456 CHEMISTR Y OF THE DIGESTIVE PROCESSES. 



temperature of the body, have recently been determined by Moore and Eock- 

 wood, 1 in the bile of the ox, pig, and dog, and in the mixed bile salts of ox 

 bile, with the following results : 



1. Pure palmitic and stearic acids are practically insoluble in ox bile at a 

 temperature of 38 to 40 C., while oleic acid is easily soluble at this tempera- 

 ture to the extent of 4 per cent. 



2. Of the mixed fatty acids of lard, beef -suet, and mutton-suet, respectively, 

 lard acids are most soluble, mutton-suet acids least soluble, while beef-suet 

 acids are intermediate. Thus in ox bile the solubilities are lard fatty acids, 

 3 '5 per cent.; beef-suet fatty acids, 2*5 per cent.; mutton-suet fatty acids, 2 

 per cent. 



3. The solubility of the fatty acids in bile is only in part due to the bile 

 salts. A strong solution (9 per cent.) of the bile salts of ox bile dissolves all 

 three mixtures of fatty acids both more feebly and more slowly than bile itself. 

 Mere removal of the " pseudo-mucin " frolh bile greatly diminishes its solvent 

 action on fatty acids. 



The same experimenters have shown that the filtered contents of the dog's 

 intestine, removed during fat absorption, are capable, in some samples, of 

 digesting and dissolving at body temperature to a clear solution as much as 

 4 per cent, of neutral fats. On cooling, the dissolved fatty material was 

 thrown out of solution as fatty acids. This experiment shows that, in the dog 

 at least, 2 fats can be dissolved and absorbed in solution as fatty acids. 



The solubilities of the mixed fatty acids in bile, stated above, are quite 

 sufficient to account for the absorption of all the fats of the food in the form 

 of dissolved fatty acids, since they exceed the concentrations in which the 

 products of carbohydrate and proteid digestion are met with in the intestine. 

 But this alone is not sufficient evidence to prove that in the normal course of 

 events all the fat is absorbed in such form. 



The acids of the fats give an acid reaction with litmus. The bile used in 

 the experiments arranged to determine the solubilities was at first strongly 

 alkaline to litmus, but after it had dissolved the fatty acids it became markedly 

 acid to that indicator. It follows, that a fluid with an alkaline reaction to 

 litmus cannot hold in solution any free fatty acids. Now, in the intestine of 

 the white rat, during active fat absorption, the reaction is commonly strongly 

 alkaline to litmus, all the way from pylorus to caecum, and is never acid to that 

 indicator for a greater distance than 6 in. from the pylorus. 3 



Further, even in the case of the dog, and in that part of the intestine where 

 the reaction is acid to litmus, there are probably soaps as well as fatty acids in 

 solution. This is shown by the behaviour towards litmus and methyl-orange 

 of the contents of this part of the intestine. The acid reaction towards litmus 

 is shown by the alkaline reaction to methyl-orange to be due to very weak 

 organic acids ; at the same time, the alkaline reaction to methyl-orange also 

 shows that there is an excess of bases present (above the amount necessary to 

 combine with the strong acids), which is combined with very weak acids. 

 The most probable conclusion, as such a state of affairs is met with during the 

 digestion of an almost purely fatty meal (beef-suet), is that these weak acids are 

 the acids of the fats (oleic, palmitic, and stearic) in combination as soaps. 

 Hence, in that part of the small intestine of the dog where the reaction is acid 

 to litmus, fat absorption is probably going on, partly in the form of dissolved 

 fatty acids and partly in the form of dissolved soaps ; in the part where the 

 reaction is alkaline to litmus, wholly in the form of dissolved soaps. 



In those animals, such as herbivora, in which the reaction of the intestinal 

 contents is strongly alkaline, it is probable that all the fat is absorbed as soaps. 



1 Loc. cit. 



2 Similar results were not obtained with filtered intestinal contents obtained from the 

 rabbit or pig. 



3 Moore and Roekwood, loc. cit. 



