90 ZOOLOGY. 



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 

 the constituent parts of animal bodies affects every structure 

 and organ, and that an entire renewal of the body occurs in 

 every seven years. This opinion is not based on direct experi- 

 ment, and seems, indeed, contrary to the fact. Many organs 

 remain for long periods stationary, although it may be ad- 

 mitted that, under peculiar circumstances, the original tissues 

 themselves may be attacked, as after long fasting. Thus, the 

 curious experiments of M. Chossat show that when birds do 

 not find in their food a sufficient proportion of calcareous 

 matters, the phosphate of lime entering into the composition 

 of their bones is taken away, little by little. 



Now, the blood furnishing, as we have seen, the materials 

 of the various humours which the animal economy constantly 

 rejects and expels from it by the route of the secretions, 

 becomes unceasingly impoverished, and might take away from 

 the organs the soluble principles they contain, if the repeated 

 introduction of foreign substances did not maintain this liquid 

 always saturated with the same principles. It results from 

 this, that this introduction of the alimentary matters into the 

 organism is necessary, not only to effect the increase of the 

 living parts, but to secure the conservation or preservation of 

 the tissues already existing, and to prevent the resorption or 

 re-absorption of their constituent materials. In brief, the 

 nutrient matter introduced continually into the blood is neces- 

 sary, no doubt, not only for the growth of the body, but 

 also to maintain all the organs in their integrity, and to pre- 

 vent their being acted on by absorption. 



Finally, the slow combustion taking place in every part of 

 the body also destroys the combustible matters : and unless 

 this destruction be met by constant renewal, it would seem, 

 from the experiments of Dumas and others, that the oxygen 

 would act on and destroy the materials composing the organs 

 themselves. From the aliments there must come the com- 



