HANDBOOK OF PHYSIOLOGY. 



ceed the repair, the animal decays; and if decay go on beyond a certain 

 point, life becomes impossible, and the animal dies. 



Growth, or the inherent power of increasing in size, although essen- 

 tial to our idea of life, is not, it must be recollected, confined to living 

 beings. A crystal of common salt, for example, if placed under appro- 

 priate conditions for obtaining fresh material, will grow in a fashion us 

 definitely characteristic and as easily to be foretold as that of a living 

 creature ; but the growth of a crystal takes place merely by additions 

 to its outside; the new matter is laid on particle by particle, and layer 

 by layer, and, when once laid on, it remains unchanged. In a living 

 structure, where growth occurs, it is by addition of new matter, not to 

 the surface only, but throughout every part of the mass. 



Again, all living structures are subject to constant decay. Thus, a 

 man's body is not composed of exactly the same particles day after day, 

 although to all intents he remains the same individual. Almost every 

 part is changed by degrees; but the change is so gradual, and the re- 

 newal of that which is lost so exact, that no difference may be noticed, 

 except at long intervals of time. A lifeless structure, as a crystal, is 

 subject to no such laws; neither decay nor repair is a necessary condi- 

 tion of its existence. That which is true of structures which never had 

 to do with life is true also with respect to those which, although they 

 are formed by living parts, are not themselves alive. Thus, an oyster- 

 shell is formed by the living animal which it incloses, but it is as lifeless 

 as any other mass of inorganic matter; and in accordance with this 

 circumstance its growth takes place layer by layer, and it is not subject 

 to constant decay and reconstruction. The hair and nails are examples 

 of the same fact. 



In connection, too, with the growth of lifeless masses there is no 

 alteration in the chemical composition of the material which is taken 

 up and added to the previously existing mass. For example, when a 

 crystal of common salt grows on being placed in a fluid which contains 

 the same material, the properties of the salt are not changed by being 

 taken out of the liquid by the crystal and added to its surface in a solid 

 form. But the case is essentially different in living beings, both animal 

 and vegetable, as the materials which serve ultimately to build them 

 up are much altered before they are finally assimilated by the structures 

 they are destined to nourish. 



The growth of all living things has a definite limit, and the law 

 which governs this limitation of increase in size is so invariable that we 

 should be as much astonished to find an individual plant or animal 

 without limit as to growth as without limit to life. 



5. The Power of Reproduction. The amoeba, to return to our former 

 illustration, when the growth of its protoplasm has reached a certain 

 point, manifests the power of reproduction, by splitting up into (or in 



