70 PHYSIOLOGY 



THE CELLULOSES. Cellulose (C 6 H 10 5 ) X is a colourless, insoluble 

 material, or mixture of materials, which compose the cell walls of the younger 

 parts of plants, and therefore forms a constituent of most of our vegetable 

 foods. It is insoluble in water or dilute acids or alkalies, its only solvent 

 being an ammoniacal cupric oxide solution. On boiling with strong acids, 

 it gradually undergoes hydrolysis and yields sugar, the nature of which 

 varies according to the source of the cellulose. In herbivorous animals cellu- 

 lose undergoes digestive changes and forms an important constituent of their 

 food. The solution of the cellulose in this case is effected by the agency, not 

 of ferments secreted by the wall of the gut, but of micro-organisms which 

 swarm in the paunch of ruminants and in the caecum of other herbivora. In 

 some cases the effective agent is a cytase present in the vegetable cells 

 themselves. Since this ferment is destroyed by boiling, cooked hay is much 

 less digestible than in the raw condition. In certain invertebrata it seems 

 probable that a true cellulose-digesting ferment, or cytase, is secreted by the 

 walls of the alimentary canal. In man cellulose undergoes practically no 

 change in digestion, and serves merely by its bulk to promote peristalsis and 

 the normal evacuation of the bowels. A further consideration of its chemical 

 properties, as well as of the closely allied vegetable materials, gums, pectins, 

 mucilages, derived for the most part from the condensation of pentose 

 molecules, may be dispensed with here. 



