PRINCIPLES CONSUMED BY THE ORGANISM. 505 



supply of material, a man should not grow indefinitely. After the being is fully devel- 

 oped, and during what is known as the adult period, the supply seems to be about equal 

 to the waste. But, after this, nutrition gradually becomes deficient, and the deposition of 

 new matter in progressive old age becomes more and more inadequate to supply the place 

 of the living nitrogenized substance. We may at this time, as an exception, have a con- 

 siderable deposition of fat, but the nitrogenized matter is always deficient, and the pro- 

 portion 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 these 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 completely regenerating its sub- 

 stance, 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 nutri- 

 tion, 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 influences 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 inter- 

 esting of these modifications, are those due to age, constituting, as they do, in early life, 

 the process of development. These 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 

 important division of the subject, the more interesting, because the nutrition and develop- 

 ment of the individual tissues are closely connected with the processes of regeneration 

 and repair after injury. We have stated, as far as possible, all that is positively known of 

 the nutrition of the fully-formed tissues of the body ; but the history of their develop- 

 ment belongs to embryology. If we were to attempt to follow the processes of regenera- 

 tion after injury, in nerves, muscles, bone, etc., we should be compelled to pass almost 

 immediately into the domain of pathology. The influences of climate, respiratory activity, 

 food, etc., have already been considered under the heads of respiration, alimentation, and 

 excretion, and will be touched upon again in connection with animal heat. 



Products of Disassimilation. It only remains now to recapitulate briefly tlie mode 

 of production of the excretions. The process of disassimilation, we are aware, always 

 accompanies nutrition, and the substances thus formed are the result of the final changes 

 of the organic constituents of the tissues. As we have seen in studying the urine, the 

 excrementitious principles proper are always associated with inorganic matter which has 

 passed through the organism ; and, while there are many effete substances that we have 

 been able to recognize, the physiological and pathological relations of which have been 

 more or less successfully studied, there are probably others which have thus far escaped 

 observation. It is almost futile to speculate upon the probable bearing which the discovery 

 of new excrementitious principles will have upon pathological conditions, while there are 

 so many which we now know only by name, their relations to the different tissues being 

 still obscure ; but, if we reason from the light thrown upon certain diseased conditions by 

 the fact that urea, the urates, and cholesterine are liable to be retained in the blood and 

 produce certain symptoms, we may safely infer that the description of new effete prin- 

 ciples will have an important influence upon our pathological knowledge as well as our 

 comprehension of physiological processes. The following are the most important excre- 

 mentitious matters, the relations of which to nutrition and disassimilation are more or 

 less fully understood : 



