUTKITION. 



Nutrition is the earliest and most constant of vital op- 

 erations. So prominent is the nutritive apparatus, that 

 an animal has been likened to a moving sac, organized to 

 convert foreign matter into its own likeness, to which the 

 complex organs of animal life are but auxiliaries. Thus, 

 the bones and muscles are levers and cords to carry the 

 body about, while the nervous system directs its motions 

 in quest of food. 



The objects of nutrition are growth, repair, and propa- 

 gation. The first object of life is to grow, for no animal 

 is born finished. Some animals, like plants, grow as long 

 as they live; 19 but the majority soon attain a fixed size. 

 In all animals, however, without exception, food is wanted 

 for another purpose than growth, namely, to repair the 

 waste which is constantly going on. For every exercise 

 of the muscles and nerves involves the death and decay 

 of those tissues, as shown by the excretions. The amount 

 of matter expelled from the body, and the amount of nour- 

 ishment needed to make good the loss, increase with the 

 activity of the animal. The supply must equal the de- 

 mand, in order to maintain the life of the individual ; and 

 as an organism can make nothing, it must seek it from 

 without. Not only the muscles and nerves are wasted by 

 use, but every organ in the body ; so that the whole struct- 

 ure needs constant renewal. An animal begins to die the 

 moment it begins to live. The function of nutrition, 

 therefore, is constructive, while motion and sensation are 

 destructive. 



