HANDBOOK OF PHYSIOLOGY. 



such as diatoms and the like, from which it extracts what it requires, 

 and then rejects or excretes the remainder, which has never formed part 

 of the body, by withdrawing itself from it. The assimilation which goes 

 on in the body of the amoeba, is to replace waste of its tissue consequent 

 upon manifestation of energy. 



The two processes of waste and repair, then, go on side by side, and as 

 long as they are equal the size of the animal remains stationary. If, 

 however, the building up exceed the waste, then the animal grows ; if 

 the waste exceed the repair, the animal decays ; and if decay go on be- 

 yond a certain point, life becomes impossible, so the animal dies. 



Growth, or inherent power of increasing in size, although essential to 

 our idea of life, is not confined to living beings. A crystal of common 

 salt, or of any other similar substance, if placed under appropriate con- 

 ditions for obtaining fresh material, will grow in a fashion as definitely 

 characteristic and as easily to be foretold as that of a living, creature. It 

 is, therefore, necessary to explain the distinctions which exist in this re- 

 spect between living and lifeless structures ; for the manner of growth 

 in the two cases is widely different. 



Differences between living and lifeless growth. (1.) The growth of a 

 crystal, to use the same example as before, takes place merely by addi- 

 tions 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, on the other hand, as, for example, a brain or a muscle, 

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

 only, but throughout every part of the mass. 



(2.) All living structures are subject to constant decay ; and life con- 

 sists not, as once supposed, in the power of preventing this never-ceasing 

 decay, but rather in making up for the loss attendant on it by never- 

 ceasing repair. Thus, a man's body is not composed of exactly the same 

 particles day after day, although to all intents he remains the same in- 

 dividual. Almost every part is changed by degrees ; but the change is 

 so gradual, and the renewal of that which is lost so exact, that no differ- 

 ence may be noticed, except at long intervals of time. A lifeless struc- 

 ture, as a crystal, is subject to no such laws ; neither decay nor repair is 

 a necessary condition of its existence. That which is true of structure 

 which never had to do with life is true also with respect to those which, 

 though 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 the constant decay and reconstruction which belong to the 

 living. The hair and nails are examples of the same fact. 



(3.) In connection with the growth of lifeless masses there is no al- 

 teration 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. A plant, like a crystal, can only grow when fresh material is 

 presented to it ; and this is absorbed by its leaves and roots ; and animals 



