94 THE PHILOSOPHY OF BIOLOGY 



therefore, reached in which the composition of the 

 mixture remains constant. 



Now there is dissociated fat in the intestine after a 

 meal, but there is only neutral fat in the wall of the 

 intestine. The fat itself cannot pass through the cells 

 forming the intestinal wall, but the glycerine and fatty 

 acid into which it is dissociated can so pass, since they 

 are soluble in the liquids of the intestine. We suppose 

 that the cells of the wall of the intestine also contain 

 the fat-splitting ferment ; this ferment in the cells 

 acts on the glycerine and fatty acid immediately they 

 enter and recombines these radicles again into neutral 

 fat, the above equation now reading from right to left. 

 But after a time this reaction in the cells will also 

 begin to reverse, for the enzyme will begin to split up 

 the synthesised neutral fat when the state of chemical 

 equilibrium in the new conditions is attained. Fatty 

 acid and glycerine will then diffuse out from the cells 

 into the adjacent lymph stream or blood stream — 

 perhaps neutral fat will also pass from the cells into 

 these liquids, we are not sure. At all events the lymph 

 and blood after a meal containing much fat are crowded 

 with minute fat globules. But why are there no fatty 

 acids or glycerine in the blood, for the latter also con- 

 tains lipase (the fat-splitting enzj^me)? The explan- 

 ation is, apparently, that either an anti-enzyme is pro- 

 duced, or that the enzyme passes into a zymoid con- 

 dition. Why also does fat accumulate in the tissues ? 

 Here, again, the activity of the enzyme, which from 

 other considerations we may regard as being universally 

 present almost everywhere in the body, must be 

 supposed to be arrested by some means. 



The conception of a catalytic agent, such as we can 

 study in pure chemistry, thus carries us a long way 

 in our description of the processes of digestion, absorp- 



