132 THE BODY AT WORK 



ingests. It could, and very possibly it does, ingest albumin and 

 fats as such. Still, the elaborate provision which is made for 

 converting albumin into diffusible peptone, and cane-sugar and 

 maltose into easily diffusible dextrose, suggests that substances 

 which will pass through membranes are more readily absorbed 

 than substances which will not. We are justified in looking 

 upon absorption as a physical problem up to a certain point. 

 But we must not dwell too much on the physical aspects of 

 the problem. If the absorption of food were merely a process 

 of diffusion, an enormous quantity of water would be required 

 to carry the diffusible products of digestion into the villi. The 

 passage of the foods is aided by the selective activity of the 

 epithelial cells, Peptonization greatly facilitates the work of 

 the epithelial cells, but it is not a condition essential to ab- 

 sorption, so far as soluble proteins are concerned. It is, how- 

 ever, essential that the proteins should be presented to the epi- 

 thelial cells in a soluble form. They could do nothing with the 

 solid fibres of meat, however much they might have been dis- 

 integrated by mastication and by the action of hydrochloric 

 acid. It is only after digestion by pepsin and by trypsin that 

 all the proteins of food are brought into solution. Digestion 

 is needed to reduce them to a condition in which the epithelial 

 cells can take them up. 



Much thought has been devoted to the question of the form 

 in which fat is absorbed. Fat in the chemical sense a pure 

 fat, that is to say is a compound of a fatty acid and glycerin. 

 Suet, lard, butter, vegetable oils, etc., are mixtures of several 

 fats. All consist of glycerin united with fatty acids. The 

 acids are stearic acid, palmitic acid, oleic acid, and others 

 of less importance. Fats are insoluble in water ; so also are 

 the fatty acids. A fatty acid combined with an alkali (in 

 place of glycerin) is a soap. Soaps are soluble in water. If 

 milk is examined under the microscope, it is found to contain 

 droplets of fat, varying in size, but all minute. The larger 

 droplets tend to rise to the surface as cream, but the smaller 

 droplets do not run together. If milk from which the cream 

 has been skimmed is sterilized, it retains its normal appearance 

 for an indefinite time. Its fat remains in droplets. In 

 technical language, milk is an emulsion. Theoretically oil 

 and water would make an emulsion, if the droplets of oil were 



