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mann, there are never more than two intercommunicating lacteals in a single
villus in the human subject (fig. 113, J) ; but both he and Frey found a copious
network in the villi of the sheep. Like the lymphatics elsewhere, the lacteals in
the villi are bounded by a delicate layer of flattened epithelial cells. These are
connected with the branched cells of the tissue of the villus (fig 110, c), and
these again with the flattened cells which form the basement membrane ; from
the latter, prolongations extend between the epithelium-cells towards the surface.

The muscular tissue within the villus was discovered by Briicke ; it consists
of a thin stratum of plain fibre-cells disposed longitudinally around the lacteal ;
on being stimulated in animals they produce an obvious retraction of the villus.




THE SMALL INTESTINE.



91



This muscular tissue is a prolongation from the muscularis mucosse. The fibre-
cells at the sides and towards the end of the villus pass from the lacteal to be





Fig. 113. INJECTED LACTEAL VESSELS IN TWO VILLI OF THE HUMAN INTESTINE.
100 DIAMETERS. (TeichmanD. )

The lacteals are represented as filled with white substance and the blood-vessels with dark, a, 6,
the lacteal vessels, single in one villus and double in the other ; c, the horizontal lacteal vessels with
which those of the villi communicate ; d, the blood-vessels, consisting of small arteries and veins with
capillary network between.

Fig. 114. VERTICAL SECTION OF THE INTESTINAL MUCOUS MEMBRANE OF THE RABBIT.
(Slightly altered from Frey. ) ip

Two villi are represented, in one of which the dilated lacteal alone is shown, in the other the blood-
vessels and lacteal are both seen injected, the lacteal white, the blood-vessels dark : a, the lacteal
vessels of the villi ; a', horizontal lacteal, which they join ; 6, capillary blood-vessels in one of the
villi ; c, small artery ; d, vein ; e, the epithelium covering the villi ; g, tubular glands or crypts of
Lieberkubn, some divided down the middle, others cut more irregularly ; i, the submucous layer.

A, cross section of three tubular glands more highly magnified.

attached to the basement membrane (fig. 110, m. m.) ; usually their attachment to
this is forked, a connective tissue corpuscle filling up the interval (Watney).



^ tr. -




B.




Fig. 115. COLUMNAR EPITHELIUM CELLS OF THE RABBIT'S INTESTINE. (E. A. S.)

A. Two cells isolated after maceration in very weak chromic acid. They are much vacuolated,
and one of them (2) has a fat-globule attached near its end ; the striated border (str. ) is well seen, and
the bright disk separating this from the cell-protoplasm ; n, nucleus with intranuclear network ; a, a
thinned out wing-like projection of the cell which probably fitted between two adjacent cells.

B. A row of columnar cells from an intestinal villus of the rabbit, str, striated border ; w, smaller
cells of the nature of lymph-corpuscles, between the epithelium cells.

Columnar epithelium cells (figs. 109, 112, 115) cover not only the villi but also
the rest of the surface of the intestine, and extend into the tubular glands. Their
general characters have already been described under the head of " Epithelium " (see
Vol. I., Part 2, pp. 199, 200).



92



ORGANS OF DIGESTION.



ep atr



They arc set upon the surface of the basement membrane often by a somewhat
flattened extremity. There is never any continuity between their attached
extremity and the branched corpuscles of the retiform tissue of the villus, such
as has often been supposed to exist ; on the contrary, the epithelium separates
with the greatest readiness from the subjacent tissue, and almost always with a
simple truncated extremity, sometimes pitted but never much branched. Between

Fig. 116. SECTION OF PART OP A RAT'S

VILLUS DURING ABSORPTION OF FAT.

(E. A. S.) Osmic acid preparation.
Highly magnified.

ep, epithelium, filled with fatty particles ;
str, striated free border ; c, lymph-cor-
puscles in the tissue of the villus, containing
fatty particles ; c', others between the epi-
thelium cells ; I, central lacteal, containing
chyle and disintegrating leucocytes.

the epithelium cells are leucocytes
in variable number, but most nu-
merous in the lower part of the
intestine and near the lymphoid
follicles. They often show indica-
tions of division by karyokinesis.
They may lie free in small (lymph)
spaces between the columnar cells.

There is frequently a well-marked
layer of granular eosinophil cells im-
mediately below the epithelium of
the villi (Hardy).

Amongst the ordinary epithelium cells are others (fig. 112, (/), the outer half
of which is filled with mucigen, and in some this has become discharged as
mucus from the cell, and the free end is ruptured {goblet cells, see Vol. I.,
Part 2, p. 200). The number of cells containing mucus varies much in different
animals, and perhaps, under different conditions, in the same animal. There are
comparatively few in the glands of the small intestine. The epithelial cells are
in all probability the primary agents in promoting the absorption of the food
materials from the interior of the gut, and the seat of the retrograde processes of

Fig. 117. SECTION OF FROG'S INTESTINE DURING ABSORP-
TION OF FAT. (E. A. S.) Osmic acid preparation.
Highly magnified.

ep, epithelium ; str, its striated border ; I, lacteal ;
c, c, lymph-corpuscles containing fine fatty particles.
The fatty particles in the epithelium cells are coarse in the
peripheral and fine in the central zone of each cell.

metabolism which the products of digestion
undergo during absorption. Most food materials
cannot be traced in microscopic specimens, but
fatty substances, from their property of becom-
ing stained with osmic acid, can be to some extent followed. The examination of
such specimens taken during digestion of a meal containing fat shows the
epithelium cells turbid with oil-droplets in their interior ; and in some animals
(frog, rat), at a subsequent stage amoeboid cells within the tissue of the villus
appear pervaded with similar but finer fatty particles, and eventually the central
lacteal becomes filled with these. It is probable that the amoaboid lymph-corpuscles





THE SMALL INTESTINE.



93



which are found so abundantly within the tissue of the villus, and even amongst
the epithelium-cells of the surface, play an important part in the transference of
such particles from the epithelium-cells to the lacteal, for at certain~stages of fat
absorption they contain abundant fatty particles, and the large amount of lymphoid
tissue in the lower end of the small intestine seems to be related to a greater power
of absorption in that part of the gut. In the transference of carbon particles
in the lungs from the interior of the alveoli to the lymphatics, which at least in
part is due to the activity of amoeboid cells, we have an analogous process.

In other animals (dogs) the absorption of fat appears to occur in a fluid form,
and here we see darkly stained streaks extending from the inter-epithelial spaces
to the borders of the central lacteal.

It has been denied by Heidenhain that the lymphoid cells of the intestine contain fatty
particles during fat absorption : he states that the particles within them which are stained
black by osmic acid are not fatty but albuminous, baing insoluble in ether. This is, however.





Fig. 118. SECTION OF THE MUCOUS MEMBRANE OF THE SMALL INTESTINE ACROSS THE CRYPTS OF

LIEBERKUHN. (Rauber.) ^^

(The blood-vessels had been injected in this preparation. )

Fig. 119. LYMPHOID OR RETIFORJI TISSUE OF THE INTESTINAL MUCOUS MEMBRANE OF THE SHEEP
(from Frey). MAGNIFIED 400 DIAMETERS.

Cross section of a small fragment of the mucous membrane, including one entire crypt of Lieber-
kulin and parts of several others : a, cavity of the tubular glands or crypts ; b, one of the lining
epithelial cells ; c, the lymphoid or retiform spaces, of which some are empty, and others occupied by
lymph-cells as at d.

not correct. Many of the particlss which the lymph-cells contain during 1 fat absorption
unquestionably dissolve in ether and other solventsof fat, although there are some cells which
contain particles which are undissolved by those re-agents. These may be of an albuminous
nature, or they may still be fatty, but so modified by the action of the osmic acid as to have
been rendered insoluble in ordinary fat solvents. In the frog, where absorption proceeds
more slowly, and can be more easily traced, no fat is to be seen anywhere but in the epithelium
cells, in the leucocytes, and in the lacteals. In the guinea-pig also, as Heidenhain has himself
shown, amoaboid cells take up the whole of the absorbed fat, after it has traversed the epithe-
lium. In other animals absorption may take place so rapidly that the absorbed fat, after being
finely divided and modified in the epithelium cells, may be set free between them without all
being immediately taken up by leucocytes. Eventually, however, most of it appears to be
removed by these cells. The agency of the epithelium cells in fat absorption has been denied
(Watney, Zawarykin), but on entirely insufficient grounds, for the absorbed fat is very easily
shown within those cells.

Glands. Two kinds of small secreting glands open on the inner surface of the
intestine, viz., the crypts of Lieberkiihn, and Brunner's glands, the last being
peculiar to the duodenum. In addition to these, numerous lymphoid nodules are
found, which are either scattered and isolated (solitary glands) or collected into
patches (Peyer's glands).

The crypts of Lieberkiihn, the smallest but most numerous of these glandular
structures, are found in every part of the small intestine, opening on the surface



94



ORGANS OF DIGESTION.



between the villi (figs. 109, 114, 118, 119). They consist of minute tubes, closed
at their attached extremity, which is slightly enlarged, and placed more or less per-
pendicularly to the surface, upon which they open sometimes two or three together.
The crypts of Lieberkiihn vary in length from 0'2 to 0'3 mm., and their diameter is
about 0'04 mm. They are longest in the duodenum. The walls of the tubes are
formed of a basement membrane, lined with a columnar epithelium (fig. 119), the
deepest cells of which those, that is to say, at the fundus of the glands, are in
some animals filled with granules (Paneth). Goblet-cells occur here and there
amongst the ordinary cells.

Although it is usually believed that mucus-secreting or goblet cells may be formed from
any of the columnar epithelium cells of the intestine, it would appear from the observations
of Bizzozero, that the granules of the cells of the gland-fundus are mucigen-granules, and that
it is these cells which give rise to the goblet cells not only of the glands themselves but also
of the surface epithelium of the mucous membrane and villi. In this process they become
gradually shifted in position, their place being taken by other cells formed by a process of

Fig. 120. SECTION THROUGH THE COM-
MENCEMENT OP THE DUODENUM AT
THE PYLORUS. (Klein.)

v, villi ; b, apex of a lymphoid
nodule ; c, crypts of Lieberkiihn ; ra,
muscularis mucosae ; s, secreting tubes
of Brunner's glands ; d, ducts of pyloric
glands of stomach ; g, tubes of these
glands cut across in mucous membrane ;
t, deeper lying tubes situated in sub-
mucous tissue, and corresponding with
Brunner's glands of the intestine.

cell-division at the base of the glands .
While this seems to be the mode of
production of the goblet or mucus-
producing cells, the ordinary co-
lumnar cells appear to be formed
near the mouths of the glands,
where karyokinetic figures are com-
mon (they are very rare on the villi
themselves), and to become shifted
along the villi as any of the columnar
cells of those organs become de-
stroyed.

Brunner's glands are small compound acino-tubular glands, which exist in the
duodenum, where they are most numerous at the upper end, in general occupying
thickly a space extending from one to two inches beyond the pylorus. A few of
them are said also to be found quite at the commencement of the jejunum. They
are imbedded in the submucous coat, and may be exposed by dissecting off the
muscular coat from the outside of the intestine. They may extend partly into
the mucous membrane between the crypts of Lieberkiihn. In structure they
somewhat resemble the small glands which are found in various parts of the lining
membrane of the mouth and elsewhere, each consisting of a number of tubular
alveoli, connected by the terminal ramifications of the duct, which latter penetrates
the muscularis mucosse, and opens upon the inner surface of the intestine. The
ducts open either between the crypts of Lieberkiihn or in some cases into the bases
of the crypts (Schaffer). In sections through the pylorus the glands of Brunner
appear like direct continuations of the pyloric glands of the stomach (Watney),
which they closely resemble in structure, but they are somewhat more complicated
and more deeply seated.

The solitary glands are soft, white, rounded, and slightly prominent bodies
0'6 mm. to 3 mm. in diameter, which are found scattered over the mucous




THE SMALL INTESTINE.



95




membrane in every part of the small intestine. They are found as well at the
mesenteric as at the free border, both between and upon the valvula^ conniventes,
and are rather more numerous in the lower portion of the bowel. These so-called

Fig. 121. SECTION OF A SOLITARY

GLAND OF THE SMALL INTESTINE.

(Cadiat.)

c, c, villi, partially deprived of
their epithelium ; d, crypts of Lieber-
kiihn ; a, solitary gland composed of
retiform lymphoid tissue, which has
become partly broken away in pre-
paring the section ; 6, epithelium
covering the apex or cupola of the
follicle ; c, e, muscularis mucosae ;
/, submucous coat.

glands are in structure similar
to the lymphoid nodules of
various parts already described,
consistiug of clumps of dense
retiform tissue, the meshes of
which are closely packed with
lymph-corpuscles, and per-
vaded by fine capillaries.
They are here and there

united at the sides with the surrounding lymphoid tissue, but are at most points
distinctly marked off from it, partly owing to the fact that their supporting
retiform tissue becomes closer and finer, partly owing to their being surrounded
by a rich plexus of lymphatic vessels ; or they even hang, as it were, into a lymphatic

Fig. 122. A SMALL PATCH OF PEYKR's GLANDS FROM THE ILEUM.

SLIGHTLY MAGNIFIED (Boehm).

(or lacteal) sinus, which may entirely surround the
nodule, except next the inner surface of the intestine.
The epithelium over the nodule often has a large
number of lymph-corpuscles between the epithelial
cells. The base of the nodule or follicle is situated in
the submucous tissue ; but it extends upwards, through
the muscularis mucosse, into the mucous membrane,
causing a bulging of this towards the interior of the
gut (as in fig. 124, d d}. The prominent part of the
follicle sometimes has villi upon it, and, placed around
very irregularly, are seen the mouths of the crypts of
Lieberkiihn.

The agminated glands or glands of Peyer
(who described them in 1077) are groups or patches
of lymphoid nodules. The groups have an oblong
figure (fig. 122), and vary from half an inch to two
or even four inches in length, and from half an inch
to about an inch in width (12 mm. to 120 mm. long

and 12 mm. to 25 mm. broad). They are placed lengthways in the intestine at that
part of the tube most distant from the mesentery ; and hence, to obtain the best
view of them, the bowel should be opened by an incision along its attached border.

The lymphoid nodules which by their aggregation make up a Peyer's patch are
in almost all respects similar to the solitary glands above described. As a rule, their





Bl




ORGANS OF DIGESTION.



surface is free from villi, and the crypts of Lieberkiihn are collected in circles
around them. Fine blood-vessels are distributed abundantly on the exterior of the

Fig. 123. PORTION OF AN INJECTED PETER'S
PATCH (from Kb'lliker). MAGNIFIED.

The drawing was taken from a prepira-
tion made by Frey of the intestine of the
rabbit. It represents the fine capillary net-
work spreading from the surrounding blood-
vessels into the interior of three lymphoid
nodules.

follicles, and give off still finer capil-
lary branches, which, supported by
the retiforrn tissue, are disposed
principally in lines converging to
the centre (fig. 123).

The lacteal plexuses, which are
abundant in the whole extent of
the intestine, are especially rich
where they surround the follicles of
Peyer's glands (fig. 124), often
forming sinuses around them, as in
the case of the solitary follicles
above described.

In all, from twenty to thirty of these oblong patches may in general be found ;
but in young persons dying in health as many as forty-five have been observed.





Fig. 124. VERTICAL SECTION OF A PORTION OF A PATCH OF PEYER'S GLANDS, WITH THE LACTEAL
VESSELS INJECTED (after Frey). 32 DIAMETERS.

The specimen is from the lower part of the ileum : a, villi, with their lacteals left white ; b, some of
the tubular glands ; c, the muscular layer of the mucous membrane ; d, cupola or projecting part of the
nodule ; c, central part ; /, the reticulated lacteal vessels occupying the lymphoid tissue between
the nodules, joined above by the lacteals from the villi and mucous surface, and passing below into g,
the reticulated lacteals under the follicles, which again pass into the large efferent lacteals, </ ; i, part
of the muscular coat.

They are larger and placed at shorter distances from each other in the lower part of
the ileum ; but in the upper portion of that intestine and in the lower end of
the jejunum the patches occur less frequently, become smaller, and are of a nearly
circular form ; they may, however, be discovered occasionally in the lower portion of



THE SMALL INTESTINE.



the duodenum. Still smaller irregularly shaped clusters of these follicles are

sometimes found scattered throughout the intestine.

The glands of Peyer are best marked in the young subject. After middle life

they become less obvious, and disappear almost completely in advanced age, their

remains being often indicated by dark colouration of the mucous membrane.

Vessels and Nerves of the
small intestine. The branches of
the mesenteric artery, having reached
the attached border of the intestine,
pass round its sides, dividing into
numerous ramifications, and frequently
anastomosing at its free border. Most
of the larger branches run immediately
beneath the serous coat ; they then





Fig. 125. SECTION OF SMALL INTESTINE WITH BLOOD-VESSELS INJECTED. (Heitzmann. )

V, villi ; G, glands ; M, muscukris mucosse ; A, areolar coat; R, circular, and L, longitudinal
muscular layers ; P, peritoneal coat.

Fig. 126. SECTION THROUGH PART OP A PEYEU'S PATCH SHOWING THE LYMPHATICS INJECTED.

(Modified from Heitzmann.)

V, villi ; MM, muscukris mucosag ; F, lymphoid follicles ; M, mucous membrane ; T, circular, and
L, longitudinal layers of tlie muscular coat ; P, peritoneal layer. The crypts of Lieberkiihn are seen
in this section.

pierce the muscular coat, supplying it with a few vessels as they pass, and ramify
in the submucous areolar layer, so as to form a close network. From this
smaller vessels pass into the mucous coat. Some of these are the arteries of the
villi, already described. Others pass to the mucous membrane and supply a
network of capillaries surrounding its glands. Other branches again pass into the
muscular coat from the vessels of the submucoga. The fine capillaries of the
muscular coat are arranged in two layers of oblong meshes, which correspond in

VOL. III., PT. 4. H



98



ORGANS OF DIGESTION.



direction with the longitudinal and circular muscular fibres. The veins accompany
the arteries, with which they correspond in general distribution.




Fig. 127. LYMPHATIC PLEXUS (I) AND s Karons PLEXUS (n) IN THE MUSCULAK COAT OF THE INTESTINE

(Auerbach).

The lymphatics of the intestine (lacteals) may be conveniently distinguished as
those of the mucous membrane aud those of the muscular coat. Those of the




Fig. 1*28. PLEXUS OF AUKKBACH BETWEEN THE TWO LAYERS OK THE MUSCULAR COAT of THK
INTESTINE. (Cadiat. ) Chloride of gold preparation.

mucous membrane form a copious plexus (figs. 124, 120), which receives the central
vessels of the villi, and pervades both the mucous and submucous layers in
the latter being of considerable size, and forming, as before mentioned, a close plexus



THE SMALL INTESTINE.



99




Fig. 129. PLEXUS OF MKISSXER, FROM THE

SUBMUCOUS LAYER OF THE INTESTINE.

(Cadiat.) Chloride of gold prepara-
tion.

a, a, ganglia ; b, b, cords of plexus ; c, a
small blood-vessel.

or a sinus around the base of each
lymphoid follicle. Another set of
lymphatics lies under the peritoneal
coat, and is especially developed
along a narrow strip at the
attachment of the mesentery. In
the muscular coat the main plexus
is situated between the circular and
longitudinal layers of fibres (fig.
127, I) ; and there are likewise close
plexuses threading the whole thick-
ness of the muscular wall. These
lymphatics of the muscular coat are
in complete continuity with those of
the mucous membrane, and pass
into larger vessels at the mesenteric
border, which again run into the
lacteal vessels of the mesentery.

The nerves of the small intes-
tine are chiefly derived from the

superior mesenteric plexus. This plexus is formed by nervous branches from the
coeliac plexus, the semilunar ganglion, and from the vagus nerve. The plexus and

Fig. 130. NERVE-ENDINGS IN THE SMALL IN-
TESTINE OF THE GUINEA-PIG. (Cajal. )
Silver chromate preparation.

a, 6, c, d, small nerve cells belonging to the
interglandular plexus of the mucous membrane ;
e, f, corresponding cells belonging to the nerve-
plexus of the villi ; M, nerve-fibres belonging
to the plexus of Meissner, distributed to the
muscularis mucosse.

plexiform branches into which it divides
cling at first very closely to the larger
divisions of the superior mesenteric
artery, and, dividing similarly with
the ramifications of the arteries, the
branches of the nerves, retaining still a
wide plexiform arrangement, pass on-
wards to the different parts of the
intestine between the two folds of the
mesentery, and finally, separating some-
what from the blood-vessels, reach the
intestine in very numerous branches,
to be distributed in its coats. Passing

first between the longitudinal and circular layer of the muscular coat, they here
form a close gangliated plexus throughout the whole extent of the intestine (fig. 127, n,
as exhibited under a low power ; also fig. 128). This, which is known as the plexus

H 2




100 ORGAKS OF DIGESTION.

of Auerbach, or the plexus myenlericus, and which is principally composed of r.on-
medullated fibres, gives off fine branches to the muscular substance, these first
forming a smaller plexus amongst the muscular fibres. Other larger branches pass
between the circular bundles of fibres to reach the submucous layer, where they form
a second gangliated plexus (plexus of Meissnei-, fig. 129), the threads of which are
much finer than those of the intermuscular plexus.

The cords of these plexuses contain two kinds of nerve-fibres, both non-
medullated. Those of the one kind pass through the ganglionic enlargements,
giving off collateral fibres which ramify around the nerve-cells : they probably
take origin in the prevertebral or other more central ganglia. Those of the other
kind are finer and more numerous ; they are processes of the cells of the ganglionic
enlargements and are probably partly passing to their distribution in the muscular
layers or in the mucous membrane respectively ; partly serving to connect
neighbouring ganglia with one another.

From Meissner's plexus nerve-fibres pass to be distributed to the muscular layer
of the mucous membrane, breaking up into fine fibrils, which take the direction of
the fibre-cells of this layer, whilst other fine fibrils form ramifications in the proper
tissue of the mucous coat and villi (fig. 130), and, according to some authors, send
branches into the epithelium penetrating between the columnar cells. Upon these
ramifications in the mucous membrane and villi, and also upon the ramifications
which are distributed to the muscular layers, small cells are found which are thought
to be of nervous nature. Similar cell-like enlargements occur on the terminal



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