2 HAND-BOOK OF PHYSIOLOGY. 



tions for obtaining fresh material, will grow in a fashion as definitely char- 

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

 therefore, necessary to explain the distinctions which exist in this respect 

 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 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. The 

 growth is here said to be superficial. 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; the growth is not superficial, but interstitial. 



(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 t after day, although to all intents he remains the same indi- 

 vidual. 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 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 condition, of its existence. That which is true of structures 

 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 encloses, but it is 

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

 this circumstance its growth takes place, not inter stitially, but 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 alter- 

 ation in the chemical constitution 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 vegeta- 

 ble. A plant, like a crystal, can only grow when fresh material is pre- 

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

 for the same purpose of getting new matter for growth and nutrition, 

 take food into their stomachs. But in both these cases the materials are 

 much altered before they are finally assimilated by the structures they 

 are destined to nourish. 



(4.) The growth of all living things has a definite limit, and the Li\v 



