DIGESTION 135 



after they passed through the wall of the alimentary canal, 

 they would produce various undesirable results. 



There is some difficulty in following droplets of fat across 

 the space which intervenes between the epithelium of a villus 

 and its lacteal radicle. It has been asserted that leucocytes 

 act as carriers, catching the droplets as they are extruded by 

 the epithelial cells, and bearing them into the radicle, where 

 they set them free. Undoubtedly, many leucocytes are present 

 in the lymph-spaces of a villus. After a meal they are found 

 charged with fat. But it is hardly in accord with what we 

 know of the character of a leucocyte to suppose that it will 

 let go fat which it has once ingested into its own body-substance. 

 A leucocyte is not a disinterested organism. If fat droplets 

 arlf floating across from the epithelium to the lacteal, leucocytes 

 are pretty certain to steal some of them. But we know of 

 no other case in which they give up what they have stolen, 

 unless it be something which is injurious to their own health. 

 Even then they usually cling to it, whether it be a germ or a 

 particle of soot, until their own dissolution sets it free. 



Neither proteins nor sugar reach the lacteal radicle. Both 

 these substance are absorbed from the lymph in the tissue- 

 spaces of the villus by the blood-capillaries and venules which 

 traverse them. The veins of the intestine unite to form the 

 portal vein, up which proteins and sugar are carried to the 

 liver, where they are stored, to be doled out into the blood- 

 stream as the tissues need them. 



Bacteria of the Alimentary Canal. The enzymes (ferments) 

 of the several digestive juices are not the only agents which 

 modify the constitution of the foods within the alimentary 

 canal. Throughout the whole of the tract conditions are in 

 many respects favourable for the growth of putrefactive 

 organisms. Mouth, stomach, small and large intestine, has 

 each its special bacterial flora. It is doubtful whether any of 

 these organisms, with the single exception of the bacteria 

 which in herbivorous animals break up cellulose, are favourable 

 to digestion. That they are not necessary has been shown 

 by an ingenious experiment on new-born animals. Guinea- 

 pigs born in an aseptic chamber, through which filtered air 

 was drawn, and fed every hour on sterilized milk, throve and 

 put on weight. When killed at the end of eight days, no germs 



