SOL UTION THEORIES OF FA T ABSORPTION. 45 3 
found the reaction of the intestinal contents to be acid all the way from pylorus 
to caecum. 
Vaughan Harley 1 tested the reaction of the upper and lower halves of 
the small intestine, and of the large intestine, in three dogs which had been 
fed on milk, and found that the reaction was acid in all three portions. 
Moore and Rockwood 2 have recently studied the reaction of the intestine 
in the dog during fat absorption to different indicators, chosen with a view 
to determining, not only the reaction, but also the character of the acids or 
bases causing that reaction. The indicators used were litmus solution, rnethyl- 
orange, and phenolphthalein. The reaction to litmus of the upper part of the 
small intestine was found to he acid, changing to alkaline at a somewhat 
variable point, situate two-thirds to three-fourths of the way from pylorus to 
caecum. The contents of the large intestine are commonly acid to litmus, 
while the reaction of the contents of the caecum lies intermediate between 
that of the contents of the ileum and that of the contents of the large in- 
testine, and may lie either faintly alkaline or faintly acid. 
The reaction at the pylorus, and for some distance below, may be nearly 
neutral or even faintly akaline to litmus; as the distance below the pylorus is 
increased, the reaction always becomes more strongly acid at first, then less 
acid again, and finally faintly alkaline at the limit described above. On 
testing with the other two indicators, it was found that the reaction was 
invariably alkaline all the way to methyl-orange, and acid all the way to 
phenolphthalein. 
These results seem at first sight confusing and contradictory, yet a 
consideration of the properties of the indicators used, not only renders them 
intelligible, but gives an indication as to the nature of the substances to 
which the contents of the intestine during fat absorption owe their reactions. 
An organic indicator only reacts to an acid which is stronger than the acid 
which it itself contains in its molecule ; to a weaker acid it is stable, and 
hence shows no acid reaction ; and in case the weaker acid is present as a 
salt, it decomposes that salt and reacts to the base with which it was com- 
bined, giving an alkaline reaction. Xow, methyl-orange is a very stable, 
phenolphthalein a very unstable, indicator, while litmus lies intermediate be- 
tween these two. Methyl-orange reacts sharply to the inorganic acids, less so 
to the stronger organic acids such as acetic acid, and not at all to carbonic acid 
and the weaker organic acids, including stearic, palmitic, and oleic acids. With 
alkaline salts of these weaker acids (carbonates, bicarbonates, and the soaps) 
it gives an alkaline reaction. Phenolphthalein reacts to traces of the weakest 
organic acids, and to carbonic acid ; to normal sodium carbonate it is alkaline ; 
to sodium bicarbonate, neutral ; with excess of carbonic acid, acid. Litmus 
reacts to even weak organic acids, but the reaction is feeble, and a considerable 
excess is necessary to give a clear reaction ; to carbonates and bicarbonates of 
the alkalies, it is alkaline. 
These considerations make it evident that the acid reaction of the 
upper part of the small intestine to litmus during fat absorption is due 
to weak organic acids, probably to dissolved acids set free from fats ; 3 
while the alkaline reaction to methyl-orange can only be due to weak 
organic acids combined with alkalies, i.e. in all probability to dissolved 
soaps. 
Since the acid reaction of the intestine during fat absorption is due 
to weak organic acids, the contention that soaps cannot be present falls 
to the ground. For the soaps would not be decomposed by these acids. 
An objection, and apparently at first sight a very serious one, to 
1 Jowrn. Physiol., Cambridge and London, 1895, vol. xviii. p. 2. 
2 Ibid., 1897, vol. xxi. p. 58. 3 Vide infra,. 
