NUTEITIVE DECOMPOSITION. 89 



for their growth and support, each choosing the molecules 

 identical with its own nature. 



It is this last act which constitutes assimilation. 



169. But nothing is known as to the real nature of this 

 act of assimilation, how brought about, how effected. Such 

 questions touch too nearly the very essence of the principle of 

 life, itself perfectly unknown in its nature. One thing is 

 certain, that in all animals possessing; a nervous system the 

 influence which this exercises over assimilation is distinct and 

 undeniable. Nor is the duration of life in the various organs 

 of the same animal the same ; the thymus gland, for example, 

 ceases to grow, and decays in the very young. The teeth 

 have their stated periods of existence ; the nails, the hair, and 

 generally the epithelial tissues, continue to grow in extreme 

 old age. 



170. The assimilating force possesses the property, 

 especially in the lower animals, of restoring parts which have 

 been destroyed; bones are reunited by bone after being 

 broken, and even large portions of them which have been lost 

 have been restored. The limb of the lizard when broken off 

 has grown again : a new foot been reproduced in crabs and 

 spiders ; in salamanders, a new eye and portion of the head 

 have been restored after the removal of the original parts by 

 amputation. Finally, earth-worms and many other annelides 

 can thus reproduce a great part of the body; and in the 

 hydra and fresh-water polyp (Fig. 4), a small fragment has 

 been found equal to the reproduction of the entire body. 



171. Moreover, various circumstances, which we have 

 not the leisure to examine here, may modify the progress of 

 the work of assimilation, render it active, retard it, or change 

 its direction. It is in this way that in certain diseases we see 

 nutrition to be almost entirely arrested, and that in others 

 certain tissues change their nature. It is also to be observed, 

 that this assimilative labour does not take place with the 

 same rapidity in all parts of the body ; to be assured of this, 

 we have only to observe the changes in form often brought 

 about by the progress of age ; for these changes depend 

 chiefly on this, that certain parts increase more rapidly than 

 others. Thus, from the moment of birth to the adult condi- 

 tion, the members of the body of man grow more rapidly than 

 the trunk ; whence it follows that, in general, this latter is a 

 portion the less considerable of the whole as the growth is 

 more prolonged. 



