NUTRITIVE DECOMPOSITION. 85 



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 ampu- 

 tation. 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. 3), 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 condition, 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. 



172. Excretion. Whilst nutrition is going on, decom- 

 position proceeds pari passu, that is to say, the separation of 

 a portion of the molecules of the tissues, and their expulsion 

 from the body. The bones themselves are thus continually 

 decomposed and recomposed ; the utricular tissue covering 

 the surface of the ligaments, mucous membranes, and 

 glandular cavities, is being continually renewed and de- 

 stroyed ; and the old epithelium gives way before new layers 

 formed beneath it in the substance of the tissues. 



Some physiologists have thought that such a renewal of 



