GROWTH. 311 



may be exactly maintained by nutrition, is not unlimited; for 

 in nearly all altered parts there appears to exist a tendency to 

 recover the perfect state ; and, in many cases, this state is, in 

 time, attained. To this we may attribute the possibility of re- 

 vaccination after the lapse of some years ; the occasional recur- 

 rence of small-pox, scarlet-fever, and the like diseases, in the 

 same person ; the wearing out of scars, and the complete restor- 

 ation of tissues that have been altered by injury or disease. 



Such are some of the more important conditions which ap- 

 pear to be essential to healthy nutrition. Absence or defect of 

 any one of them is liable to be followed by disarrangement of 

 the process ; and the various diseases resulting from defective 

 nutrition appear to be due to the failure of these conditions, 

 more often than to imperfection of the process itself. 



GROWTH. 



Growth, as has been already observed, consists in the increase 

 of a part in bulk and weight by the addition to its substance 

 of particles similar to its own, but more than sufficient to re- 

 place those which it loses by the waste or natural decay of its 

 tissue. The structure and composition of the part remain the 

 same ; but the increase of healthy tissue which it receives is 

 attended with the capability of discharging a larger amount of 

 its ordinary function. 



While development is in progress, growth frequently pro- 

 ceeds with it in the same part, as in the formation of the 

 various organs and tissues of the embryo, in which parts, while 

 they grow larger, are also gradually more developed until they 

 attain their perfect state. But, commonly, growth continues 

 after development is completed, and in some parts, continues 

 even after the full stature of the body is attained, and after 

 nearly every portion of it has gained its perfect state in both 

 size and composition. 



In certain conditions, this continuance or a renewal of 

 growth may be observed in nearly every part of the body. 

 When parts have attained the full size which in the ordinary 

 process of growth they reach, and are then kept in a moderate 

 exercise of their functions, they commonly (as already stated) 

 retain almost exactly the same dimensions through the adult 

 period of life. But when, from any cause, a part already full- 

 grown in proportion to the rest of the body, is called upon to 

 discharge an unusual amount of its ordinary function, the 

 demand is met by a corresponding increase or growth of the 

 part. Illustrations of this are afforded by the increased thick- 

 ening of cuticle at parts where it is subjected to an unusual 



