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the soaps formed.

3. The maximum of quantity and quality of emulsion formed coin-
cides with those conditions under which no formation of a membrane
can be demonstrated.^

There has been much discussion as to the factors at work in the formation
and conservation of emulsions. Brlicke Avas of the opinion that it was the
dissolved soap Avhicli conferred on the solution the power of holding the
globules apart, after they had been mechanically formed in it. Gad supposed
that the breaking up of the globules into smaller ones was due to a want of
correspondence of the rate of solution and diffusion of the soap formed into
the outer solution Avith the rate of diffusion of fatty acids towards the sur-
face of the globule. In case fatty acid diffuses from the inner part of the fat
globule towards the common surface of oil globule and solution more quickly
than it can be dissolved by the solution, a film or membrane of soap will form
around the globule. This film Avill not form at all parts of the globule equally,
and this will give rise to amoeba-like movements (due to differences in surface
tension). 2 Gad also supposes that the ultimate microscopic globules are sur-
rounded by soap films Avhich keep them from coalescing.

Quincke attributes the formation of the emulsion to the differences in sur-
face tension produced by the formation of a soap solution round the globule ;
and he also assumes the existence of films of soap (solid or in solution) around
the ultimate oil globules in the emulsion, Avhich have the property of keeping
the globules from coalescing.

There is no doubt that soap formation is an accompaniment to the forma-
tion of an emulsion of rancid oil in an alkaline solution, and it is easy to see
hoAV the formation of such a soap film, at accidentally varying rapidity, at
different points on the surface of a globule of oil, will cause variations in
surface tension at these points, and so cause the oil globule following the soap
film which covers it, to be drawn out into various shapes and split up. Such
surface tension phenomena may be observed Avhen two liquids which mix,
such as alcohol and water, are brought together. ]\Iore mixing takes place at
one point than another ; as a consequence, the mutual surface tension is less at
one point than another, and those rapid, streaming movements are produced
which may always be observed when alcohol and water are mixed. Similarly,
when a small piece of a substance like camphor is placed on water, rapid
shooting movements take place, due to an accidentally unequal solution of the
substance at different points in the circumference, and consequently varying
surface tension, as a result of which the piece of camphor is rapidly moved
about from place to place. In an exactly similar maimer a globule of rancid
oil will be pulled about, altered in shape, and broken up in an alkaline solu-
tion, from accidental variations in the strength of soap solution at different
points on its surface, causing variations in surface tension and corresponding
movements.

It is much more difficult to see how any permanent film of insoluble soap
can be formed round the ultimate globules, or even a film of soap solution of
different concentration from the rest of the menstruum in which the globules
float. From Gad's conclusions, it should be observed that in the cases Avhere
emulsion takes place best and most quickly, no such soap film can be observed,
so that this soap film cannot be experimentally demonstrated ; it is merely a
theoretical thing, devised from the supposed necessity of having sometbing to
keep the globules from coalescing. A proteid membrane surrounding tbe fat
globules in milk Avas supposed to have a similar office, but microscopically or
' Vide ivfra. - The Avords in parenthesis are added.



DIGESTION AND ABSORPTION OF FA TS. 44 7

otherwise no sucli membrane fan be demonstrated, and its existence is very
doubtful 1

A cloud is an emulsion, an emulsion of water particles in air, and no one
has ever supposed that the water particles are surrounded by membranes which
keep them apart. The prevention of coalescence is the result of the action of
several factors, of which our knowledge is not yet perfect. 1. One such
factor is the magnitude of the suspended drops ; the bigger the drops the more
rapidly they will come together, and fall (or rise) out of solution. ^ The more
mechanical agitation an emulsion is given, the longer it will persist under
otherwise unfavourable circumstances. 2. Another factor is the viscosity of
the menstruum ; the greater this is the more slowly will the finely-divided
globules be able to move through the fluid, under the influence of differences
in specific gravity or mutual attraction, so as to pass out of solution or
coalesce. 3. Another factor is the comparative specific gravities of the fat
and menstruum. 4. Still another is the mutual surface tension between
globule and menstruum; the greater this is, the greater Avill be the tend-
ency to diminution of surface, and hence to coalescence. On the other
hand, if the mutual surface tension were zero, the two fluids would mix in all
proportions.

It has been objected, by those who believe in the existence of a film around
the fat globules, to the contention that the altered nature of the menstruum is
sufiicient to account for the permanency of the emulsions obtained Avitli fats
and alkaline solutions, that a permanent emulsion cannot be obtained by
shaking up neutral fat Avith a soap solution. But the conditions in the two
cases are essentially diflerent. Keutral fats and fatty acids mix together in a
rancid fat or oil in all proportions. When such a mixture is submitted to the
action of alkali, the soap formation takes place where the fatty acids are, that
is, intimately mixed with the neutral fat. So that soap is formed everywhere
at the surface of the mass, and, dissolving, carries away (in the surface tension
diffusion streams above described) the intimately admixed fat from the main
mass in a very finely subdivided condition. If the proper conditions exist in
the solution, these minute fat particles Avill not coalesce again. Such a result
is brought al)out by the viscosity and reduction in surface tension Avhich the
solution acquires by means of the dissolved soap. On the other hand, when
neutral fat is shaken up Avith soap solution, no such disintegrating agency
comes into action, and the only thing to replace it is the mechanical subdivision
due to shaking. As v. Frey points out, the smaller the diameter of the fat
globules, the greater is the mechanical force necessary to subdivide them ; and
it is probable that by no amount of agitation can so fine a subdivision be
reached as is naturally attained by the formation of the soap amongst the fat.
By very prolonged and vigorous agitation, v. Fre)^ has obtained " mechanical
emulsions " of very considerable stability, even Avith neutral fats and Avater.
The very fine subdivision of the fat, and the increased viscosity of the men-
struum occasioned by the dissolved soap, are hence quite sufficient to explain
the permanency of emulsions of rancid oils and fats in alkaline solution.

Formation of emulsions in the intestine. — The formation of an
emulsion of fats in the intestine was already knoAvn to Eberle^ in 1834,
but Avas first brought into prominence by the classic researches of
Claude Bernard.'^ Bernard was unacquainted with our modern theories
of the formation of emulsion, and did not associate this process with

^ See V. Frey, Arch. f. Anat. u. Physiol., Leipzig, 1881, S. 382 ; Soxhlet, Landwirthsch.
Versuchss/at., 1876, Bd. xix.

- See V. Frey, loc. cit. ^ "Physiologie d. Verdauung," Wllrzljurg, 1834.

•* Comiot. rend. Acad. d. sc, Paris, 1849, tome xxviii. p. 249 ; Arch. gen. de med., Paris,
1849, S^r. 4, tome xix. p. 60 ; "Memoire sur le pancreas," Paris, 1856.



448 CHEMISTR Y OF THE DIGESTIVE PROCESSES.

the production of fatty acid by pancreatic juice, although he was the
discoverer of this saponifying action. He states that, when neutral oil
is shaken up with pancreatic juice, an instantaneous emulsion takes
place ; and, secondly, when neutral oil is submitted to the prolonged
action of pancreatic juice, fatty acids are developed. Bernard con-
sidered the formation of emulsion in the intestine as a more important
process than saponification, due to a ferment action, and speaks of a
"ferment emuls(f." It is now certainly known that fatty acids are
always formed in the intestine after the ingestion of fat, but an emulsive
ferment is no longer beheved in. The rapidity of fresh pancreatic juice
in forming fatty acid is remarkable; thus Eachford,^ in very favour-
able cases, found that a sufficient amount of fatty acid to form a spon-
taneous emulsion (5 "5 per cent.) is formed in presence of bile and
hydrochloric acid at room temperature in two minutes. This very
rapid action explains the error into which Bernard fell.^

Pancreatic juice obtained from a permanent fistula has less emulsive
power than that from a temporary fistula ; it is also poorer in proteid, and,
according to Kiihne,^ the emulsive power does not depend upon the alkali,
for faintly acid juice is capable of producing emulsion. Minkowski is of the
opinion that it is chiefly to the proteid that 'emulsion is due, basing his
opinion on the observation, made by Abelmann * in his laboratory, that after
excision of the pancreas no fat except that of milk is absorbed ; unless minced
pancreatic tissue be taken with the food, when other fats are also absorbed.
These observations have been confirmed by Sandmeyer.^

Some observers ^ hold that emulsification does not occur at all inside the
intestine, and others "^ state that a considerable amount of emulsification takes
place, but that the granules of fat in the emulsion are not nearly so small as
those found in the chyle.

Cash ^ found, in four experiments on dogs, that there was no emulsion
in the intestine during active fat absorption. Moore and Eockwood,^ in six
out of sixteen experiments, obtained a similar result, but in the other ten
experiments found emulsions in the intestine, containing fat globules of
various dimensions, some of considerable size, but many exceedingly minute.
These results indicate that in the dog at least, fats can be broken up and
absorbed without undergoing previous emulsification. Still it should be
-borne in mind that these two difi'erent conditions of the intestine in the
dog during fat absorption may be phases of the same process. The contents
of the stomach are not discharged continuously into the duodenum, but from
time to time the pyloric sphincter is relaxed, and a portion of the contents
of the stomach ejected. It may well be that the condition of no emulsion is

■^ Journ. Physiol., Cambridge and Loudon, 1891, vol. xii. p. 92.

^ The statement that the fat-splitting action of the pancreatic enzyme is very slow, and
hence that probably only a small percentage of fat is so decomposed in the intestine
(see Bunge, "Lehrbuch," Aufl. 3, S. 175), nndoubtedly arises from most observers using
not pancreatic juice but pancreatic extracts, in which the easily decomposable fat-splitting
enzyme was only present in traces. Rachford's results with pancreatic juice clearly indicate
that the pancreatic secretion is capable within the time of digestion of a i'atty meal of
decomposing all the fat into fatty acids and glycerin.

3 "Lehrbuch d. physiol. Chem.," 1868, S. 122.

•^ Inaug. Diss., Dorpat, 1890.

■' Ztschr.f. Biol., Miinchen, 1895, Bd. xxxi. S. 40.

^ Cash, Arcli.f. Anat. u. Physiol, Leipzig, 1880, S. 323; Altmann and Krclil, ihid.,
1889, Anat. Abth., Supp. Bd. S. 86; 1890, Anat. Abth., S. 97.

■^ Heidenhain, Arch. f. d. ges. Physiol., Bonn, 1888, Supp. Heft, Bd. xliii. S. 88,
Other recent observers who describe an emulsion in the intestine are, Lebedeff, Arch. f.
Anat. u. Physiol., Leipzig, 1883, S. 504 ; Lewin, Arch. f. d. ges. Physiol., Bonn, 1896, Bd.
Ixiii. S. 180!

^ Loc. cit. " Journ. Physiol., Cambridge and London, 1897, vol. xxi. p. 74.



EMULSION THEORIES OF FA T ABSORPTION. 449

that existing immediately after such a discharge from the stomach, while the
emulsion condition is a later stage.

In whatever form fats may be absorbed from the intestine, it is certain
that previous emulsifieation must greatly assist the digestive fluids, by
exposing an infinitely greater surface to their action. It is also certain that
in a great many cases, if not in all, previous emulsifieation does take
place.

Emulsion theories of fat absorption. — It was for a long time a
popular theory that only a small fraction of fat is split up in the intes-
tine into fatty acid and glycerin ; and that by means of the small
amount of acid so formed, aided by that present in the fat as it leaves
the stomach, the remainder of the fat is converted into a fine emul-
sion which passes as such into the villi, and reaches the central lacteal.^
Such a statement may be found in most text-books, but the progress of
recent work has had a tendency to cast grave doubts on its truth, and
to show that, at least as a general statement, it is erroneous. The theory
does not rest on any direct observation of the amount of fat which
leaves the intestine as emulsified fat, compared with that which leaves
it in other forms, such as soap, glycerin, and emulsified fatty acids,
— such a direct observation, in the present state of our knowledge, is
impossible, — but on indirect evidence, which is briefly as follows : —

1. The presence of a very small percentage of fatty acid is all that
is necessary in presence of an alkaline solution to perfectly emulsify
neutral fat.

2. This small amount of free fatty acid can readily be furnished by
the action of the pancreatic enzyme even on neutral fats, and to aid this
action all fats contain already some fatty acid mixed with them. The
alkaline juices poured into the intestine are capable of supplying the
alkali necessary for emulsifieation.

3. When an animal is killed during active fat digestion, the lacteals
invariably contain a white milky emulsion, consisting mainly of neutral
fats with a small percentage of alkaline soaps.

Therefore the most natural conclusion is that a fine emulsion is
formed in the intestine which passes in some manner into the lacteal.
The greater part of the fat is only physically, not chemically,
altered in digestion, and passes through the whole process as a neutral
fat.

The weak point in the emulsion theory of absorption always was,
how the fat globules got into the interior of the villus and made their
way to the lacteal. Although the fat granules in an emulsion are of
microscopic dimensions, they are still large compared to the dissolved
molecules of serum or egg albumin which are unable to pass into or out
of the intestine through the epithelial cells. If fat granules pass into
the epithelial cells at all, it must therefore be by means of a
special kind of absorption in bulk by these cells, and not by a process
even of selective diffusion from solution. Such an absorption by bulk is
easily carried out by a cell of which the protoplasm is capable of free
contraction, such as the amoeba, or leucocyte, but it is difficult to conceive
how it can take place with a fixed cell, such as those which line the
intestine. Impressed, perhaps, with the necessity of some such proto-
plasmic movement, some observers have looked earnestly for proto-

•^ This theory was first stated by Brlicke, Sitzungsh. d. h. Akad. d. Wissensch., Wien,
1870, Bd. Ixi. Abth. 2, S. 362.

VOL. I. — 29



450 CHEMISTR Y OF THE DIGESTIVE PROCESSES.

plasmic processes from the epithelial cells, and one or two ^ fancied, they
had discovered such appearances, but their observations have not been
confirmed, and are undoubtedly erroneous. If the epithelial cells of the
intestine possessed the power of absorbing in bulk fat granules, there
is no obvious reason why other food particles, such as granules of
starch or proteid, should not be similarly absorbed, but no such
absorption has ever been observed, nor are they capable of absorbing
finely subdivided granules of coloured matter, such as carmine.

The mucous membrane of the intestine contains an immense
number of lymph corpuscles.^ These are found not only in the lymph-
oid nodules, which occur so abundantly as solitary glands and Peyer's
patches, but in the intestinal villi, even between the epithelial cells, where
they may approach quite close to the free surface, and abundantly in the
adenoid tissue underlying them. Now, such lymph corpuscles are capable
of enveloping and so absorbing fat granules, and have been credited with
an important function in the removal of fat from the intestine by so
doing. It was stated by Zawarykin ^ that when fat absorption is going
on, fat granules are to be found only in these lymphoid cells and not in
the cells of the columnar epithehum. This statement is undoubtedly
erroneous, for it is easy, from an animal killed after a meal rich in fats,
to obtain sections showing the columnar cells filled with fat globules.

" During active fat absorption, especially if the amount of fat in the
chyme is relatively large, the columnar epithelial cells become filled
with globules of fatty matter. These globules are of variable size, and
may occur in all parts of the cell, but they are generally largest in the
part between the nucleus and the thickened border, and are often quite
small near the attached end of the cell." *

It is evident, then, that the greater part of the fat, if not the whole
of it, must be absorbed by the epithelial cells from the intestine. It
is also very improbable that these cells take up the fat in the form of
an emulsion. As has already been stated, the structure of the cell is
unsuitable for such a function, and, in addition, fat granules have never
been observed in the broad striated border. This almost amounts to a
demonstration that the fat passes through the border of the cell in some
soluble form, and is afterwards thrown down in a particulate form, as the
resid-t of a process of cell metabolism.

Emulsion theories of fat absorption are therefore being gradually
replaced by theories of absorjotion in solution. These theories must
next be discussed, but before doing so reference may be made to another
emulsion theory of fat absorption introduced by Munk.

Theory of I. Munk. — Munk^ showed that fatty acids can be
emulsified under exactly the same conditions as rancid fats, and further
that these fatty acids are capable of absorption, and can completely take
the place in the animal economy of neutral fats, being in great measure

1 V. Tlianlioff'er, Arch. f. d. gcs. Physiol., Bonn, 1874, Bd. viii. S. 391 ; Fortuuatow,
ibid., 1877, Bd. xiv. S. 285.

- These wandering cells ( Wcmderzellen) were first described as occuiTing in the epithelium
by Ebertli {Wiirzb. rued. Ztschr., 1864, S. 23); Arnstein {Virclww's Archiv, 1867, Bd.
xxxix. S. 537) first mentioned the presence of fat granules in them.

'■>Arch.f. d. ges. Physiol., Bonn. 1883, Bd. xxxi. S. 231.

•* Schafer, Internat. Monatschr. f. Anat. ii. Histol., Leijizig, 1885, Bd. ii. S. 6.

^ Verhandl. d. Bed. 'tried. Gesdlsch., March 1879 ; Arch. f. Anat. u. Physiol., Leipzig,
1879, S. 371 ; Virchoiv's Archiv, 1880, Bd. Ixxx. S. 10; ibid., 1884, Bd. xcv. S. 409; Ztschr.
f. physiol. Chem., Strassburg, 1885, Bd. ix. S. 568; Arch. f. Anat. u. Physiol., Leipzig,
1890, Supp. Bd., S. 138. See also v. Walther, ibid., 1890, S. 329.



SOL UTION THE OKIES OF FAT A B SORPTION. 4 5 1

converted into fats somewhere on their way from the intestine to the
thoracic duct. He is hence of the opinion that in the normal course of
digestion a considerable but indeterminate amount of fat may be
absorbed in the form of emulsified fatty acids.

Munk's experimental results as to the absorption and synthesis
during the process of absorption of the fatty acids, are of the highest
importance ; but it in no wise follows from them that the fatty acids
are absorbed in the form of an emulsion. Such a theory is subject to
the same objections as have above been urged against the older theory
of absorption as emulsified fats. The fatty acids are probably taken
up from the intestine hj the epithelial cells in some soluble form, and
synthesised to neutral fats- in these cells.

Solution theories of fat absorption. — Theory of absorption as
soaps. — One of the most important theories of fat absorption in soluble
form is, that the neutral fats are split up by the action of the pancreatic
enzyme into fatty acids and glycerin, that the fatty acids unite with a
part of the alkali of the intestinal secretions to form alkaline soaps
which are soluble in water, and that the alkaline soaps and glycerin
are absorbed in solution by the epithelial cells, and there synthesised
back to neutral fats. This theory is supported by a good deal of
experimental evidence. Eadziejewski^ showed that alkaline soaps were
absorbed ; Perewoznikoff,^ that a mixture of alkaline soap and glycerin
was absorbed and synthesised to neutral fat. The lacteals had the usual
milky appearance seen after a fatty meal ; microscopic preparations,
stained with osmic acid and with alkanna, showed in the tissue of the
villi, and in the epithelial cells, fat globules of varying size. Will,^ work-
ing under G-riinhagen's direction, confirmed these results by histological
observations on the frog; further, he showed that the presence of
glycerin was unnecessary. Will made two kinds of experiments. In
one he fed the frogs, which had previously been deprived of food,
with the materials to be tested ; in the other, he injected the materials
into the living but cut out intestine, and then examined teased speci-
mens stained with osmic acid. In both series the same results were
obtained, on feeding with a mixture of pure palmitic acid and glycerin,
or of potassium palmitate and glycerin ; at the end of twenty-four
hours an examination of the villi showed a formation of fat, by the
presence everywhere of large distinct fat globules. Injection of palmitic
acid alone into the intestine also led to the appearance of fat globules
in the epithelium,* but these were not nearly so numerous as in the
cases in which the palmitic acid was mixed with glycerin. As
Salkowski and Munk had shown that fatty acids can be emulsified
under certain conditions,^ Will proceeds to show that this could not be
the case in his experiments, and that the fat globules blackening with
osmic acid in the epithehal cells are not free fatty acid. The free fatty
acids only become emulsified when melted, and as pure palmitic acid

1 Virchow's ArcMv, 1868, Bd. xliii. S. 271 ; 1872, Bd. Ivi. S. 211.

^ Centralhl. f. cl. oned. Wissensch. , Berlin, 1876, S. 851.

^ Arch. f. cl. ges. Physiol, Bonn, 1879, Bd. xx. S. 255. See also v. Krelil, Arch. f.
Anat. It. Physiol., Leipzig, 1890, Anat. Abth. , S. 97.

•* In thus showing the formation of fat from fatty acid alone, Will anticipated I. Mimk,
but to Munk belongs the merit of clearly showing from the chemical standpoint that the
organism, probably the epithelial cells, can furnish the glycerin radicle for the synthesis
of neutral fats from the fatty acids.

'' Sitz'ungsb. cl. Perl, physiol. Gesellsch., March 1879 ; Virchow's Archiv, 1880, Bd. Ixxx.
This was a re-discovery of a fact known to Marcet many years previously, see p. 444.



452 CHEMISTRY OF THE DIGESTIVE PROCESSES.

only melts at 62^ C, such a thing could not occur in the frog's intestme.
Moreover, a microscopic examination of the intestinal contents at the
end of an experiment showed only amorphous masses of fatty acid and no
emulsified globules. Will concludes that the fatty acid must be absorbed
as a soap and not as an emulsion.

That the mucous membrane of the small intestine is capable of
taking part in such a synthetical process, is shown by experiments of
Ewald,^ who dried the mucous membrane of a dog's intestine, which had
been killed in a condition of hunger, at a low temperature after the
method introduced by Brown and Heron,^ and showed that this was
capable of inducing the formation of neutral fat, from a mixture in
proper proportions of soap and glycerin.

This experiment shows that, provided glycerin and soap are formed
in the intestine, there is an agency provided for synthesising them
back into neutral fats. Let us next consider what the probabihties are
that such a complete decomposition, into fatty acids and glycerin
followed by solution of the fatty acids as alkaline soaps, takes place in



Online LibraryE. A. (Edward Albert) Sharpey-SchäferText-book of physiology; (Volume v.1) → online text (page 64 of 147)