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found that the portal vein during proteid digestion contained no greater a
percentage of these bodies than the arterial blood, and Neumeister'^ found
that the portal vein, while absorption of peptone was going on, did not
contain a trace of this material. Neumeister also circulated defibrinated
blood, to which peptone had been added through a liver immediately

^ In many of the papers referred to in this section, "peptone" is used to signify wliat
would to-day be called a mixture of albumose and peptone ; this has usually been trans-
lated by albumose and peptone, or by albumose.

" Arch. f. Anat. u. Physiol., Leipzig, 1880, S. 33. See also Hofmeister, Ai'ch. f. exper.
Path. u. Pharmakol., Leipzig, 1885, Bd. xix. S. 17.

" Ztschr. f. Biol., Mtinchen, 1888, Bd. xxiv. S. 277, Neumeister caught the blood
from the carotid in ammonium oxalate to prevent clotting ; laked by shaking with ether ;
removed ether ; saturated with ammonium sulphate; filtered; reduced filtrate by evapor-
ating to a small bulk, filtering from time to time from crops of crystals ; and tested in final
filtrate for albumoses by the biuret test with negative results. Control experiments showed
that even a trace of albumose added to the blood intentionally could be easily identified.

■* When large amounts are injected, the fall in arterial blood pressure is so great that
secretion of urine is arrested. Even in such a case the albumose does not remain in the
blood, but passes into the lymph (Shore, Joitrn, Physiol., Cambridge and London, 1890,
vol. xi. p. 549).

^ Ploz and Gyergyai, Arch. f. cl. ges. P/iysiol., Bonn, 1875, Bd. x. S. 536 ; Hofmeister,
Ztschr. f. pliysiol. Ghem., Strassburg, 1881, Bd. v. S. 131 ; Schmidt-Midheim, Arch. f.
Anat. It. Physiol., Leipzig, 1880, S. 33 ; Fano, ibid., 1881, S. 281 ; Shore, Journ. Physiol.,
Cambridge and London, 1890, vol. xi. p. 528. A similar effect follows subcutaneous
injection (Hofmeister, loc. cit.).

« Arch./. Anat. u. Physiol., Leipzig, 1880, S. 33.

"^ Sitzungsb. d. phys.-med. Gcsellsch. zv, tVurzhurg, 1889, S. 65; ZfscJir. /. Biol.,
Mtinchen, 1888, Bd. vi. S. 287.



440 CHEMISTR Y OF THE DIGESTIVE PROCESSES.

after its removal from the body, and proved that the peptone remained
unchanged. Also, when a small quantity of x^eptone or albumose was
slowly injected into a mesenteric vein, tliis was not assimilated, but
appeared afterwards in the urine, showing that it had not been altered
by the liver. Shore ^ circulated peptone not only through the liver, but
through the spleen also, by injecting into a splenic artery, and arrived at
similar results ; practically, all the peptone appeared again in the urine
unchanged.

These results show that the albumose and peptone do not even enter
the 'portal cii^culation as such : the only remaining place where they can
undergo modification is in the wall of the intestine itself, and the
following experiments show that this is the seat of change.

Ludwig and Sahdoli ^ separated a loop of intestine in the dog with
the attached portion of mesentery, and injected a gramme of albumose and
peptone m 10 per cent, solution, ligaturing the piece of intestine at both
ends. The piece of isolated intestine was maintained alive by circulating
through it warm defibrinated blood diluted with normal sahne, by means
of a cannula inserted into that branch of the mesenteric artery which
had supplied the loop (anastomosing arterial branches being excluded by
ligatures), the blood, after circulating, flowing away by the corresponding
branch of the mesenteric vein. After four hours of perfusion in this manner,
the piece of mtestine and the defibrinated blood having been all the time
maintained at the body temperature, the remaining contents of the intes-
tine and the circulating fluid were examined for albumose. The intestine
contained about half a gramme of coagulable proteid, and only traces of
albumose, while the defibrinated blood contained no albumose whatever.
Therefore the albumose must have disappeared in the intestinal wall.

Hofmeister^ investigated the organs of dogs killed during proteid
digestion as to their content of albumose, and found it present in the
mucosa (only) of the stomach and intestine, as well as in small quantities
in the blood, and in four out of ten cases in the spleen ; in the other
organs it was entirely absent. He also showed experimentally that this
albumose underwent a rapid change.

A fresh stomach was divided into two symmetrical halves, or a piece of
small intestine longitudinally into two similar pieces.

The surface of the mucous membrane was washed clean with saline, then
one of the two pieces in each case was thrown immediately into boiling water,
while the other was similarly treated after being first kept for some time in a
moist chamber at 40' C. More peptone was always found in the first piece
than in the second, and when the second piece had been kept for a sufficient
time (1 to 2 hours) at body temperature, previous to placing in boiling water, it
was found to contain no albumose whatever. In another experiment, while
one piece was thrown, as before, immediately into boiling water, the second was
thrown for some minutes into water at 60' C, and then kept as before at 40°
C. for two hours j the result now obtained was that both pieces contained an
equal amount of albumose. Since most enzymes would not be aff'ected by such
a preliminary treatment, Avhile living cells would be destroyed, this indicates
that the cells of the mucosa do not owe their activity to contained enzymes.

'^ Journ. Thy^iol., Cambridge and London, 1890, vol. xi. p. 559; Verhandl. d. X.
internat. med. Cong., Berlin, 1891, Bd. ii. Abtli. 2, S. 31.

"^ Arch. f. Anat. u. Physiol., Leipzig, 1880, Supp. Band, S. 112.

' Ztsclir. f. physiol. Cham., Strassburg, 1882. I'd. vi. S. 51 ; Arch. f. exper. Path. u.
PharmakoL, Leipzig, 1885, Bd, xix. S. 8,



ABSORPTION OF PR O TEIDS. 44 1

Neumeister ^ states that albumoses and peptones dissolved in
whipped blood can be changed by mere contact with pieces of living
intestine, the rapidity of change being increased when a slow stream of
ail' is driven through the mixture, so as to luring the pieces of intestine
into rapid contact with different portions of the blood and albumose.

Hofmeister ^ observed a considerable increase in the number of
leucocytes in the intestinal wall during digestion of proteids, and argued
from this that these took a considerable share in proteid absorption and
in the conversion of albumoses and peptones in the adenoid tissue of
the intestinal wall, and in the mesenteric lymphatics. There is little
experimental ground for belief in such a theory. In the first place,
proteid is not absorbed to any appreciable extent by the lymphatics ;
secondly, albumoses are not changed, as Hofmeister ^ himself has shown,
in the blood, which contains plenty of leucocytes ; thirdly, Heidenhain *
has shown that the amount of leucocytes in the wall of the intestine
(and the amount of active mitosis in these) is too small to render them
adequate for such a purpose. Finally, Shore ^ has shown that, after
slow injection of a small amount of peptone (-049 grms.) into a lym-
phatic of the hind-limb in a dog, this can be detected again in the course
of twenty minutes in the chyle flowing from a fistula of the thoracic
duct, showing that it has traversed the lymphatic system unchanged.

All these experiments go to prove that albumoses and peptones are
modified during their passage through the epithelial cells by the action
of living protoplasm. What substances are formed from them is not
known by direct experiment, but it is highly probable that the process
is one of conversion backwards into coagulable proteid. It is known
that coagulable proteid can be artificially obtained from peptone and
albumose,^ and that albumose and some forms of pe^jtone used as foods
can replace coagulable proteid in maintaining nitrogenous equilibrium.
It is dilScult to see how such a result can be attained otherwise than by
a formation of coagulable proteid from albumose and peptone.

The percentage of any proteid foodstuff, which is absorbed from the
alimentary canal, may be deduced fairly accurately from a comparison
of the amount of nitrogen in the food with that of the urine and faeces
when such a food is taken into the system.

Experiment shows that the various forms of proteid are utilised by
the organism in widely varying degrees. It does not necessarily follow
that a food of which the nitrogenous part is only partially absorbed is
on that account to be despised as an adjunct to other classes of
nitrogenous food ; vegetable proteid is absorbed much more imperfectly
than that from animal sources ; but vegetable food, amongst other
things, is valuable for the consistency and bulk it gives to the food,

1 " Lehrbuch der physiol. Chem.," Jena, 1893, Th. 1, S. 251; Ztschr. f. Biol.,
Miincheu. 1890, Bd. xxvii. S. 324.

"- Arcii. f. exper. Path. u. Pharmakol., 1885, Bd. xix. S. 32; 1886, Bd. xx. S. 291 ;
1887, Bd. xxii. S. 306. See also Pohl, ihid., 18S8, Bd. xxv. S. 31 ; Heidenhain, Ao'ck. f.
d. ges. Physiol., Bonn, 1888, Supp. Heft., Bd. xliii. S. 72.

^ Hofmeister {loc. cit., Bd. xix.) is of the opinion that the portion of "peptone" which
he believes enters the blood nnchanged is converted in the tissue, "peptone" being found
during digestion in the arteries but not in the veins. The presence of any albumose or
peptone, even in the arteries, is, according to more recent observers, however, very doubt-
ful.

■* Loc. cit.

^ Journ. Physiol., Cambridge and London, 1890, vol. xi. p. 553.

6 See p. 400.



442 CHEMISTR Y OF THE DIGESTIVE PROCESSES.

and for the mechanical stimulation its presence gives to the intestinal
movements.

The small amount of vegetable proteid absorbed, compared with that
of animal proteid, is in part due to the envelope of indigestible cellulose
by which it is surrounded, in part to the shorter stay in the intestine
due to its action in causing increased peristalsis, and in part to its own
less digestible character.

The percentage of various kinds of plant proteid absorbed also varies
considerably ; thus the proteids of some leguminous plants and cereals are
absorbed nearly as perfectly as those of animal origin, while in most
others (potato, lentil) it is much less complete (22 to 48 per cent. less).
The percentage of the nitrogen of meat or egg appearing again in
the faeces in man, only amounts to 2'5 to 2 '8 per cent., that of milk to
6 to 12 per cent.

Considerable tracts of the aUmentary canal can be removed or
thrown out of action without causing the death of the animal or even
causing serious impairment in absorption.

The stomach was first removed by Czerny ^ in dogs : one animal
was preserved alive after such an operation for five years : in the course
of two months after the operation it recovered to quite a normal con-
dition, and ate, digested, and absorbed all kinds of food. It was finally
killed for examination by Ludwig and Ogata, and the dissection
showed that only a very small portion of the cardiac end of the stomach
remained.

Ludwig and Ogata ^ further investigated the course of digestion and
absorption when gastric digestion is excluded, by another method. They
made a fistula beyond the pylorus and inserted into the beginning of the
duodenum a small thin rubber ball, attached to a rubber tube, by means
of which it could be distended with water under pressure, so as to
occlude the intestine from the stomach. In this way gastric juice could
be prevented from entering the duodenum, and by feeding from the
fistula the effect of intestinal digestion alone be studied. The food was
usually completely digested and absorbed, and the faeces presented a
normal appearance. Eaw meat was digested much more efficiently than
boiled, connective tissue was not so completely digested as in normal
dogs, but nevertheless two injections of meat per diem sufficed to keep
the animal in equilibrium.

The stomach has also recently been removed in dogs by F. de Filhpi,^
who found no disturbance in metabolism and no increase in intestinal
putrefaction in spite of the absence of hydrochloric acid.

The same experimenter also removed in a bitch 1"9 metres of the
small intestine (almost the entire length), and found no metabolic
disturbance, except that the absorption of fat was diminished ; the animal
lived, and afterwards brought up a litter of pups in this condition.
The author suggests that the large intestine here vicariously took on the
absorptive functions of the small intestine.

Complete or partial extirpation of the pancreas, or ligature of its
duct, causes more or less disturbance of proteid digestion and absorption,
but not so much as might be expected, in view of the most important
proteolytic function of tlie secretion of tins gland.

^ " Beitriige z. operativen Cliirurgie," Stuttgart, 1878, S. 141.
'Arch. f. Anal. u. PhysloL, Leipzig, 1883,'^ S. 89.
^Deutsche med. Wchnschr., Leipzig, 1894, No. 40, S. 780.



DIGESTION AND ABSORPTION OF FATS 443

Minkowski and Abelmann^ found, after complete removal of the
gland in dogs, that on an average 44 per cent, of proteid was absorbed ;
after partial removal, 54 per cent. The amount of aljsorption was much
increased on giving raw pancreas with the food. Sandmeyer ^ obtained
similar results. On removal of all but one-fifth to one-fourth of the gland
(the portion remaining behind not being in communication with the
intestine), 60 to 70 per cent, of proteid was still absorbed, and, on adding
a supply of finely-minced pancreas to the food, the absorption of proteid
became almost normal.



DIGESTION AND ABSOEPTION OF FATS.

The pancreatic juice is the only digestive secretion which contains an
enzyme possessing a chemical action on the neutral fats.^ This action
consists in splitting the fats into fatty acids and glycerin,* and may
be demonstrated in one of the following ways : —

1. A neutral fat is first obtained, e.g., by thoroughly shaking olive
oil with sodium carbonate solution and ether, pipetting off the ethereal
layer, filtering if necessary, and finally allowing the ether to evaporate,
when a neutral fat is left behind. This is mixed either with fresh
pancreatic juice, or an extract of the fresh gland prepared as already
described, and the mixture, after being coloured blue by the addition
of litmus, is placed in a bath at 37° to 40° C. The alkaline reaction is
seen gradually to change into an acid one.

2. Instead of adding litmus, after the mixture of neutral oil and
pancreatic juice, or extract, has digested for some time (half to two hours),
sodium carbonate solution is added (which converts the free fatty acids
formed into soaps), and the unattached fat is removed by repeated
extraction with ether. The residue is next treated with dilute
sulphuric acid, setting free again the fatty acids, which are extracted
with fresh ether, and recovered after its removal by evaporation.

3. The formation of free fatty acid may be also qualitatively shown,
by removing water from the fresh, finely-divided gland, with 90 per cent,
alcohol, drying it with filter paper, and then covering it with a neutral
ethereal solution of butter, obtained by shaking up milk or cream with
ether and a solution of caustic soda. When this material is kept for a
short time at 37° to 40" C, a distinct odour of butyric acid appears ; and if
the mixture has been previously rendered blue by litmus, this turns red.

Form in which fats are absorbed from the intestine. ^ — ^There has
been much discussion as to the extent to which the decomposition
of the fats by the pancreatic enzyme, as above described, takes place
in the intestine ; and also as to the subsequent fate in the intestine
of the fatty acids formed therein. According to the views held on

1 " Ueber die Ausnutzuiig der ISTahrurigsstoffe nacli Pancreasextirpation," Inaiig. Diss.,
Dorpat, 1890 ; Juhresb. il. d. Fortsclir. d. Thier-Cheon., Wiesbaden, 1890, Bd. xx. S. 45.

'Ztschr.f. Biol, Miinchen, 1895, Bd. xxxi. S. 35.

^ Fats are said to undergo a certain amount of decomposition into fatty acids in the
stomach (Marcet, Proc. Roy. Soc. London, 1858, voh ix. p. 306 ; Cash, Arch. f. Anat. 21.
Physiol., Leipzig, 1880, S. 323) ; the cause of this decomposition is unknown, but it is
probably bacterial during the first stage of gastric digestion.

'^ Bernard, Compt. rend. Acad. d. sc, Paris, tome xxviii. ; Arch. gc'n. de mc'd., Paris,
1849; "Memoire sur le pancreas," Paris, 1856; " Lecons de physiologic expferinientale,"
tome ii. p. 256. For the chemical equations representing such a decomposition, see
Chemistry of the Fats, p. 19.



444 CHEMISTR Y OF THE DIGESTIVE PROCESSES.

these subjects by different experimenters, various theories have been
propounded as to the form in which fats leave the intestine. These
theories may be divided into two classes — (a) Those in which it is held
that the fats are absorbed in particulate form, as emulsified fats or
fatty acids ; (b) those in which it is held that the fats are absorbed in
solution as fatty acids or as soaps.

Emtdsification.- — All fat or oil which has not been specially
neutralised contains a slight amount of free fatty acid. On long
standing in contact with air, the amount of this fatty acid is increased,
probably by bacterial action; when this proceeds beyond a certain
limit, the fat is said to become rancid.

If such a rancid oil, or fat melted by gently warming, be briskly
shaken up with a solution of an alkaline carbonate {e.g. a 0-25 per cent,
solution of sodium carbonate), it becomes suspended permanently in the
alkaline solution in the form of very minute particles or globules,
and so forms what is known as a permanent emulsion. But if the
rancid oil be previously carefully neutralised {e.g. by mechanically
shaking for some hours with a saturated solution of barium hydrate
at 95° C, and then pipetting off),^ no amount of shaking with a solution
of an alkaline carbonate afterwards will cause it to yield a permanent
emulsion; the fluid on standing will quickly settle into two distinct
layers. Neither can a lasting emulsion be obtained by shaking up a
rancid oil or fat with distilled or acid water ; some free fatty acid and
some alkali must be simultaneously present. In other words, the
necessary conditions for the formation of a soap must be satisfied.^

Emulsifying action of alkaline salts and hile. — Attention was 'first
drawn to the action of alkaline salts in promoting emulsion by Marcet ^
in 1857 ; this author investigated the effect of both disodic phosphate
and of bile on fatty acids and on neutral fats ; his results have not
obtained, even in English text-books, the attention they deserve, and
seem in part to have become forgotten. The results with bile and
fatty acids have an important bearing on more recent researches, to be
subsequently described, and for this reason are here quoted at length.

Disodic phosphate, " when mixed with pure stearic and margaric acids
prepared from sheep's fat, and heated, produced a perfect emulsion, resemhhng
milk ; on cooling, a substance solidified, consisting of fatty acids with more or
less soda, soap, and a small quantity of phosphate of soda ; therefore the
formation of the emulsion had been attended with that of a small proportion
of soap. When neutral fats were heated, suspended in a solution of phosphate
of soda, no emulsion ocrAtrred ; the fats fused, and, on cooling, solidified under
the form of a hard cake ; the warm mixture, although strongly shaken, was
not converted into an emulsion, hut the minutely divided globules of fat rose
to the surface, uniting with each other, and solidified on cooling; the fluid
remained perfectly clear.

"The next subject for inquiry was to determine whether bile exerts
a similar action on fatty acids and neutral fats. On heating and agitating
gently a mixture of fresh sheep's hile and fatty acid (margaric, stearic, and

1 Racliford, Journ. Physiol., Cambridge and London, 1891, vol. xii. p. 73.

- Only formation of "artificial emulsions," if the expression may be used, from rancid
oils is referred to here ; it will be seen later that a jiancreatic emulsion can be formed and
persist in presence of an acid reaction due to fatty acids.

" Compt. rend. Soc. de hiol., Paris, 1857, p. 191 ; Proc. Roy. Soc. Jjondon, 1858, vol, ix.
p. 306 ; Med. Times and Gdz., London, 1858, N. S., vol. xv'ii. p. 209. The extracts are
taken from the last quoted Journal.



DIGESTION AND ABSORPTION OF FA TS 445

oleic acids), prepared from sheep's fat, as soon as the latter had begun fusing it
disappeared, and fiyiallij the whole of the fatty acid loas dissolved ; on standing,
however, it was observed that a very few extremely minute globules of fat rose to
the surface. As soon as the mixture had been allowed to become colder than the
temperature of fusion of the fatty acids, it assumed a turbid appearance
throughout, which gradually increased, the fluid becoming Avhite and milky,
slightly coloured by the bile; finally, if the fat present was in sufficient
proportion, the whole mass was converted into a semifluid paste, possessed of
a light green colour, and adhering so strongly to the sides of the vessel that it
could be turned upside down without letting out its contents.

" On diluting this remarkable emulsion with water, its consistency only
Avas altered, becoming thinner, but no decomposition occurred ; on heating
the diluted mass, the emulsion was dissolved ; it disappeared, but no globules
of fat could be seen floating on the surface beyond the few minute specks
previously mentioned. Besides this physical action of bile on fatty acids, the
phenomenon was accompanied by a chemical decomposition ; for the bile,
which was neutral or slightly alkaline before the experiment, had become
strongly acid after being treated with the fatty acid.

"An experiment was now instituted to determine whether a similar
phenomenon takes place when bile and neutral fats are mixed together.
Indeed, it was hitherto generally admitted that bile had no action on neutral
fats. The results of my observations confirm this view, for in no case could I
succeed in obtaining an emulsion and chemical decomposition, by heating bile
with pure sheep's fat or with oil, having a neutral reaction ; on agitating the
hot mixture the globules of fat were broken up, but on standing they rose to
the surface, the bile being unaltered in its appearance and reaction. Conse-
quently, bile exerts no action on neutral fats."

Since these experiments of Marcet, many observers have busied
themselves with the nature and mode of formation of emulsions.^

Brlicke foiuid that the presence of a certain amomit of free fatty acid
v^as sufficient to emulsify the remaining neutral fat, and stated that the
provision of a sufficient amount of free fatty acid to emulsify the rest
was probably the chief function of the fat-splitting property of the pan-
creatic juice. He obtained emulsion of fats containing fatty acids with
diluted egg albumin, with bile, and especially with solutions of sodium
carbonate and of borax. Gad discovered spontaneous emulsion, and
carried out exact experiments on the most favourable conditions for the
formation of emulsions. A spontaneous emulsion means the formation
of a permanent emulsion without any mechanical assistance by shaking ;
such as occurs when a drop of oil containing a sufficient percentage of
free fatty acid (5-7 per cent.) is placed on an alkaline solution of suit-
able strength (^ per cent, sodium carbonate).

The following are the main conditions which influence the formation
of spontaneous emulsions, according to Gad : — •

1. The power of different fats to form emulsions by contact with the
same fluid depends {a) on the amount of free fatty acid in the fat, (h) on
the solubility of the soaps formed from these fatty acids, (c) on the
viscosity of the fat.

2. The power of the same fat to form emulsions in contact with

^ Ktihne, " Physiol. Chem.," 1866, S. 129 ; Brilcke, Sitzungsb. d. k. Akad. d. Wissensch.,
Wien, 1870, Bd. Ixi. Abth. 2, S. 362 ; J. Steiner, Arch./. Anat. u. Physiol., Leipzig, 1874,
S. 286 ; J. Gad, ibid., 1878, S. 181 ; G. Quincke, Arch. f. d. ges. Physiol., Bonn, 1879, Bd.
xix. S. 129 ; v. Frey, Arch. f. Anat. u. Physiol., Leipzig, 1881, S. 382; Rachford, Journ.
Physiol., Cambridge and London, 1891, vol. xii. p. 72.



446 CHEMISTR Y OF THE DIGESTIVE PROCESSES.

different fluids depends («) on the degree of alkalinity of the fluid, (&) on
their chemical composition, in so far as this influences the solubility of



Online LibraryE. A. (Edward Albert) Sharpey-SchäferText-book of physiology; (Volume v.1) → online text (page 63 of 147)