GROWTH. 39 1 



of it has gained its perfect state in both size and composi- 

 tion. 



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 ordi- 

 nary 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 thickening of cuticle at parts 

 where it is subjected to an unusual degree of occasional 

 pressure or friction, as in the palms of the hands of persons 

 employed in rough manual labour ; by the enlargement 

 and increased hardness of muscles that are largely exer- 

 cised ; and by many other facts of a like kind. The 

 increased power of nutrition put forth in such growth is 

 greater than Liight be supposed ; for the immediate effect 

 of increased exercise of a part must be a greater using of 

 its tissue, and might be expected to entail a permanent 

 thinning or diminution of the substance of the part. But 

 the energy with which fresh particles are formed is suffi- 

 cient not only to replace completely those that are worn 

 away, but to cause an increase in the substance of the 

 part the amount of this increase being proportioned to 

 the more than usual degree in which its functions are 

 exercised. 



The growth of a part from undue exercise of its functions 

 is always, in itself, a healthy process ; and the increased 

 size which results from it must be distinguished from the 

 various kinds of enlargement to which the same part may 

 be subject from disease. In the former case the enlarge- 

 ment is due to an increased quantity of healthy tissue, 



