70 PHYSIOLOGY 



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 cages 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 tharj hay 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. 



