548 THE ABSORPTION OF FOOD [CH. XXXVI. 



other particles, and it is certain that the epithelial cells do not 

 protrude pseudopodia from their borders (this, however, does occur 

 in the endoderm of some of the lower invertebrates) ; moreover, fat 

 particles have never been seen in the striated border of the cells. 



The difficulty has now been solved. Munk and, later, Moore and 

 Eockwood, showed quite conclusively that in the intestine fat is 

 completely broken down into glycerin and fatty acids ; preliminary 

 emulsification is advantageous for the formation of these substances, 

 but is not essential. Fat, therefore, is entirely absorbed as glycerin 

 and fatty acids; the latter, however, in great measure are first 

 converted into soaps, that is, compounds of the fatty acids and 

 alkalis. These soluble cleavage products pass readily through the 

 striated borders of the intestinal epithelial cells; and these cells 

 perform the synthetic act of building them into fat once more, the 

 fat so formed appearing in the form of small globules, surrounding 

 or becoming mixed with the protoplasmic granules that are ordi- 

 narily present. Another remarkable fact which Munk made out is 

 that after feeding an animal on fatty acids the chyle contains fat. 

 The necessary glycerin must have been formed by protoplasmic 

 activity during absorption. 



Bile aids the digestion of fat, the bile salts acting in the same way 

 as the co-enzyme of pancreatic lipase ; bile also is a solvent of fatty 

 acids, and it probably assists fat absorption by reducing the surface 

 tension of the intestinal contents; membranes moistened with bile 

 allow fatty materials to pass through them more readily than would 

 otherwise be the case. In cases of disease in which bile is absent 

 from the intestines, a large proportion of the fat in the food passes 

 into the faeces. 



The faeces on an ordinary mixed diet contain comparatively 

 little food residues, and a small quantity is excreted even during 

 starvation. Voit and Hermann showed independently that an 

 intestinal loop which had been emptied and separated from the 

 rest of the bowel contained a few days later, material identical with 

 faeces, and consisting of intestinal juice, desquamated epithelium 

 cells, and bacteria. The increase in the amount of fasces which 

 occurs when food is taken, even when the food is free from cellulose, 

 is due to the mechanical and chemical stimulation which leads to 

 an increase in the succus entericus, and in the shedding of epithelial 

 cells. The faeces contain about 1 per cent, of nitrogen, but this 

 is chiefly contained in the bodies of bacteria, and the disintegrated 

 epithelial cells. Addition of protein to the diet makes practically 

 no difference to the nitrogen in the faeces under normal conditions. 



The addition of cellulose to the diet increases the bulk of the 

 faeces, partly because much of the cellulose is excreted unchanged, 

 partly because it stimulates the mucous membrane to secrete more 



