THE CARBOHYDRATES 77 



changes and forms an important constituent of their food. The solu- 

 tion of the cellulose in this case is effected by the agency, not of fer- 

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

 swarm in the paunch of ruminants and in the coecum of other herbi- 

 vora. 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. 



