CHAPTER IV. 



THE METABOLIC PROCESSES OF THE BODY. 



444. 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 trans- 

 formed into the other. There remains the further task to inquire 

 into the manner in which the energy set free in this transforma- 

 tion 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 assimi- 

 lated 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 inter- 

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

 knowledge of the matter is at present very imperfect. 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 conse- 

 quence, perhaps somewhat loosely, speak of them as taking place 

 in the blood. 



