316 HAND-BOOK OF PHYSIOLOGY. 



which is subject to them. It has but to be placed in appropriate condi- 

 tions, and destruction and repair will continue indefinitely. Force, too, 

 is manifested, but there is nothing present which -can transform it into 

 vital shape, and so there is no life. 



Hence, our notion of the constant decay which, together with repair, 

 takes place throughout life, must be not confined to any simply mechanical 

 act. It must include the idea, as before said, of laying up of force, and 

 its expenditure its transformation too, in the act of being expended. 



The growth, then, of an animal or vegetable, implies the expenditure 

 of physical force by organized tissue, as a means whereby fresh matter is 

 added to and incorporated with that already existing. In the case of 

 the plant the force used, transformed, and stored up, is almost entirely 

 derived from external sources; the material used is inorganic. The result 

 is a tissue which is not intended for expenditure by the individual which 

 has accumulated it. The force expended in growth by animals, on the 

 other hand, cannot be obtained directly from without. For them a 

 supply of force is necessary in the shape of food derived directly or indi- 

 rectly from the vegetable kingdom. Part of this force-containing food 

 is expended as fuel for the production of power; and the latter is used 

 'as a means wherewith to elaborate another portion of the food, and incor- 

 porate it as animal structure. Unlike vegetable structure, however, 

 animal tissues are the seat of constant change, because their object is not 

 the storing up of power, but its expenditure; so there must be constant 

 waste; and if this happen, then for the continuance of life there must 

 be equally constant repair. But, as before said, in early life the repair 

 surpasses the loss, and so there is growth. The part repaired is better 

 than before the loss, and thus there is development. 



The definite limit which has been imposed on the duration of life has 

 been already incidentally referred to. Like birth, growth, and develop- 

 ment, it belongs essentially to living beings only. Dead structures and 

 those which have never lived are subject to change and destruction, but 

 decay in them is uncertain in its beginning and continuance. It de- 

 pends almost entirely on external conditions, and differs altogether from 

 the decline of life. The decline and death of living beings are as definite 

 in their occurrence as growth and development. Like these they may be 

 hastened or stayed, especially in the lower forms of life, by various influ- 

 ences from without; but the putting off of decline must be the putting 

 off also of so much life; and, apart from disease, the reverse is true also. 

 A living being starts on its career with a certain amount of work to do 

 various infinitely in different individuals, but for each well-defined. In 

 the lowest members of both the animal and vegetable creation the prog- 

 ress of life in any given time seems to depend almost entirely on external 

 circumstances; and at first sight it seems almost as if these lowly-formed 

 organisms were but the sport of the surrounding elements. But it is 



