390 NUTRITION. 



structure 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 recurrence of small-pox, scarlet- 

 fever, and the like diseases in the same person ; the wearing 

 out of scars, and the complete restoration of tissues that 

 have been altered by injury or disease. 



Such are some of the more important conditions which 

 appear to be essential to healthy nutrition. Absence or 

 defect of any one of them is liable to be followed by dis- 

 arrangement 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 replace those which it loses by the waste or 

 natural decay of its tissue. The structure and composi- 

 tion of the part remain the same ; but the increase of 

 healthy tissue which it receives is attended with the capa- 

 bility of discharging a larger amount of its ordinary 

 function. 



While development is in progress, growth frequently 

 proceeds 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, com- 

 monly, 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 



