or cavity of the tube ; d, mouths of the tubes opening
into the general cavity of the intestine; e, blind,
extremities of the tubes, corresponding to the sub-
mucous areolar tissue.
blind extremity. Its average diameter is
about 3-30^ f an i ncn except at its orifice,
where it is somewhat wider. The lower part
of the tube is often slightly enlarged : and
rarely it appears to bifurcate. But while it is
doubtful whether these appearances can be
depended upon *, it is certain that they are
not sufficiently marked or frequent to alter
the above general description, -j-
This cylindrical tube is composed of base-
ment membrane and epithelium. The former
constituent needs no special description. The
latter is a single layer of short columnar cells.
It clothes the whole interior surface of the
tubes ; and becomes continuous, at their upper
extremities, with the epithelium covering the
villi, the constituent cells of which generally
appear to be somewhat longer in shape. The
cylindrical cavity bounded by these cells has
* See foot-note to p. 321.
t A more valid exception to the above statement
may be found in the upper part of the duodenum
of s'ome of the domestic Mammalia ; in whom many
of these tubes divide, a little below the surface, into
three or four smaller ones. This condition may be
regarded as a transition from the structure of either
the pyloric tubes, or the clustered glands of Brunn,
to that of the ordinary intestinal tube. From the
appearances offered by the cylindrical epithelium
that lines their interior, the first of these conjec-
tures may be regarded as the more probable of the
two.
a diameter which amounts to about one-fourth
the width of the entire tube.
The arrangement of these tubes so precisely
recalls that of the gastric glands into which
their structure appears sometimes to merge,
as scarcely to require any separate descrip-
tion. Like these, they are placed vertically
side by side, in a sparing quantity of dense
fibrous matrix ; and are imbedded by their
lower extremities in a layer of a similar ap-
pearance. The latter contain much unstriped
muscle, the characters of which can be seen
Fig. 257.
Vertical and longitudinal section of the small intestine
in the lower part of the jejunum, showing the general
arrangement of its coats. (Magnified 50 diameters.)
cr, villi ; b, intestinal tubes ; c, submucous areolar
tissue ; d, circular fibres of the muscular coat ; e,
longitudinal fibres, external to these, covered by
peritoneum. ^
even more distinctly than in the analogous
gastric structure. The aggregate mass of these
vertical tubes forms the great bulk of the
mucous membrane. So that a vertical section
of this tunic exposes a dense pallisade of
tubes, the depth of which corresponds to the
thickness of the membrane: while a trans-
verse one shows that the interstices of the
cylinders are only occupied by a scanty matrix
enclosing numerous vessels.
The chief interruption to their presence is
caused by the racemose glands, villi, and fol-
licles, which will shortly be described. The
ducts of the first of these three structures
merely occupy a certain portion of space
which would otherwise be taken up by follicles.
But the two latter encroach upon the surface
of the mucous membrane much more con-
siderably. And since it is only between the
villi that we find the intestinal tubes, so the
number of such tubes which occupy the inter-
vals of these processes must necessarily cor-
respond to the thickness with which the latter
are strewed over the surface. Over the more
projecting parts of the follicles, the tubes are
also absent ; in a circular space which is sur-
rounded by a ring of apertures. The latter
belong to the inflected upper extremities of
348
STOMACH AND INTESTINE.
those tubes which immediately encircle each
follicle.
When fresh, these tubes always exhibit the
structure just described ; their only contents
being a clear, structureless, homogeneous fluid.
But from their minute size, it is obvious that
this fluid can never be obtained from their inte-
rior in sufficient quantity for any trustworthy
analysis. While unless the secerning process
were extremely rapid, even the secretion
poured forth into the tube would often be
mixed with those coarsely filtered contents
of the intestine which can enter its upper orifice
from the general cavity of the alimentary canal.
And as regards all fluids found in the gene-
ral cavity of the intestine, we ought never to
forget that to procure them in a state of abso-
lute purity is impossible. Under normal cir-
cumstances, the fluid present in any part of the
bowel can only be regarded as a complex mix-
ture of several ingredients, all of which are
probably themselves undergoing a continual
metamorphosis. Could we deduct from the
contents of the intestine all chyme, bile, and
pancreatic secretion, the residue would be
strictly an intestinal juice. And by far the
larger quantity of such a juice would be com-
posed of the secretion of the intestinal tubes.
Now we shall hereafter find that analogy sup-
plies us with some plausible conjectures re-
specting the fluid secreted by the duodenal
glands. While the closed follicles which
abut on the cavity of the bowel can scarcely
furnish sufficient fluid seriously to affect
the composition of any mixture which it may
contain. Hence whatever the share taken
by the vtlli, the secretory office of these tubes
might apparently be to some extent determined
from an examination into the chemical and
physiological properties of even such an im-
pure or mixed intestinal juice.
The reader will, however, hardly be sur-
prised when he is informed, that these condi-
tions have never yet been fulfilled; and hence,
that a satisfactory account of this interesting
fluid remains at present impossible. But he
must not therefore think the above allusions
superfluous. For it is only by a reference to
these conditions of experiment, that we can
judge how far we ought to accept the state-
ments made by various recent observers re-
specting this fluid.
Thus Frerichs* obtained intestinal juice
from fasting cats and dogs, in whom a few
inches of intestine had been emptied, and tied
at both ends, about five hours before thev
were killed. Lehmann -j- procured it from a
fistula of the small intestine, which had fol-
lowed an operation for hernia in the human
subject ; and in which another fistula, higher
up, gave passage to the ordinary mixed con-
tents of this part of the alimentary canal.
Zander J instituted fistulae in animals. And,
finally, Bidder and Schmidt, who adopted
Frerichs' method without obtaining one drop
* Op. cit. p. 850.
t Op. cit. vol. ii. p. 112.
j Koelliker, Op. cit.
Lehmann, Loc. cit. and Bd. iii. s. 335.
of intestinal juice, carefully compared the mix-
ture withdrawn from simple fistula?, with a
very small quantity of a purer fluid which was
yielded by dogs in whom the pancreatic and
biliary ducts had been tied, and the gall bladder
made to discharge its contents externally.
According to all these observers, the
intestinal juice is a transparent, viscid, and
strongly alkaline fluid. It contains nuclei,
and round or columnar nucleated cells;
an abortive cell-growth, the admixture of
which does not substantially affect the struc-
tureless character of the secretion. Of its
composition and reactions we can only say,
that it appears to contain mucus and the
ordinary salts; which together form a solid
residuum, that amounts to about 2 per cent.
of the whole quantity of fluid.*
As regards the physiological properties of
the intestinal juice, it has the power of con-
verting starch into grape sugar. But however
obvious the usefulness of this capacity, it is
possessed in an equal degree by so many other
animal substances, that it can hardly be re-
garded as the specific purpose or function of
this secretion.-}-
But the recent observations of Zander,
together with those of Bidder and Schmidt,
claim for this secretion a much more impor-
tant office : an office which would entitle
the whole of the small intestine to that ap-
pellation of a " ventrlculus succentiirlatus, "
which was formerly bestowed on the duode-
num. These observers agree in the state-
ment, that the intestinal juice dissolves protein-
compounds, both in and out of the body.
And from the careful quantitative researches
of Bidder and Schmidt, it would follow, not
only that its solvent powers upon these sub-
stances are from three to four times greater
than those of the gastric juice itself, but that
in the Dog, about half the daily albumen of a
flesh diet is habitually left untouched by the
stomach, to undergo solution in the intestine
by the secretion.
Against such a conclusion I would suggest
the following arguments, which together in-
duce me to think that this doctrine ought not
at present to be accepted. That a large organ
like the stomach, with a definite and com-
plicated structure, should so incompletely
discharge its single chemical function, is a
paradox which alone involves a great improba-
bility. This suspicion becomes still stronger
when we consider that, under normal circum-
stances, gastric juice is always conveyed from
the stomach into the intestine during the pro-
cess of gastric digestion ; while it is evident
that none of the experiments by these observers
quite exclude the possibility of such a transit.
Nay more, if we suppose what is surely not
* Bidder and Schmidt observed a much larger
quantity in the mixed fluid ; but point out that the
fixed contents would be raised by the addition of
the more concentrated secretions of the liver and
pancreas. Hence I prefer quoting the estimate
deduced by Lehmann from what was probably a
purer fluid.
f Compare the remarks on the pancreatic fluid,
in a subsequent part of this article.
STOMACH AND INTESTINE.
349
impossible that the juice carried onwards
into the intestine is there concentrated by
the partial absorption of its watery part, some
of the strange quantitative results obtained by
Bidder and Schmidt cease to be altogether in-
explicable. It may indeed be urged, that the
alkaline character observed in the intestinal
juice sufficiently proves that its digestive pro-
perties are not derived from the stomach.
But although the addition of a caustic alkali
destroys the efficiency of gastric juice, still
such a process seems very different from that
absorption of acid, or that gradual admixture
of a dilute alkaline solution, by which a similar
reaction would probably be communicated in
the living intestine. And, finally, is it like the
ordinary economy of Nature, that an elaborate
secretion should pass the pylorus, to be at once
annihilated, and then replaced by a second and
equally complex antagonist juice ? On such a
supposition, indeed, there are many animals in
whom almost all the gastric juice would be
wasted. For example, there is great reason to
suppose that the sojourn of food in the Horse's
stomach is so brief, that anything like the
stomach digestion of Carnivora is impossible.
But are we therefore entitled to assert that
this organ is utterly useless?
Such considerations appear to render it
more probable, that the gastric juice may
retain its digestive efficacy after passing
through the pylorus ; and that the presence
of this secretion in the small intestine suffi-
ciently explains the solvent powers of the
juice which is found in this situation.
But we are not left to such arguments alone
to disprove the solvent powers ascribed to the
intestinal juice by the above observers. They
receive a still more direct contradiction from
the experiments made by Frerichs* and by
Lehmann.f These authorities concur to state,
that neither in nor out of the body can it
dissolve the protein compounds. And Leh-
mann's case may be regarded as affording
much more than an ordinary negative re-
sult ; since in it, all communication between
the stomach and the fistulous aperture made
use of, seems to have been excluded.
Finally, we may recall to the reader that
close parallel which was observed in the action
of the gastric juice and the infusion of sto-
mach ; how, allowing for dilution and im-
purity, we found the latter behaving just like
the former. Now, in striking contrast to this
significant fact, numerous observers J agree in
representing the infusion of intestine as utterly
incapable of that solvent action attributed by
Bidder and Zander to its secretion. Indeed,
Koelliker and Goll have found the capa-
city of digesting protein-compounds so inti-
mately connected with the structure of the
proper gastric tubes, as to be almost lost in
the pyloric extremity of the Dog's stomach ;
where these begin to assume the characters
* Loc. dt.
t Is>c. cit.
I Kotlliker, Valentin, Todd and Bowman, and
others.
of intestinal tubes by losing their oval gastric
cells.
Hence all these circumstances throw great
doubt on the alleged solvent powers of the in-
testinal juice ; and render it impossible for us
at present to decide what is the exact digestive
office which it fulfils. And we are almost as
ignorant of its quantity as of its quality. But
it is probably secreted by the small intestine
in much greater amount than by the large.
According to Bidder and Schmidt, it is poured
out most freely about five or six hours after a
meal. And drinking soon increases its amount,
without causing any con verse diminution of its
concentration. Its strongly alkaline reaction
may be conjectured to have some relation to
that large quantity of acid, which is appa-
rently withdrawn from the chloride of sodium
contained in the blood of the stomach, in
order to furnish the gastric juice. Indeed, a
liberation of soda or some other alkaline base,
appears almost implied in that of the hydro-
chloric acid. But hitherto no exact analysis
has informed us to what particular substance
the alkaline character of the intestinal juice
is immediately due. And it is only after a
careful comparison of the composition and
quantity of this secretion with those of the less
alkaline bile and pancreatic fluid that we
should be entitled to conjecture, how far the
neutralization* of the acid peptone constitutes
a special function of the intestinal juice. Still,
from the great extent of secreting surface
which yields this juice, we can hardly doubt,
that it takes a large share in this neutralizing
process, which was formerly attributed chiefly
to the bile. It probably thus forms part of
that cycle of alternate decomposition and
recomposition, which appears to be under-
gone by the chloride of sodium.
The vascular arrangements by which these
intestinal tubes are supplied with blood, so
closely resemble those of the stomach -tubes,
as to Vender any special description of them
superfluous. Like the tubes themselves, the
vessels are chiefly concerned with secretion.
But while we are left in doubt as to the pre-
cise degree or kind of that absorptive function
which the vessels of these tubes possess, in
common with those of all such mucous sur-
faces, we are perhaps justified in attributing
a special capacity of absorption to the plexus
of large capillaries, which here, as in the
former organ, lies immediately beneath the
epithelium, around their open extremities.
The loops of this superficial plexus are
generally more simple than in the stomach.
They encircle the mouth of each tube with
what is often only a single ring of capillary
(b fig. 258.) ; except in the neighbourhood of
the solitary or agminate follicles, where they
resemble the analogous gastric vessels in
forming more complex meshes (a Jig. 20.)
They communicate very freely with the capil-
* It is impossible to state whether this neutra-
lization of the gastric acid takes place during the
sojourn of the gastric juice in the intestine, or after
its absorption into the capillary veins around the
canal.
350
STOMACH AND INTESTINE.
laries of the neighbouring villi. And the
venous radicles of these latter processes usually
unite with the branches formed by their con-
t Fig. 2.58.
Capillaries occupying the surface of the mucous mem-
brane of the small intestine ; as seen on examining
an injected specimen by reflected light, with a mag-
nifying power of about 50 diameters.
a, b, capillaries around the orifices of the intes-
tinal tubes. At a their meshes are more numerous
and complex than at b, where they are almost re-
duced to single capillaries ; c, calibre or cavity of
the intestinal tube.
flux in a small vein ; that sinks vertically
through the mucous membrane, to join the
sub-mucous plexus which gives origin to the
portal vein.
Villi. The interior of almost all the small
intestine presents to the naked eye a texture
very like that of velvet. For it is soft and
shaggy : yields readily to pressure : and, on
close examination, is evidently composed of
innumerable short filaments, which are placed
more or less vertically to the general inner sur-
face of the tube. These filaments, the dense
arrangement of which on a common surface
causes this general velvety appearance, are
thence usually named villi. Their form, and
their situation, or office, might also be denoted
by the name of intestinal or chyliferous papilla.
We have seen that, in the stomach, the con-
fluent ridges intervening between the tubes are
here and there raised into slight projec-
tions. These are rendered more prominent by
artificial injection of the subjacent vessels,
or even by that afflux of blood which ordi-
narily attends the digestive act. In the pylo-
ric extremity of the organ, these projections
become more distinct. And just at its termi-
nation, some of them often assume the form
of bluntly triangular and flattened folds.
In the upper part of the duodenum, the
villi begin; by processes which somewhat
resemble the gastric elevations just alluded
to, and occupy an analogous situation with
respect to the intestinal tubes. At first, they
may be described as flattened folds, the out-
line of which is a very obtuse triangle, that
has a broad base about four or five times
its height ( T o tn of an inch). In the lower
part of the duodenum, this rudimentary form
for the most part disappears ; and the villi,
which are still more or less flattened, have
about twice the length, and half the width, of
those present in the upper part. But it is
in the upper part of the jejunum that they
attain their greatest number ; being placed so
closely together that their interstices scarcely
Fig. 259.
Villusfrom the upper part of the jejunum, as seen in
the fasting state. Magnified 140 diameters.
a, epithelium of the villas ; 6, parenchyma or sub-
stance of the same.
equal their own bulk. Here they also acquire
their maximum length, which ranges from
about ^yth to ^th, or even T ^th or T \th of
an inch. Their form, however, is still that of a
flattened cone (compare Jig. 259. and Jig. 257.
p. 347.) ; the breadth of the base of which is
about Jth, and the depth about -j^th, its height.
In the remainder of the intestine, the length of
the villi gradually recedes to that which they
possess in the lower part of the duodenum ;
while their number also diminishes to a some-
what smaller extent. Throughout all this
extent, the shapes and sizes of contiguous
villi often present great varieties. But as a
rule, the lower we descend in the examina-
tion of the intestine, the greater is the number
of cylindrical- forms we meet with. While
towards the extremity of the ileum, the gradual
diminution of their size renders many of them
scarcely more than ^^o^ 1 f an m ch in dia-
meter.
The villi cover the whole surface of the
mucous membrane of the small intestine, in-
cluding its valvulae conniventes ; and they
extend to the free margin of the valve
which marks the commencement of the caecum
and colon. The only exception to their pre-
STOMACH AND INTESTINE.
351
sence occurs in the agminate follicles, or
" I'eyer's patches." Here they are absent
over the several follicles which together form
each patch ; and become short, (0, Jig. 272,
p. 358.) blunt, irregular, or even confluent,
where they occupy their interstices.
We have seen that each of the valvulae
conniventes is a doubled fold of membrane,
separated by a layer of areolar tissue. While
the minute intestinal tube may almost be re-
garded as a mere membranous lamina, which
is involuted so as to surround a cylindrical
cavity, and is packed in a sparing fibrous invest-
ment. But the villus constitutes, as it were, a
solid process of the mucous membrane. In
accordance with this structure, it consists of
an epithelium, a basement membrane, a stroma
or basis of fibrous tissue, unstripecl muscle,
and numerous blood-vessels. And in ad-
dition to these constituents, which may be
found under various modifications throughout
the svhole intestinal mucous membrane, the
interior of each villus encloses one or more
branches of the lacteal vessels which con-
tain the chyle.
The epithelium of the villi (a, fig. 259. and
a, jigs. 264, 265,266.) consists of a single layer
of cylindrical cells, which, as regards size,
shape, and general appearance, closely
resemble those seen on the ridges between
the tubes of the stomach. They are, how-
ever, even more delicate in their structure,
as well as more conical in their shape.
Fis.
And their contents are, even during fasting,
somewhat darker and more granular. The
nucleus, which occupies the same situation
in both these varieties of cylindrical epi-
thelium, contains a single bright spot, or
nucleolus : in rare instances, this appears to
be double.
The basement-membrane (at b, Jig. 260.)
does not require any special mention. As
in the gastric ridges, it is very closely at-
tached to the subjacent structures, espe-
cially to the vessels. But its continuity
with the similar structure forming the in-
testinal tubes sufficiently indicates that it
is really a distinct membrane. And it is
often demonstrated to be such by the action
of water ; which, after transuding it from the
outer surface, raises the membrane, in the shape
of a delicate transparent bulla,from the general
mass of the villus beneath.
The blood vessels of the villi are extremely
numerous.
Small arteries, (a a, Jig. 260.) of about T ^^th
of an inch in diameter, pass between the in-
testinal tubes. The base of each villus re-
ceives one, two, or more of these, according
to its size. They now pass upwards in
the substance of the process, at some dis
tance from its surface; and rapidly diminish
by giving off numerous capillaries, into which
their own trunks entirely merge at about the
middle of the height of the villus.
The ultimate capillaries themselves (c c t
260.
Vessels of two villi, injected. Magnified 100 diameters.
a a, arteries entering the basis of each villus near its centre; vv, veins seen in the same situation; c,
capillaries lying immediately beneath the limitary membrane ; d, tortuous capillaries occupying the
free extremity of one villus ; b, limitary or basement membrane of the villus, denuded of its epithelium
Jig. 260.) are, on an average, about rd of the
above diameter. They constitute a net-work,
which lies directly under the basement mem-
brane ; and covers the whole villus so thickly,
as to give it a vivid red colour in injecte'd
specimens. The shape and complexity of this
usually such, that the length of its meshes is
five or six times their width. The capillaries
are distinguished by their being apt to ex-
hibit a wavy and tortuous course (d, Jig.
260.) which often [causes their real length
greatly to exceed 'that of the villus itself.
network is liable to great variety ; but is This character is especially marked at the
352
STOMACH AND INTESTINE.
free extremity of the villas: to the contrac-
tion of the muscular layer of which it would
appear to be chiefly, though not wholly, due.
The veins (vv,fg.260.) come off from this
network by the gradual union of capillaries
in the upper half of the villus, so as to form
two or more venous trunks. These are
usually about double the width of the corre-
sponding arteries : they run at a distance
from them ; and often lie rather nearer to
the surface of the villus. Below, these
trunks become confluent in the single vein
of the process; which, passing vertically
downwards, terminates by joining one of the
numerous veins belonging to the venous plexus
around the orifices of the intestinal tubes.
And this latter network also joins that of the