CHEMICAL BASIS OF THE ANIMAL BODY. 1215 



The change which glycogen in the liver undergoes post- 

 mortem and presumably also during life is strikingly different 

 from that which has been described above. Whereas by ordi- 

 nary enzymic hydrolysis, maltose is the chief final product 

 obtained, there is now no doubt that in the liver little if any 

 maltose is formed, the so-called liver-sugar being apparently 

 identical with true dextrose. 



6. Cellulose (C 6 H 10 O 5 ) W . 



Although true cellulose is never found as a constituent of 

 the animal tissues, it possesses no inconsiderable interest for the 

 physiologist in view of the fact that in the herbivora a large 

 amount of cellulose is digested and absorbed so that it does not 

 reappear externally in the excreta. In man also there is no 

 doubt that some digestion and absorption of cellulose may occur, 

 the process being facilitated by the fact that in those more suc- 

 culent vegetables and fruits in which it is taken by man, the 

 cell-walls are comparatively non-1 ignified and hence more easily 

 acted upon by the digestive agents. Further, although at pres- 

 ent but little is known as to how the digestion of cellulose is 

 brought about in the alimentary canal, there is increasing evi- 

 dence of the possible existence of a specific enzyme to whose 

 solvent action the change is due. But as yet this evidence rests 

 almost entirely upon experiments with and observations on vege- 

 table organisms. 



Cellulose is related to starch and in some cases plays the 

 part of a store of reserve material, being dissolved, presumably 

 by some enzyme, and utilized during germination. 



By treatment with strong sulphuric acid cellulose may be 

 dissolved with the formation of a dextrin-like product : on dilut- 

 ing with water and boiling it is finally converted into a sugar 

 which is apparently identical with dextrose. 



As already stated, cellulose is undoubtedly digested in the 

 alimentary canal more especially of herbivora, but also to a less 

 extent of man. We know however but little of the real nature 

 of the digestive processes which are involved in this. It may 

 take place under the influence of putrefactive organisms, and 

 this accords with the marked occurrence of marsh-gas in the 

 gases of the intestine of herbivora and its increased presence in 

 the intestine of man when largely fed with a vegetable diet. 

 On the other hand it is possible that the digestion may turn out 

 to be due to some definite enzyme, but as yet no such enzyme 

 has been obtained with certainty from the secretions or tissues 

 of the alimentary canal. 



7. Tunicin (C 6 H 10 5 V 



This substance constitutes the chief part of the mantle of 



