THE HISTORY OF GLYCOGEN. 437 



CHAPTER IV. 

 THE METABOLIC PEOCESSES OF THE BODY. 



372. WE have followed the food through its changes in the alimen- 

 tary 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 comparatively raw food which enters the blood, in 

 order that it may be assimilated with the least possible labor 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 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. 



How little we know concerning the metabolism of the master tissues 

 themselves was shown when we were dealing with these tissues in an earlier 

 part of this work ; but success in the study of these can hardly be ex- 

 pected until our knowledge is increased as regards the changes which the 

 blood undergoes before it reaches and after it leaves the muscle or the 

 nerve. The fact that a large part of the absorbed food is carried through 

 the liver before it is thrown on the general circulation leads us to suppose 

 that in this large organ important metabolic processes are carried on ; and 

 observation with experiment confirms this view. Important as the secre- 

 tion of bile may be, the other metabolic functions of the liver are of still 

 greater importance. 



THE HISTORY OF GLYCOGEN. 



373. If the liver of a well-fed animal be removed immediately after 

 death, rapidly divided into small pieces, thrown into boiling water, rubbed 

 up and boiled, a decoction may be obtained which after careful neutraliza- 

 tion and filtration will be tolerably free from proteid matter. Such a 

 decoction is remarkably opalescent, milky in fact in appearance, much more 

 so than a similar decoction from muscle or other tissue, and remains opal- 

 escent even after repeated filtration. Treated with iodine, the solution 

 turns a brownish-red, port-wine red color, not unlike that given by dextrin 



