decay, and if examined at two different periods of its duration, does not 

 contain one of the same molecules. The experiment performed with 

 madder, which dyes red the bones of animals among whose food it is 

 mixed, proves, most unquestionably, this incessant decomposition of ani- 

 mated and living matter. One has only to interrupt, for a sufficient 

 length of time, the use of that plant, to make the uniformly red colour 

 assumed by. the bones completely disappear. Now, if the hardest and 

 most solid parts, most calculated to resist decay, are undergoing a per- 

 petual motion of decomposition and of regeneration ; there can be no 

 doubt, that this motion must be far more rapid in those whose power of 

 cohesion is much inferior; for example, in the fluids. 



Attempts have been made to determine the period a.t which the body is 

 completely renovated; it has been said, that an interval of seven years 

 \vas required for one set of molecules to disappear and be replaced by 

 others; but this change mj^st go on more rapidly in childhoofl and in 

 youth. It must be slower at a mature age, and must require a consider- 

 able time, at a very advanced period of life, when all the parts of the body 

 become, in a remarkable degree, fixed and firm in their consistence while 

 the vital powers become more languid. There can be no donbt, that the 

 sex, the habit, the climate in which we live, the profession we follow, our 

 mode of life, and a variety of other circumstances, accelerate or retard 

 it; so that it is absolutely impossible to fix, with any degree of certainty, 

 its absolute duration. 



CVIII The parts of our body, in proportion as they undergo decay, 

 are repaired only by means of homogeneous particles exactly like them- 

 selves : were it otherwise, their nature, which always remains the same, 

 would be undergoing perpetual changes. 



When, in consequence of the successive changes which it has undergone 

 from the action of the organs of digestion, of absorption, of the circu- 

 lation, of respiration, and of secretion, the nutritive matter is animalized 

 or assimilated to the body, which it is to nourish, the parts which it 

 moistens, retain it and incorporate it to their own substance. This nu- 

 tritive identification is not performed alike in the brain, in the muscles, 

 in the bones, &c. Each of them appropriates to itself, by a real process 

 of secretion, whatever it meets with fitted for its nature, in the fluids con- 

 veyed to it by the different kinds of vessels, but especially by the arteries, 

 it leaves unaffected, the remaining heterogeneous particles. A bone is 

 a secretory organ, around which the phosphate of lime is deposited ; the 

 lymphatic vessels which, in the process of nutrition, perform the office 

 of excretory ducts, remove that saline substance, when it has lain suffi- 

 ciently long in the cells of its tissue. The same happens to the muscles, 

 with regard to fibrina, and to albumen with regard to the brain ; every 

 part appropriates to itself, and converts into a solid form, those fluids 

 \vhich are of the same nature, in virtue of a power of which the term af- 

 finity of aggregation, used in chemistry, gives an idea, and of which it is 

 perhaps the emblem. 



The nutrition of a part requires that it should be possessed of sensibi- 

 lity and motion ; by tying the arteries and nerves of a part, it cannot be 

 nourished, nor can it live. The blood which flows along the veins, the 

 fluid conveyed by the absorbents, contain, in a smaller proportion than 

 arterial blood, vivifying the reparatory particles. It is even commonly 

 thought, that the lymph and venous blood contain no directly nutritive 

 particles. As to the share which the nerves take in the process of mi- 



