390 NUTRITION. 



and the deposition of new matter in progressive old age is 

 more and more inadequate to supply the place of the living 

 nitrogenized substance. We may at this time, as an excep- 

 tion, have a considerable deposition of fat, but the nitrogen- 

 ized matter is always deficient, and the proportion of inert 

 inorganic matter combined with it is increased. 



There can be little if any doubt that the forces which 

 induce the regeneration or nutrition of parts reside in the 

 organic nitrogenized substance, and that they give to the 

 parts their characteristic functions, which we call vital ; the 

 inorganic matter being passive, or having, at the most, 

 purely physical functions. If, therefore, as age advances, 

 the organic matter be gradually losing the power of com- 

 pletely regenerating its substance, and if its proportion 

 be progressively diminishing, while the inorganic matter is 

 increasing in quantity, a time will come when some of the 

 organs necessary to life will be unable to perform their 

 office. When this occurs we have death from old age, or 

 physiological dissolution. This may be a gradual failure of 

 the general process of nutrition, or it may attack some one 

 organ or system. Why death is thus certain to occur, we do 

 not know, any more than we can explain why and how 

 animals live. 



The modifications in nutrition due to the very varied in- 

 fluences that may be brought to bear upon it present a most 

 extended subject for discussion ; but we shall not touch upon 

 any of these influences that are not purely physiological. 

 Among the most interesting of these modifications, are those 

 due to age, constituting, as they do, in early life, the process 

 of development. They will be treated of fully in connection 

 with the subject of generation. It is evident, also, from 

 what we have already said, that each tissue and organ has 

 its own conditions of nutrition and development ; and this 

 constitutes another interesting division of the subject, the 

 more so, because the nutrition and development of the indi- 

 vidual tissues are closely connected with the processes of 



