FOOD DIGESTION AND RESPIRATION 77 



urea in the liver by the removal of water, a reversal of the action 

 of the enzyme urease, described elsewhere (E., p. 182). 



C0 2 +H 2 



Loss, probably by bacterial action. 



FIG. 2. Diagram of the Nitrogen Cycle from the Atmosphere through 

 Plants and Animals back to the Atmosphere. 



In Fig. 2 we have a diagrammatic representation of the 

 nitrogen cycle, as it may be called. 



The Large Intestine 



The importance of this last part of the alimentary canal varies 

 in different animals ; in those living on flesh, nearly all of the useful 

 constituents have been absorbed by the time the large intestine 

 is reached. The chief process in it is a further taking up of water 

 from the semi-liquid indigestible contents, so that they become 

 more solid before final rejection. In those animals which take a 

 large proportion of vegetable food, a certain further process of 

 digestion goes on in the large intestine, mainly by the action 



