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the intestine.

The idea that only a small fraction of the fats is decomposed in
the alimentary canal into fatty acids and glycerin, has arisen from
repetition of the emulsion theory only, and not from any experimental
observation of lack of intensity of action of the fat-splitting ferment.
Hoppe-Seyler^ found that most of the fatty matter in both small and
large intestine was composed of stearic and palmitic acids accompanied
by very little neutral fat, and concludes that the decomposition into the
fatty acids and glycerin is much greater than is usually supposed.
Eachford^ states that pancreatic juice must act verj^ rapidly on fats,
under the favourable conditions found in the duodenum, and is capable,
imless checked or retarded in some manner, of splitting all the fats of
the food into fatty acids and glycerin in the time required for intestinal
digestion.

It may be concluded, then, that there is sufficient fat-splitting power
provided in the intestine for the complete conversion of the fats into
fatty acids ; and it has been already pointed out that, on feeding with
fatty acids or with soaps, these are absorbed, and converted into fats in
the process. It only remains to consider, in connection with the soap
theory, whether, in the natural process of fat digestion and absorption,
it is probable that the fatty acids so set free combine with alkahes to
form soaps, or whether they are absorbed in some other soluble form.

It has been objected to the theory of absorption in the form of
soaps, that the reaction of the small intestine in the dog during fat
absorption is not alkahne, but acid ; that soaps cannot persist in
presence of an acid reaction, and hence that fats cannot be absorbed
as soaps.

Cash^ investigated the reaction of the intestine in three experiments on
dogs, in Avhich the animals were fed on a mixture of starch and fat, and in
three similar exijeriments in which the animals were fed on fat alone. He

'^ Arch. f. Anat. u. Physiol., Leipzig, 1883, Supp. Bd., "Festschrift f. du Bois-Rey-
mond," S. 302, Vorlaufige Mittlieiluiig.
- Proc. Hoy. Soc. London, 1880, p. -393.
^ Firchotu's Archiv, 1863, Bd. xxvi. S. .534, Aiimerkung.
■* Journ. Physiol., Cambridge and London, 1891, vol. xii, p. 92.
^Arch.f. Anat. u. Physiol., Leij^ig, 1880, S. 323.



SOL UTION THE OKIES 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^ 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- 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, methyl-
orange, and phenolphtlialcin. The reaction to litmus of the upper part of the
small intestine was found to be 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 csecum lies intermediate between
that of the contents of the ileum and that of the contents of the large in-
testine, and may be 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 residts 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 ; wdth 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 v^eak organic acids, probably to dissolved acids set free from fats ; ^
w^hile 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

^ Journ. Physiol., Cambridge and London, 1895, vol. xviii. j). 2.
" Ibid., 1897, vol. xxi. ^. 58. ^ Vide infra.



454 CHEMISTR Y OF THE DIGESTIVE PROCESSES.

absorption in the form of soaps, is that urged by I. Munk, namely, the
enormous quantity of alkali which would be required for such a purpose.
Munk ^ reckons that to so combine with the fatty acids of 200 grms. of
fat, about 40 grms. of sodium carbonate (ISTaaCOg) would be required.
Now a dog of 25 kilos, can easily digest from 200 to 350 grms. of fat
in twenty-four hours.^ Supposing only 200 grms. are digested, and that
all this is absorbed as soaps and glycerin, about 40 grms. of sodium
carbonate will be required for the purpose ; now the total blood only
contains, in such an animal, alkali equivalent to 6 grms. of NagCOg ; if
the other fluids of the body be supposed to contain an amount of alkali
equivalent to another 6 grms. of sodium carbonate, the total alkalinity is
equivalent to that of 12 grms. of sodium carbonate.^ Therefore, to
suffice for the absorption of the fatty acids as soaps, from three to four
times the total alkali of the body must pass out in the intestinal
secretions, and be reabsorbed with the fatty acids, during twenty-four
hours. This is obviously impossible ; therefore the fats are not absorbed
as soaps and glycerin.

This objection of Munk loses, however, most of its weight, when
the probable processes taking place, in case fats are absorbed as soaps
and glycerin, and synthesised again to neutral fats in the epithelial
cells, are carefully considered. In the synthesis of fat from soap and
glycerin within the cell, alkali is again set free in exactly equal amount
to that in which it was used up in the intestine, and this alkali must be
got rid of by the cell in some manner. Why should it not be sent back
again into the intestine, and act as a carrier to a fresh quantity of fatty
acid as soap into the cell ? In such a fashion a very small amount of
alkali would suffice to explain the carriage of all the 200 grms. of fat as
dissolved soap and dissolved glycerin into the epithelial cells.

It might possibly be further objected that soaps are only present in
small quantity in the intestinal contents. But this applies also to
alkali alljumin, propeptones, peptones, and sugars ; in fact, to all the
products of the digestion of both proteids and carbohydrates. If soaps
are normally absorbed by the epithehal cells, it is probable that these
cells possess a selective capacity for soap absorption, as they do for many
other products of digestion, and hence that the soaps are absorbed as
they are formed, and never allowed to accumulate in appreciable
quantity in the intestine.

There is, then, no proof that soaps cannot be formed in the intestine,
nor is there any impossibility or improbability in the way of all the fats
being first decomposed into fatty acids, then converted into soluble
soaps and absorbed as such.

Theory of absorption as dissolved fatty acids. — Another theory is, that
the fats are absorbed in the form of dissolved fatty acids.

The fatty acids of the fats are practically insoluble in Avater, but are
soluble to a certain extent in bile, the solubility increasing with rising temper-
ature. Strecker* stated, in 1848, that taurocholic acid possesses the property
of dissolving fat, fatty acids, and cholesterin in considerable quantity. This
fact is mentioned by Strecker in connection with the difficulties attending the

^ Virchovjn ArcUv, 1880, Bd. Ixxx. S. 11 ; 1884, Bd. xcv. S. 408.
" Petteiikofer and Voit, Zischr. f. Biol., Miinclien, 1873, Bd. ix. S. 30.
" These figures must only be taken as argumentative data, overstepping the truth, and
not as truly indicating the total alkalinity.

■* Ann. d. Chem., Leipzig, 1848, Bd. Ixv. S. 29.



SOL UTION THEORIES OF FA T ABSORPTION. 45 5

preparation of taurocliolic acid in a pure condition from l)ile. He did not
pursue the subject further on its own account, and his statement is in part
erroneous, for the neutral fats scarcely dissolve at all in bile. In 1858, Marcet^
published the results already described, showing the great solubility of the
fatty acids in bile when heated above their melting points.. Latschinoff^
described a variable compound, or rather mixture, formed by taurocholic acid
with a mixture of stearic and palmitic acids, which possesses certain crystallo-
graphic properties, but no definite chemical composition.

Altmann,^ mainly on histological grounds, concluded that fats are
not absorbed as an emulsion, but in some soluble form.

Krehl,^ under Altmann's direction, obtained sections of the intestine,
stained by osmic acid, from animals killed at varying times after feeding
on fat (olive oil and cream). These preparations showed a gradual
increase in the size of the globules with the advancement of the period
of digestion. Also, it was observed that in the earlier stages the
small fat globules showed a clear centre, surrounded by a dark ring.
From these appearances it was judged that the formation of the fat
granules was a gradual one from solution, and not from drops of fat
emulsion. In considering the soluble form in which the fats are
absorbed, Altmann rejects the idea that they are absorbed as soaps,
chiefly on the ground that the reaction in the small intestine of the dog
is acid, so that it cannot contain dissolved soaps ; ^ yet from such a por-
tion of intestine, with an acid reaction and containing a clear fluid, the
charged lacteals are often to be seen conveying away absorbed fat.
Altmann cites the statements as to the solubility of the fatty acids in
bile already mentioned,*^ and adds an experiment of his own, in which he
shows that a considerable, but not too great, quantity of a solution of com-
mercial glycerin soap, and then excess of hydrochloric acid, may be added
to a solution of sodium glycocholate or taurocholate without producing
any precipitation of either fatty or bile acid. From these data, and the
observation of Munk that the fatty material found in the dog's intestine
during fat digestion may contain as much as 12 per cent, of free fatty
acids, Altmann argues that the free fatty acids are dissolved in the
intestine by the bile acids. As the fatty acids so dissolved are absorbed,
fresh amounts of the neutral fats are decomposed, and the free fatty
acids so formed pass into solution to replace those removed by absorp-
tion. So that there is a cyclic process involving the decomposition of fats,
solution of fatty acids in the bile acids present, absorption of these fatty
acids by the intestinal cell, and regeneration of neutral fat within the
cell, accompanied by the appearance of fat granules.

Altmann did not quantitatively determine the amount of solubility of
fatty acids in bile acids, bile, or intestinal fluid. The solubility in bile varies
greatly with temperature, as is shown by Marcet's experiments.'^ At the
temperature of the body the solubility is much less than at the temperature of
fusion of the fatty acids, but is still considerable ; while at ordinary atmo-
spheric temperature (14° to 15° C.) the solubility is very slight.^

The solubilities of the fatty acids, and mixtures of these at or near the

1 See p. 444.

- Ber. d. deidsch. cheon. Gssellsch., Berlin, 1880, Bd. xiii. S. 1911.

'^ Arch. f. Anat. tt. Physiol., Leipzig, 1889, Auat. Abth., Siipp. Bd. S. 86.

4 i5ic^., '1890, Anat. Abth., S. 97.

^ This objection is discussed under the soap-absorption theory. See p. 453.

6 Except those of Marcet. '^ See p. 444.

® Moore and Rockwood, Journ. Physiol., Cambridge and London, 1897, vol. xxi. p. 58.



456 CHEMISTR Y OF THE DIGESTIVE PROCESSES.

temperature of the body, have recently been determined by Moore and Rock-
wood/ 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, resjjectively,
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 " from 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, ^ 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 2:)rove that in the normal course of
events all the fat is absorbed in such form.

The acids of the fats give an acid reaction Avith 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 Avay from pylorus to c£ecum, and is never acid to that
indicator for a greater distance than 6 in. from the pylorus.^

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 jDresent (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.

•^ Loc. cit.

2 Similar results were not obtained with filtered intestinal contents obtained from the
rabbit or pig.

^ Moore and Rockwood, loc. cit.



PASSAGE OF THE EAT TO THE LACTEALS. 457

If a rabbit be killed some hours after a meal of oats, a certain amount of fat is
shown to be in process of absorption by tlie whiteness of the lacteals, but the
reaction of the contents of the small intestine is always markedly alkaline.

It is probable, then, that in all animals a great part of the fat is
absorbed dissolved in the form of soaps ; but in some animals a part
is also absorbed as dissolved fattj acids, while in others the entire
quantity leaves the intestine in the form of soaps.

These various theories as to the form in which fats enter the epithelial
cell, may be summarised as follows : —

Emulsion theories. — 1. A small percentage of the fat is sjDlit up into
fatty acids and glycerin, the fatty acids unite with the alkaline basis of the
mixed secretions present in the intestine, and the rest of the fat is thereby
converted into an emulsion, which is absorbed by the columnar cells.

2. A considerable part of the fat is split up into fatty acids and glycerin,
and absorbed as emulsified fatty acids and glycerin, which are synthesised
to neutral fats by the columnar cells.

Solution theories. — 1. All the fat is split up into fatty acids and glycerin;
the fatty acids combine with alkaline bases to form soluble soaps ; these and
the dissolved glycerin are absorbed in solution, and synthesised to neutral
fats in the columnar cells.

2. All the fat is split up into fatty acids and glycerin ; the fatty acids
are dissolved as such by the intestinal fluid (the bile being that constituent
which gives this solvent property to the fluid), these dissolved fractions of the
fat are absorbed by the columnar cells, and by these are synthesised again to
neutral fats.

3. The processes indicated under solution theories 2 and 3 probably
mutually replace each other to a variable extent in some animals, but in
others absorption takes place entirely in the form of soaps.

Passage of the fat from the epithelial cells to the lacteals.—

In whatever form the fat passes into the columnar cells, it is certain
that it is here converted again into fat. During active fat absorption
these cells become gorged with fat globules of varying dimensions. It
is agreed by all observers that this fat passes from the epithelium to
the lacteals in the form of an emulsion, but there is some difference of
opinion as to the fashion in which it is conveyed.

It has already been stated that the tissue of the villi, especially
during active fat absorption, contains immense numbers of leucocytes.
These are found not only in the subepithelial tissue, but between the
epithelial cells. The number in this position is greatly increased during
absorption, and at this time lymphoid cells occur also in the lacteals,
but " are found more numerously in the lacteals of the villi than in
those which are more deeply seated, and, most numerously of all, near
the blind end of the lacteal. That they pass into this vessel from
the surrounding lymphoid tissue is certain, for a lymphoid cell may
often be seen, fixed by the reagent employed for hardening the tissue,
in the act of passing through the wall of the lacteal." ^ After a meal
containing fat, these lymphoid corpuscles contain granules, which stain
black with osmic acid ; many of these are soluble in ether, so that they
are unquestionably composed of fat.

^ Schafer, Internat. Monatsclir. f. Anat. ii. Histol., Leipzig, 1885, Bd. ii. S. 6. The
greater part of the description of the carriage of fat by leucocytes, between epithelium and
lacteal, given in the text, is abstracted from this source.



458 CHEMISTR Y OF THE DIGESTIVE PROCESSES.

These appearances led Schafer^ to express the view that the
lymphoid corpuscles have an important function in taking up the fat
from the epithelial cells, and carrying it towards and into the lacteal,
where they set the fat free by disintegrating. No fat particles are, as a
rule, found between the epithelium and the central lacteal, save such as
are embedded in lymphoid corpuscles. Nor is there any channel of
communication between the epithelial cell and the lacteal, as was
formerly supposed, by which the fat globules might be carried into the
lacteal. The epithelial cells never penetrate the basement membrane,
nor are they continued into the cells of the retiform tissue beneath.
Wiemar ^ admits the presence, during fat absorption, of fat granules in
the leucocytes, but from the small amount of fat so found, compared
with that in the epithelial cells, considers that the leucocytes can only
be of secondary importance. In this connection it should be noted that
Schafer ^ has pointed out that the relative amount of fat granules in
leucocytes and epithelial cells varies with the activity of absorption.
" When the absorptive activity is feeble, or when the amount of fat in
the chyme is relatively small, 4;here may be little or no fat in the
columnar epithelial cells, although the amoeboid cells between them may
be gorged with fat granules. In frogs fed with lard in the spring,
fatty globules are still abundant in the columnar epithelial cells on the
eighth day after the feeding, whereas, in frogs similarly fed in November,
the greater part of the fat was discharged 'p&r anum, by the third day,
very little being absorbed, and what was being taken up during that
time was only to be found in the amoeboid cells, none at all being
present in the epithelial cells themselves." This seems to indicate that,
when the rate of absorption is slow, the amoeboid cells are able to keep
pace with it, but when the supply is too abundant for this, the columnar
cells act as temporary storehouses, and become filled with granules,
which are afterwards carried off by the amoeboid cells.

Heidenhain* ascribes only a secondary importance to the leuco-
cytes. He gives as grounds for this opinion — (1) That in newly-born
puppies, which have already sucked, and in which milk absorption is
going on, there are scarcely any leucocytes present in the epithelium, so
that there is no constant connection between fat absorption and the
presence of leucocytes. (2) Leucocytes containing granules, which stain
black with osmic acid, are to be found in the crypts of Lieberkiihn, mto
which fat cannot enter from the intestine. (3) The material which is
stained black with osmic acid is chiefly something else than fat, since
it stains with acid-fuchsin, and cannot be washed out of adhesively
mounted sections by ether or xylol.^ Heidenhain^ admits, however,
that in the guinea-pig fat is undoubtedly present in considerable
quantity in the amoeboid cells during fat absorption.

Heidenhain '' still adheres to the emulsion theory of absorption, but

^ Quain's "Anatomy," Sth edition, 1876, vol. ii. p. 363; " Pract. Histology," 1876,



Online LibraryE. A. (Edward Albert) Sharpey-SchäferText-book of physiology; (Volume v.1) → online text (page 65 of 147)