Arthur Sheridan Lea Sir Michael Foster.

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present), peptone, pepsin and bile salts. The precipitate is ledis-
solved in an excess of bile or solution of bile-salts ; but the pepsin
though redissolved remains inert towards proteids. This precipi-
tation actually does take place in the duodenum, and we shall
speak of it again later on.

With regard to the action of bile on fats, the following state-
ments may be made. Bile has a slight solvent action on fats, as
seen in its use by painters. It has by itself a slight but only
slight emulsifying power : a mixture of oil and bile separate after
shaking rather less rapidly than a mixture of oil and water.
With fatty acids bile forms soaps. It is moreover a solvent of
solid soaps, and it would appear that the emulsion of fats is
under certain circumstances at all events facilitated by the pres-
ence of soaps in solution. Hence bile is probably of much greater
use as an emulsion agent when mixed with pancreatic juice than
when acting by itself alone. To this point we shall return.
Lastly, the passage of fats through membranes is assisted by
wetting the membranes with bile, or with a solution of bile-salts.
Oil will pass to a certain extent through a filter-paper kept wet
with a solution of bile-salts, whereas it will not pass or passes
with extreme difficulty through one kept constantly wet with
distilled water.

Bile possesses some antiseptic qualities. Out of the body its
presence hinders various putrefactive processes; and when it is
prevented from flowing into the alimentary canal, the contents
of the intestine undergo changes difl'erent from those which take
place under normal conditions, and leading to the appearance of
various products, especially of ill-smelling gases.

These various actions of bile seem to be dependent on the bile
salts and not on the pigmentary or other constituents.

Pancreatic Juice.

§ 209. Natural healthy pancreatic juice obtained by means of
a temporary pancreatic fistula difl'ers from the digestive juices of
which we have already spoken, in the comparatively large quantity
of proteids which it contains. Its composition varies according to
the rate of secretion, for, with the more rapid flow, the increase of
total solids does not keep pace with that of the water, though the
ash remains remarkably constant.

By an incision throngh the linea alba the pancreatic duct or (ducts)
can easily be found either in the rabbit or in the dog, and a c^innula
secured in it. There is no difficulty about a temporary fistula; but



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Ceip. 1.3 TISSUES AND MECHANISMS OF DIGESTION. 359

with permanent fistulse the secretion is apt to become altered in nature^
tndto lose many of its characteristic properties. Some, however, have
incceeiled in obtaining permanent fistulse without any impairment of
the secretion.

Healthy pancreatic juice is a clear, somewhat viscid fluid,
frothing when shaken. It has a very decided alkaline reaction,
and contains few or no structural constituents.

The avera^^e amount of solids in the pancreatic juice (of the
i^) obtained from a temporary fistula is about 8 to 10 p.c. ; but
in even thoroughly active juice obtained from a permanent fistula,
is not more than about 2 to 5 p.c, -8 being inorganic matter;
and this is probably the normal amount. The important con-
stituents of quite fresh juice are albumin, a pecuUar form of
pHJttiid allied to myosin, giving rise to a sort of clotting, a small
amount of fats and soaps, and a comparatively large quantity of
sodium carbonate, to which the alkaline reaction of the juice is
dae,and whic!i seems to be peculiarly associated with the proteids.
Since, as we shall presently see, pancreatic juice contains a
ferment acting energetically on proteid matters in an alkaline
medium, it rapidly digests its own proteid constituents, and, when
kept, speedily changes in character. The myosin-like clot is
di'^olved, and the juice soon contains a peculiar form of alkali-
albumin (precipitable by saturation with magnesium sulphate) as
well as small quantities of leucin, tyrosin and peptone, which seem
to bi the products of self-digestion and are entirely absent from
the perfectly fresh juice.

§210. Action on Food-stuffs. On starch, pancreatic juice
a-^ts with great energy, rapidly converting it into sugar (chiefly
nuluise). All that has been said in this respect concerning
saliva might be repeated in the case of pancreatic juice, except
that the activity of the latter is far greater than that of the
f inner. Pancreatic juice and the aqueous infusion of the gland
are always capable of converting starch into sugar, whether the
•tnimal from which they were taken be starving or well fed. From
the juice, or, by the glycerine method, from the gland itself, an
amylolytic ferment may be approximately isolated.

On prot^uis pancreatic juice also exercises a solvent action, so
fiir similar to that of gastric juice that by it proteids are converted
into peptone. If a few shreds of fibrin are thrown into a small
quantity of pancreatic juice, they speedily disappear, especially at
« teaiperature of 35® C, and the mixture is found to contain
peptone. The activity of the juice in thus converting proteids
into peptone is favoured by increase of temperature up to 40® or
thcreaWt*», and hindered by low temperatures ; it is permanently
d«stmyed by lioiling. The digestive powers of the juice in fact
•lepend, like those of gastric juice, on the presence of a ferment
which, as we have already said, may be isolated much in the



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360 TRYPTIC DIGESTION. [Book n

same way as pepsin is isolated, and to which the name trypsin has
been given.

The appearance of fibrin undergoing pancreatic digestion is
however different from that undergoing peptic digestion. In the
former case the fibrin does not swell up, but remains as opaque as
before, and appears to suffer corrosion rather than solution. But
there is a still more important distinction between pancreatic and
peptic digestion of proteids. Peptic digestion is essentially an
acid digestion ; we have seen that the action only takes place in
the presence of an acid, and is arrested by neutralisation. Pan-
creatic digestion, on the other hand, may be regarded as an alka-
line digestion ; the action is most energetic when some alkali is
present ; and the activity of an alkaline juice is hindered or de-
layed by neutralisation and arrested by acidification at least with
mineral acids. The glycerine extract of pancreas is under all
circumstances as inert in the presence of free mineral acid as that
of the stomach in the presence of alkalis. If the digestive mix-
ture be supplied with sodium carbonate to the extent of 1 p.c,
digestion proceeds rapidly, just as does a peptic mixture when
acidulated with hydrochloric acid to the extent of -2 p.c Sodium
carbonate of 1 p.c. seems in fact to play in tryptic digestion a
part altogether comparable to that of hydrochloric acid of -2 p.c. in
gastric digestion. And just as pepsin is rapidly destroyed by
being heated to about 40^ with a 1 p.c. solution of sodium carbo-
nate, so trypsin is rapidly destroyed by being similarly heated
with dilute hydrochloric acid of -2 p.c. Alkaline bile, which
arrests peptic digestion, seems, if anything, favourable to tryptic
digestion.

Pancreatic digestion and gastric digestion agree in that by
both proteids are converted into peptones. Naturally in the alka-
line pancreatic digestion no bye products allied to acid-albumin,
such as parapeptone, make their appearance ; there are however
various bye products on which we need not dwell. Albumoses
are not conspicuous in pancreatic digestion, they are very rapidly
carried on to the further stage of peptone.

In one respect there is an essential difference between gastric
and pancreatic digestion. In gastric digestion the products are
not carried beyond the proteid stage ; in pancreatic digestion part of
the proteid is changed into something which is no longer proteid.

During the pancreatic digestion of proteids, two remarkable
nitrogenous crystalline bodies, leucin and tyrosin make their appear-
ance. When fibrin (or other proteid) is submitted to the action of
pancreatic juice, the amount of peptone which can be recovered
from the mixture falls far short of the original amount of proteids ;
and the longer the digestive action, the greater up to a certain point
is this apparent loss. If a pancreatic digestion mixture be freed
from the bye products by neutralisation and filtration, the filtrate
yields, when concentrated by evaporation, a crop of crystals of



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Chap, i.] TISSUES AND MECHANISMS OF DIGESTION. 861

tjTosin. If these be removed the peptone may be precipitated
from the concentrated filtrate by the addition of a large excess
of alcohol and separated by filtration. The second filtrate upon
being concentrated by evaporation yields abundant crystals of
leucin and traces of tyrosin. Thus by the action of the pancreatic
juice a considerable amount of the proteid, which is being di-
gested, is so broken up as to give rise to products which are no
longer proteid in nature. From this breaking up of the proteid
there arise leucin, tyrosin, and probably several other bodies, such
as fatty acids and volatile sutetances. We said that in gastric
digestion more than one kind of peptone was probably formed,
and the same may be said of pancreatic digestion. We may now
add that in both gastric and pancreatic digestion two kinds of
peptone are probably formed, one of which resists the action of
trypsin, and undergoes no further change, but the other of which,
whether arising from gastric or pancreatic digestion undergoes
further change by the action of trypsin and it is this which
is the source of the leucin and other bodies of which we are
speaking.

As is well known, leucin and tyrosin are the bodies which
make their appearance when proteids or gelatin are acted on by
dilate acids, alkalis, or various oxidising agents. Leucin is a body,
which in an impure state crystallizes in minute round lumps with
an obscure radiate striation, but when pure, forms thin glittering
flat crystals. It has the formula CeH^jNO, or CaHjo-NH, (CO.OH)
and is amido-caproic acid. Now caproic acid is one of the " fatty
acid " series, so that leucin may be regarded as a compound of
ammonia with a fatty acid. Tyrosin, C»H„N03, on the other
hand, belongs to the "aromatic" series; it is a phenyl compound,
and hence allied to benzoic acid and hippuric acid. So that in
pancreatic digestion the large complex proteid molecule is split
up into fatty acid and aromatic molecules, some other bodies
of less importance making their appearance at the same time.
We infer that the proteid molecules are in some way built up
out of "fatty acid" and "aromatic" molecules, together with
other components, and we shall later on see additional reasons
for this view.

Among the supplementary products of pancreatic digestion
may be mentioned the body indol (CsH^N), to which apparently
the strong and peculiarly faecal odour which sometimes makes its
appearance during pancreatic digestion is due. Indol, however,
nnlike the leucin and tyrosin, is not a product of pure pancreatic
digestion, but of an accompanying decomposition due to the action
of organised ferments. A pancreatic digestive mixture soon be-
comes swarming with bacteria, in spite of ordinary precautions,
when natural juice or an infusion of the gland is used. When
isolated ferment is used, and atmospheric germs are excluded, or
when pancreatic digestion is carried on in the presence of salicylic



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362 TRYPTIC DIGESTION. [Book n.

acid, or thymol, which prevent the development of bacteria and
like organisms but permit the action of the trypsin, no odour is
perceived, and no indol is produced.

On the gelatiniferous elements of the tissues in the condition
in which they actually exist in the tissue previous to any treat-
ment pancreatic juice appears to have no solvent action. The
fibrillae and bundles of fibrillse of ordinary untouched connective-
tissue are not digested by pancreatic juice, which in this respect
afifords a striking contrast to gastric juice. But when they have
been previously treated with acid or boiled so as to become con-
verted into actual gelatine, trypsin is able to dissolve them, appar-
ently changing them much in the same way as does pepsin.
Trypsin unlike pepsin, will dissolve mucin. Like pepsin, it is
inert towards nuclein, horny tissues, and the so-called amyloid
matter.

On fats pancreatic juice has a twofold action. In the first
place it emulsifies fats. If hog's lard be gently heated until it
melts and be then mixed with pancreatic juice before it solidifies
on cooling, a creamy emulsion, lasting for almost an indefinite
time, is formed. So also when olive oil is shaken up with pancre-
atic juice, the separation of the two fluids takes place very slowly,
and a drop of the mixture under the microscope shews tliat the
division of the fat is very minute. An alkaline aqueous infusion
of the gland has similar emulsifying powers. In the second place
pancreatic juice splits up neutral fats into their respective acids and
glycerine. Thus palmitin (or tripalmitin) (CisHsi . CO . 0)8 . CsH,
is with the assumption of SHgO split up into three molecules of
palmitic acid 3(Ci5H8i. CO . OH) and one of glycerine (CsH5)(0H)j;
and, so with the other neutral fats. If perfectly neutral fat be
treated with pancreatic juice, especially at the body-temperature,
the emulsion which is formed speedily takes on an acid reaction,
and by appropriate means not only the corresponding fatty acids
but glycerine may be obtained from the mixture. When alkaU
is present, the fatty acids thus set free form their corresponding
soaps. Pancreatic juice contains fats, and is consequently apt after
collection to have its alkalinity reduced ; and an aqueous infusion
of a pancreatic gland (which always contains a considerable amount
of fat) very speedily becomes acid.

Thus pancreatic juice is remarkable for the power it possesses
of acting on all the food-stuffs, on starch, fats and proteids.

The action on starch, the action on proteids, and the splitting
up of neutral fats appear to be due to the presence of three distinct
ferments, and methods have been suggested for isolating them.
The emulsifying power, on the other hand, is connected with the
general composition of the juice (or of the aqueous infusion of the
gland), being probably in large measure dependent on the alkali
and the alkali-albumin present. The proteolytic ferment trypsin
as ordinarily prepared seems to be proteid in nature and capat^i



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Chap, i.] TISSUES AND MECHANISMS OF DIGESTION. 363

of giving rise, by digestion, to peptone ; but it may be doubted, as
in the case of pepsin and other ferments, whether the pure ferment
has yet been isolated. There are no means of distinguishing the
amylolytic ferment of the pancreas from ptyalin. The term pan-
creatin has been variously applied to many dififerent preparations
from the gland, and its use had perhaps better be avoided.

The action of pancreatic juice, or of the infusion or extract of
the gland, on starch, is seen under all circumstances, whether the
animal be fasting or not. The same may probably be said of the
action on fats. On proteids the natural juice, when secreted in a
normal state, is always active. The glycerine extract or aqueous
infusion of the gland, on the contrary, as we have already explained,
§ 200, is active in proportion as the trypsinogen has been converted
into trypsin.

Succus Entericus.

§ 211. When, in a living animal, a portion of the small
intestine is ligatured, so that the secretions coming down from
al«ve cannot enter its canal, while yet the blood-supply is
maintained as usual, a small amount of secretion collects in its
interior. This is spoken of as the succus entericus, and is supposed
to be furnished by the glands of Lieberkiihn, of which we shall
presently speak.

Saccus entericus may be obtained by the following method, known
AS that of Thiry modified by Vella. The small intestine is divided in
two places at some distance (30 to 50 cm.) apart. By fine sutures the
lower end of the upper section is carefully united with the upper end
of the lower section, thus as ft were cutting out a whole piece of the
small intestine from the alimentary tract. In successful cases, union
Wtween the cut surfaces takes place, and a shortened but otherwise
satiiifactory canal is re-established. Of the isolated piece the two
end* are separately brought through incisions in the abdominal wall
and their mouths carefully fastened in such a manner that each mouth
of the piece opens on to the exterior. During the process of healing
two fistulse are thus established, one leading to the beginning of and
the other to the end of a short piece of intestine quite isolated from
the rest of the alimentary canal ; oy means of these openings a small
quantity of fluid can be obtained.

The quantity secreted is said to be considerably increased by the
administration of pilocarpi n.

Saccus entericus obtained from the dog by the above method
ia a clear yellowish fluid having a faintly alkaline reaction and
containing a certain quantity of mucus. It is said to convert
stirch into sugar, and proteids into peptone (the action being very
similar to that of pancreatic juice), to split up neutral fats, to
emulsify fats and to curdle milk. It is also said to invert cane-
•ugar rapidly, and by a fermentative action to convert cane-sugar



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864 SUCCDS ENTERICUS. [Book n.

into lactic acid, and this again into butyric acid with the evolution
of carbonic acid and free hydrogen.

According to the above results, succus entericus is to be re-
garded as an important secretion acting on all kinds of food. But
even at the best, its actions are slow and feeble. Moreover many
observers have obtained negative results, so that the various state-
ments are conflicting. Besides, we have no exact knowledge as to
the amount to which such a secretion takes place under normal
circumstances in the living body. We may therefore conclude
that, at present at all events, we have no satisfactory reasons for
supposing that the actual digestion of food in the intestine is, to
any great extent, aided by such a juice.

Of the possible action of other secretions of the alimentary
canal, as of the caecum and large intestine, we shall speak when
we come to consider the changes in the alimentary canal.

§ 212. Gallstones, Concretions, often of considerable size,
known as gallstones are not unfrequently formed in the gall
bladder, and smaller concretions are sometimes formed in the bile
passages. In man two kinds of gallstones are common. One kind
consists almost entirely of cholesterin, sometimes nearly free from
any admixture with pigment, sometimes more or less discoloured
with pigment. Gallstones of this kind have a crystalline structure,
and when broken or cut shew frequently radiate and concentric
markings. The other kind consists chiefly of bilirubin in combi-
nation with calcium. Gallstones of this kind are dark coloured
and amorphous. Less common than the above are small dark
coloured stones, having often a mulberry shape, consisting not of
bilirubin itself, but of one or other derivative of bilirubin. Gall-
stones consisting almost entirely of inorganic salts, calcic carbon-
ates and phosphates, are also occasionally met with. In the lower
animals, in oxen for instance, bilirubin gallstones are not uncom-
mon, but cholesterin gallstones are rare.

A gallstone appears always to contain a more or less obvious
' nucleus,' around which the material of the stone has been de-
posited, and which may be regarded as the origin of the stone ;
the real cause of the formation of the stone lies however in certain
changes in the bile, by which the cholesterin, or bilirubin, or other
constituent ceases to remain dissolved in the bile. But we cannot
discuss this matter here.



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SEC. 4. THE SECRETION OF PANCREATIC JUICE
AND OF BILE.



§ 213. The Secretion of Pancreatic Juice, Although in some
cases, as that of the parotid of the sheep, the flow of saliva is
continuous or nearly so, in most animals, as in man, the inter-
mitience of the secretion is very nearly absolute. While food is
in the mouth saliva flows freely, but between meals only just
suflScient is secreted to keep the mouth moist, and probably the
greater part of this is supplied not by the larger salivary but by
the small buccal glands. The flow of pancreatic juice, on the
other hand, is much more prolonged, being in the rabbit continu-
ous, and in the dog lasting for twenty hours after food. But this
contrast between the secretion of saliva and that of pancreatic
juice is natural, since the stay of food in the mouth even during
m protracted feast is relatively short, whereas the time during
which the material of a meal is able in some way or other to affect
the pancreas is very prolonged.

The flow though continuous, or nearly so, is not uniform. In
the dog the flow of pancreatic juice begins immediately after food
has lieen taken, and rises to a maximum which may be reached
within the first, or as in the case furnishing the diagram given in
Fig. 83 the second hour, but which more commonly is not reached
until the third or fourth hour. This rise is then followed by a
fall, after which there is a secondary rise, reaching a second maxi-
mum at a very variable time but generally between the fifth and
9e^'enth hours. This second maximum, however, is never so high
as the first.

The second rise may be due to material absorbed from the
intt^tines being carried in the circulation to the pancreas and so
directly exciting the gland to activity, much in the same way as,
in the case of the stomach, the absorption of digested material
promotes the flow of gastric juice, see § 194 ; and a similar absorp-



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366



SECRETION OF PANCREATIC JUICE. [Book n.



tion may contribute to the first rise also, but it is more probable
tliat so marked and sudden a rise as this is carried out by some




Fig. 83. Diagram illubtrativg the influence op Food on the Seoretion
OF Pancreatic Juice. (N. O. Bernstein )

The abscissas represent hours after taking food ; the ordinates represent in c.c
the amount of Sjecretion in 10 min. A marked rise is seen at B immediately after
food was taken, with a secondary rise between the 4th and 5th hours afterwards.
Where the line is dotted the observation was interrupted. On food being again
given at C, another rise is seen, followed in turn by a depression and a secondary rise
at the 5th hour. A very similar curve would represent the secretion of bile.

nervous mechanism. The details of this mechanism have how-
ever not as yet been satisfactorily worked out.

Stimulation of the medulla oblongata, or of the spinal cord,
will call forth secretion in a quiescent pancreas, or increase a
secretion already going on. On the other hand a secretion already
going on may be arrested by stimulation of the central end of the
vagus, and the stoppage of the secretion which has been observed
as occurring during and after vomiting is probably brought about
in this way. This effect however is not confined to the vagus,
it occurs also after stimulation of other afferent nerves, such
as the sciatic.

§ 214. The Secretion of Bile. The act of secretion of bile by
the liver must not be confounded with the discharge of bile from
the bile-duct into the duodenum. When the acid contents of the
stomach are poured over the orifice of the biliary duct, a gush of
bile takes place. Indeed, stimulation of this region of the duo-
denum with a dilute acid at once calls forth a flow, though
alkaline fluids so applied have little or no effect. When no such



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Chap.!.] TISSUES AND MECHANISMS OF DIGESTION. 367

acid fluid is passing into the duodenum no bile is, under normal
circumstances, discharged into the intestine. The discharge is
due to a contraction of the muscular walls of the gall-bladder
and ducts, accompanied by a relaxation of the sphincter of the
orifice ; both acts are probably of a reflex nature, but the details
of the mechanism have not been worked out.

The secretion of bile on the other hand, as shewn by the
results of biUary fistuhe, is continuous ; it appears never to cease.
WTien no food is taken the bile passes from the liver along the
hepatic and then back along the cystic duct (the flow being aided



Online LibraryArthur Sheridan Lea Sir Michael FosterA text book of physiology → online text (page 43 of 148)