312 NUTRITION. 



degree of occasional pressure or friction, as in the palms of 

 the hands of persons employed in rough manual labor ; by 

 the enlargement and increased hardness of muscles that are 

 largely exercised ; and by many other facts of a like kind. 

 The increased power of nutrition put forth in such growth is 

 greater than might 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 sufficient 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 in- 

 crease 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 

 kind of enlargement to which the same part may be subject 

 from disease. In the former case, the enlargement is due to 

 an increased quantity of healthy tissue, providing more than 

 the previous power to meet a particular emergency ; the other 

 may be the result of a deposit of morbid material within the 

 natural structure of the part, diminishing, instead of augment- 

 ing, its fitness for its office. Such a healthy process of growth 

 in a part, attended with increased power and activity of its 

 functions, may, however, occur as the consequence of disease 

 in some other part; in which case it is commonly called Hy- 

 pertrophy, i. e., excess of nutrition. The most familiar ex- 

 amples of this are in the increased thickness and robustness 

 of the muscular walls of the cavities of the heart in cases of 

 continued obstruction to the circulation ; and in the increased 

 development of the muscular coat of the urinary bladder 

 when, from any cause, the free discharge of urine from it is 

 interfered with. In both these cases, though the origin of the 

 growth is the consequence of disease, yet the growth itself is 

 natural, and its end is the benefit of the economy ; it is only 

 common growth renewed or exercised in a part which had 

 attained its size in due proportion to the rest of the body. 



It may be further mentioned, in relation to the physiology 

 of this subject, that when the increase of function, which is 

 requisite in the cases from which hypertrophy results, cannot 

 be efficiently discharged by mere increase of the ordinary 

 tissue of the part, the development of a new and higher kind 

 of tissue is frequently combined with this growth. An exam- 

 ple of this is furnished by the uterus, in the walls of which, 

 whei| it becomes enlarged by pregnancy, or by the growth of 



