CHAPTER IV. 

 THE METABOLIC PROCESSES OF THE BODY. 



356. 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 otherwise modified albumin), sugar (lactic 

 acid), and fats, accompanied by various salts and water. We 

 have further seen that the waste products which leave the body 

 are urea, carbonic acid, salts and water. We have now to 

 attempt to connect together the food and the waste products ; 

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

 one is transformed into the other. There remains the further 

 task 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 com- 

 paratively raw food which enters the blood, in order that it may 

 be assimilated 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 be removed from 

 the body as speedily as possible. There can be no doubt that 

 manifold intermediate changes of this kind do take place in the 

 body ; but our knowledge of the matter is at present very imper- 

 fect. In a few 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. 



How little we know concerning the metabolism of the master 

 tissues themselves was shewn when we were dealing with these 



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