CHEMICAL BASIS OF THE ANIMAL BODY. 99 



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



Although true cellulose is never found as a constituent of the 

 animal tissues, it possesses no inconsiderable interest for the physio- 

 logist in view of the fact that in the herbivora a large amount of 

 cellulose is digested and absorbed so that it does not reappear ex- 

 ternally 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 succulent vegetables and 

 fruits in which it is taken by man, the cell-walls are comparatively 

 non-lignified and hence more easily acted upon by the digestive agents. 



The lignification of the cell-wall which has taken place in those plant-tissues to 

 which the name ' woody ' is ordinarily applied is due to the presence of a substance 

 called lignin. Very little is known of it as a chemical individual : it appears to 

 contain more carbon than does cellulose. Its discrimination from cellulose depends 

 on the fact that it is coloured yellow by the action of Schulze's reagent (see below) 

 and deep brown by that of iodine and sulphuric acid. When treated with phloro- 

 glucin and strong hydrochloric acid it turns red ; it is coloured bright yellow by 

 the action of aniline sulphate or chloride and the subsequent addition of hydrochloric 

 acid. 



Further, although at present but little is known as to how the 

 digestion of cellulose is brought about in the alimentary canal, there 

 is increasing evidence 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 of 

 vegetable organisms ] . 



Cellulose is related to starch and in some cases (Date, Phytelephas) 

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

 by some enzyme, and utilised during germination. The cell- wall of 

 vegetable cells is composed of cellulose, which in young cells is pure 

 and much less resistant to various reagents than it is in the older cells 

 where it has become lignified and incrusted with other substances. 

 When pure it is soluble in one reagent only, viz. Schweizer's, which is 

 a solution of hydrated cupric oxide in ammonia 2 . When treated with 

 strong sulphuric acid cellulose is changed and yields. a substance which 

 is coloured blue by iodine; a similar colouration is observed on the 



1 Brown and Morris, Jl. Chem. Soc. Vol. LVII. (1890), p. 497. Contains 

 references to other literature. 



2 Prepared as follows. Sulphate of copper in solution to which some ammonium 

 chloride has been added is precipitated with caustic soda : the hydrated cupric oxide 

 thus obtained is washed and dissolved to saturation in 20 p.c. ammonia. It may 

 also be prepared by pouring strong ammonia on to copper turnings, the requisite 

 oxidation of the copper being effected by drawing a current of air through the 

 fluid in which the turnings are immersed. (Cross and Bevan, Cellulose, 1885, 

 p. 6.) 



a "2 



