CHAPTER V. 

 THE METABOLIC PHENOMENA OF THE BODY. 



WE have followed the food through its changes in the alimentary 

 canal, and have seen it enter into the blood, either directly or by the 

 intermediate channel of the lacteals, in the form of peptone (or other- 

 wise modified albumin), sugar (lactic acid), and fats, accompanied 

 by various salts. We have further seen that the waste products 

 which leave the body are urea, carbonic acid and salts. We have 

 now to attempt to connect together the food and the waste pro- 

 ducts ; to trace out as far as we are able the various steps by which 

 the one is transformed into the other, and to inquire into the 

 manner in which the energy set free in this transformation is 

 distributed and made use of. 



The master tissues of the body are the muscular and nervous 

 tissues ; all the other tissues may be regarded as the servants of 

 these. And we may fairly presume that, besides the digestive and 

 excretory tissues which we have already studied, many parts of the 

 body are engaged either in further elaborating the comparatively 

 raw food which enters the blood, in order that it may be as- 

 similated with the least possible labour by the master tissues, or in 

 so modifying the waste products which arise from the activity of 

 the master tissues that they may by removed from the body as 

 speedily as possible. There can be no doubt that manifold inter- 

 mediate changes of this kind do take place in the body ; but our 

 knowledge of the matter is at present very imperfect. In one or 

 two instances only can we localize these metabolic actions and 

 speak of distinct metabolic tissues. In the majority of cases we 

 can only trace out or infer chemical changes, without being able 

 to say more than that they do take place somewhere; and in 

 consequence, perhaps somewhat loosely, speak of them as taking 

 place in the blood. 



