pletely acid to plienolphthalein, showing that the acid reaction to litmus in the upper part is due to wealc organic , while the alkaline reaction in the lower is due to fixed alkali, accompanied by dissolved carbonic acid. The alkaline reaction to methyl-orange in the upper pai't, where it is acid to litmus and phenolphthalem, shows that in that part there is an excess of bases, above that quantity necessary to combine with all the inorganic acids, which are combined with very weak organic acids (probably fatty acids), for methyl-orange is a stable indicator, and does not react to such acids, while litmus, and, still more so, phenolphthalein, are indicators which are affected by these acids. In the lower third or thereabouts, where the reaction is alkaline to litmus, there cannot be any fatty acids present in solution.
Any fat absorbed as free fatty acid in solution must, therefore, be taken up from the upper two-thirds or three-fourths of the intestine where the reaction is acid to litmus, but even here a considerable part is probably being absorbed in solution as soaps, as is shown by the reaction being at the same time alkaline to methyl-orange. In the lower part all the fat absorbed must be taken up as soaps.
During fat absorption in the white rat,* the reaction of the contents of the small intestine is commonly alkaline to litmus from pylorus to caecum, and is never acid for a greater distance than 2 or 3 in. below the pylorus ; in this animal, therefore, nearly all the fat must be absorbed in solution as soaps.
We have not investigated the reaction of the intestinal contents in other animals during fat absorption, but in the rabbit, during carbohydrate absorption, it is strongly alkaline all the way, from pylorus to ceecurn, and in the pig the mixed contents during the absorption of a mixed meal (meal and oats) had a strong alkaline reaction. As already stated, the filtered contents in these animals do not perfectly dissolve fat, and the portion dissolved must be in the form of soap, because the reaction remains alkaline to litmus after solution. In such animals it is probable that the greater part of the fat must be absorbed as soaps.
The main objections which have been urged’ against absorption of fats as soaps are, first, absorption in presence of an acid reaction in the dog, in which case it was supposed impossible that soaps could be present simultaneously in solution,f and, secondly, that the
- In this* animal the intestinal contents are usually semi-solid. Care was taken
to mix them so as not to obtain the alkaline surface reaction sometimes described. On thorough mixing an alkaline reaction was obtained.
f The acid reaction is also commonly supposed to preclude the possibility of the formation of an emulsion, and Cash (‘Arch. f. Anat. u. Physiol.,’ 1881, p. 386), in experiments chiefly made to determine this point, failed to find any emulsion within the dog s intestine. In ten out of sixteen experiments we obtained more or less emulsion, and in five of these, in almost the entire length, a perfect emulsion, containing immense numbers of minutest fat globules, possessing a marked acid
