able curvature, which has the shape of the
italic letter S. From the lower end of this
"sigmoid" flexure, it passes obliquely towards
the median line ; where it terminates in a
straight, short tube, that runs vertically
through the pelvis to the outlet of the anus.
Hence the entire segment of large intestine has
the shape of a horse-shoe; and forms a large
bend, which is concave downwards, and passes
almost round the confines of the abdomen
before ending at the inferior extremity of this
cavity. While its general arrangement is
such, that the intestinal canal, which diverges
from the median line at the lower end of
the oesophagus, returns to it shortly before
terminating in the posterior or lower segment
of the trunk.

An accurate measurement of the length and
width of this tube is opposed by the diffi-
culties already alluded to in the case of the
small intestine. My own observations would
indicate an average length of from four to six
feet, and a mean diameter of about If to 2^
inches: the two measurements usually vary-
ing inversely to each other, except in the cases
of extreme distention or contraction, when
both respectively increase or decrease simul-
taneously. Hence the large intestine has
about a quarter the length, and twice the
width, of the small. From such an estimate
we may conclude that, while its capacity is
almost equal to that of the narrower tube,
its active surface is scarcely half as large.
And even this great difference is much in-
creased by the absence of villi and valvuloe
conniventes from the interior of the large in-
testine.

Like the rest of the canal, the wall of the
large intestine is composed of the serous, mus-
cular, and mucous coats; and of vessels, nerves,
and lymphatics, which are distributed to them.

The nature and arrangement of these
tunics vary, however, in the several parts of
the tube. And these differences, aided by
others which affect its size, shape, and situa-
tion, subdivide the large intestine into the
following segments : the ccecum ; the vermi-
form appendix; the colon, in which we dis-
tinguish an ascending, transverse, and descend-
ing portion, and a sigmoid flexure ; and, finally,
the rectum. The anatomy of each of these
will demand a brief notice.

The ccecum (c,Jigs. 276, 277.) (formerly blind
gut, Eng.; blind Darm, Germ.) is the first and
largest of these segments. Its arrangement may
be described as due to the fact, that the small
intestine, instead of being simply continuous



STOMACH AND INTESTINE.



363



with the large (like the stomach with the duo-
denum), opens into it at right angles to its axis,
and at some distance from its commencement;
so as to leave a blind extremity of the larger
tube at the site of their mutual junction. The
size of this cnl-dc-sac generally exceeds that of
the remainder of the bowel ; it being larger than
any other part of the alimentary canal, with
the single exception of the stomach. When
moderately distended, its diameter ranges from
2 to 3 inches ; and its length is about as
much. Its vertical extent is, however, somewhat
arbitrary ; since, though defined in part of its
circumference by the aperture of the ileum
and by the ilio-caecal valve, it is elsewhere
only limited by an imaginary line drawn
around the tube at the level of the latter
orifice.

The situation of the caecum, in the left iliac
fossa, allows it to vary considerably in size,
without undergoing any marked change of its
relations. Bound down as it is to the fascia
over the iliacus muscle by peritoneum and
loose areolar tissue, its enlargement merely
causes it to displace such portions of the small
intestine as may hitherto have shared the oc-
cupation of the iliac fossa. After its distention
has removed these from its anterior surface,
it reaches the anterior wall of the belly in the
iliac region ; where its size, shape, and con-
tents can be more or less recognized during
life, by the ordinary means of physical investi-
gation.

The above dimensions render it obvious
that the shape of the caecum is somewhat
globular. This shape is, however, modified by
the arrangement of its muscular layers; which
here begin to offer a peculiarity that is main-
tained throughout the whole of the colon. The
uniform external or longitudinal layer present
in the small intestine is here contrasted by one
which is separated into three flattened bands,
that occupy the side of the tube at nearly equal
distances from each other. In the caecum one
of these (and the larger of the three) is ante-
rior ; one posterior ; and one external. And
all three of them become continuous above
with the corresponding bands around the as-
cending colon. Between these slips of muscle,
the bowel presents a more or less dilated and
projecting external surface ; which is again sub-
divided by transverse constrictions into subor-
dinate pouches or sacculi. On laying open the
bowel, and removing the mucous membrane
from its inner surface, it may be seen that these
transverse constrictions are in reality formed
by the circular muscular coat ; which gives off
projections or incomplete septa, that compli-
cate the general cavity of the tube, by adding
a number of supplementary cells. These cells
are arranged in three vertical rows ; which are
separated by ridges, that correspond to the
external depressions formed by the longitu-
dinal bands above mentioned. Between these
bands, the " haustra" or pouches of the bowel
possess a muscular tunic of very inconsider-
able thickness : the transverse or circular
layer being reduced to a thin membranous
lamina ; and the longitudinal being, as before



stated, altogether absent. The close relation
of these longitudinal and transverse septa to
the length and width of the bowel is well
shown by the effect of cutting across or re-
moving its three bands, and then distending the
tube by artificial inflation. This obliterates
the " falciform folds " or transverse septa ;
and thus converts the sacculated intestine
into a canal, the length and diameter of which
are nearly double of what it formerly pos-
sessed when retained in its proper shape by
its longitudinal bands or " taeniae."

The serous covering of the ccecum is chiefly
remarkable from the closeness with which it
generally attaches the bowel to the fascia over
the iliacus muscle. When the tube is but
moderately distended, it covers only its ante-
rior surface. Extreme contraction can, how-
ever, render it a more complete covering; and
may even produce it into a kind of meso-
caecum behind the bowel. While conversely,
great distention of the tube reduces the peri-
toneum to a partial investment ; which occu-
pies but a third, or even less, of the intestinal
surface.

The mucous membrane of the coecum differs
in no essential respect from that of the re-
mainder of the large intestine, the structure
of which is continued up to the very edge of
the valve which severs it from the ileum.

The caecum has three apertures: one, a
large opening by which its cavity is directly
continuous with the colon ; a second, which
communicates with the small intestine, and is
guarded by a double valve ; and a third, which
opens into the slender vermiform appendix.

lleo-ccBcal valve. The opening into the
ileum is situated at the upper border of the
coecum ; on its left side, and a little poste-
riorly. The structures which bound and define
this opening are collectively termed the ileo-
ccecal or the ileo-colic valve : although these
names ought in strictness to be limited to
those separate portions of the entire intestinal
valve which their etymology would indicate.

The arrangement of the intestinal tunics
in this valve is best seen by inflating and dry-
ing that part of the intestine, which includes,
together with the last inch or two of ileum, the
caecum, and the commencement of the colon.
On cutting out a piece of such a dried prepara-
tion, so as to gam a view of its interior, we
see the valve as represented in the accom-
panying figure (Jig. 275.) The small in-
testine, generally inclining slightly upwards as
well as backwards, passes towards the caecum,
at what is thus a rather acute angle. In-
stead, however, of opening into the bottom of
one of the sacculi of the caecum, it selects for
its entry the exact site of the deepest and
most projecting of those transverse constric-
tions which project into the cavity of the large
intestine. This constriction occupies the
inner side of the bowel ; and is, as it were,
split up by the entering ileum into two laminae ;
an upper and a lower, an ileo-colic and an
ileo-caecal (e,f,fig. 275.) While, at the same
time, the hitherto cylindrical calibre of the
small intestine is gradually reduced to a hori-



364



STOMACH AND INTESTINE.



zontal slit or fissure, as it enters this
fold.

Fig. 275.




Caecum inflated, dried, and opened, to exhibit the
arrangement of its valve.

a, termination of the ileum ; b, ascending colon ;
c, caecum ; d, transverse constriction projecting into
the caecum from its inner surface; ef, valve sepa-
rating the small from the large intestine ; e, its ho-
rizontal ileo-colic lamina ; f, its more oblique ileo-
czecal lamina; g, the vermiform appendix of the
caecum.

Such a description at once explains the
form of the valve: how each of its seg-
ments, for example, constitutes a crescentic
membrane, the plane of which meets that of
its fellow at an acute angle, and the free
edge of which is directed outwards ; and
how both end anteriorly and posteriorly in a
commissure or fold, that gradually decreases
in depth as it passes either forwards or back-
wards round the intestine.

As regards the details of its construction,
each segment of the valve is chiefly formed
by the prolongation of a corresponding por-
tion of the circular muscular fibres of the
ileum, together with a few proper to the
large intestine. The assistance afforded to
these by the attachment of the peritoneum
and the longitudinal fibres of the ileum, to
the fixed margin of each segment, is well
shown by the effect of dividing the latter
structures. For after such an injury, mode-
rate traction draws out the valve into a sur-
face, which is directly continuous with the
lower end of the small intestine ; and at the
same time converts its horizontal slit into a
large elliptical aperture. The difference be-
tween the ileo-cascal and ileo-colic portions
consists chiefly in the fact, that the plane of
the former is more oblique, and its margin
more concave, than that of the latter.

The mechanism of this valve may be easily
deduced from its structure. In all states short
of actual distention, the passive contraction
of its muscular walls no doubt insures their



contact, and shuts off the cavity of the ileum
from that of the caecum. While any approach
towards a more active dilatation of the large
intestine whether of the caecum, or colon
at once brings about a close apposition of
the two portions of the valve. And what-
ever aid may be given to this mutual apposi-
tion of the surfaces of the valve bv its own
active muscular contraction during life, no-
thing is more certain than that its closure is
essentially independent of any such vital pro-
cess. For the gradual and equable distention
of the caecum with liquid in the dead subject
can also produce this result.

Nor is it difficult to understand how such
a closure is effected. The passage of the con-
tents of the large intestine, over either plane
of the valve, presses it against the opposite
one, so as at once to close its orifice. Be-
sides this, the free margins of the valve form
segments of a larger circle than its attached
ones. Hence they are disproportionately
tightened by the same distending force. In
this way, the double curve of each lamina is
soon reduced to a straight line, that brings it
into exact apposition with its antagonist. So
that, within all ordinary limits, the greater
the dilating force, the more closely are the
two lips of the valve applied to each other.

The only valid exceptions to this rule may
be found in those cases in which the ileum
and caecum are filled simultaneously. Such a
process of distention necessarily occurs in all
cases of mechanical obstruction of the diges-
tive canal below this valve; as a result of the
downward flow of the contents of the small
intestines. And since it obviously distends
the aperture by the application of a counter-
force from the side of the ileum, its mecha-
nical action is so simple as to require no
further explanation. Its effect may indeed
be seen in the caecum, as usually inflated and
dried.*

The function of this valve therefore offers
a complete contrast to that by which the
stomach opens into the small intestine. For
while it affords little or no obstacle to an
onward transit of the contents of the canal,
it resolutely bars the way to all regurgitation:
an action which we have already seen is
exactly reversed by the pylorus. And even
in the absence of information respecting the
details of its active contraction, its structure
entitles us to conjecture, that the greater
part of its efficiency depends upon a passive,
and therefore permanent mechanism ; and
not, as is the case with the pylorus, on an in-
termittent (and vital) shortening of its mus-
cular fibres.

The use of the caecum is evidently that of
forming a receptacle, in which the contents of
the small intestine may sojourn for a certain
time, before passing onwards into the colon.

* Hence it is scarcely a superfluous caution to
add, that in examining such a preparation we ought
always to recollect, that the patulous orifice thus
seen is in reality an abnormal one, which does not
illustrate the mechanism of the valve in the
healthy living body.



STOMACH AND INTESTINE.



365



For not only are its shape, size, and direc-
tion such as admirably adapt it to this pur-
pose, but its development in different species
and individuals closely corresponds to the
degree in which such a delay is advantageous
to digestion. Thus the large caecum of the
Herbivora is contrasted, in the Carnivora, by
one of but inconsiderable size and development.
While there are grounds for conjecturing, that
the habitual use of a vegetable diet is capable
of increasing its size in the human subject.
In all of these respects, however, its develop-
ment does but parallel that of the remainder
of the large intestine. We may therefore defer
considering the nature of its secretion, and
the changes undergone by its contents, until
these segments of the bowel have also been
noticed.

The vermiform appendix (g,j%. 275.) which
is so named from its resemblance in shape and
size to a worm, is a small, smooth, cylindrical
tube ; that opens into the caecum below (and
rather posterior to) the aperture of the small
intestine. Its length varies from one to four
or five inches : its diameter from about a fourth
to a third of an inch. Its attached end of course
shares the situation of the contiguous part of
the caecum. Its distal extremity is usually
free ; and may hence be found in almost any
situation which its length, and that of the
short mesentery that binds it down, together
allow it to take. Its opening into the caecum
is often partially occluded by a kind of trans-
verse fold or valve.

As regards its structure, the vermiform ap-
pendix exhibits all three of the ordinary coats.
Its peritoneum is derived from that of the
caecum, and often forms a short fold or me-
sentery which is prolonged up a part of its
length. Its muscular stratum is of uniform
and considerable thickness, and is continuous
with the three longitudinal bands which give
the ceecum its sacculated shape. Its mucous
membrane is occupied by tubes and follicles,
like those of the colon. And the latter
structures are often present in such extra-
ordinary numbers, as to constitute almost
a continuous layer of these minute closed
sacs. The calibre of the tube, which is in
general disproportionately small, is occupied
by a sparing quantity of glairy mucus ; and
occasionally, by small fragments of the ordi-
nary intestinal contents.

The use of the vermiform appendix is un-
known. It has been suggested to be a mere
relic of the umbilical duct of the foetus : an
erroneous view, to which allusion will hereafter
be made in speaking of the development of the
intestinal canal. It is almost peculiar to Man ;
in whom its situation often causes it to receive
small solids in their transit through the caecum,
with the result of their becoming impacted in
its narrow cavity. This accident is some-
times followed by inflammation and perfora-
tion of the tube, causing fatal peritonitis.

The colon* (formerly Great gut, Eng.;

* This Greek word, which has been adopted into
most of the modern languages, is derived indiffe-



Grimmdarm, Germ.), which forms by far the
greater part of the large intestine, extends from
the ilio-caecal valve to the rectum. Starting
from the right iliac fossa, it passes vertically
{a, Jig. 274.) up the posterior wall of the belly,
and on the right side of the spine, until it
reaches the under surface of the liver. A
sudden turn at a right angle marks the end
of this ascending portion, and the beginning
of its transverse part. The latter segment,
though tolerably horizontal, forms an arch
(t,fig. 274.) with the convexity forwards, so as
to pass around the projecting spine and aorta.
Below the spleen it merges, by another rectan-
gular bend, into the descending colon (d,jig.
274.) This takes much the same course on the
left side of the abdomen as the ascending colon
does on the right ; and opposite to the crest of
the ileum, it ends by becoming continuous
with the sigmoid flexure (s,fig. 274.), The
latter portion is attached by a short mesen-
tery to the left iliac fossa; and it terminates
in the rectum, at a point corresponding to the
left sacro-iliac symphysis.

The relations of each of these segments to
the adjacent textures and organs may be
easily deduced from their course as described
above.

Thus the ascending colon lies on the right
kidney and quadratus lumborum muscle, from
which it is only separated by loose areolar
tissue. On its left side, is the psoas muscle ;
and above it, the vertical portion of the duo-
denum. In front, it is covered by coils of in-
testine ; or, if sufficiently distended to thrust
these away, by the anterior wall of the belly.

The transverse colon is almost always in
contact with the omentum and abdominal pa-
rietes, which it touches in the horizontal line
that marks the mutual limit of the umbilical
and epigastric regions. Above it, is the first
portion of the duodenum; with the stomach,
liver, gall bladder, and spleen. Below it, are
the coils of the small intestine. Behind it, lie
the second and third portions of the duo-
denum, the latter covering the aorta. To its
posterior surface is attached the transverse
mcso-colon; which connects this part of the in-
testine with the wall of the belly, by a double
fold of peritoneum, that splits to enclose the
tube. The double lamina formed by the re-
union of these two layers of serous membrane
in front of the bowel, is continuous, at the
lower border of the great omentum, with the
similar process that descends from the great
curvature of the stomach.*

The descending colon, like the ascending,
lies on the left kidney and the left quadratus
lumborum muscle, and is covered by a va-
riable quantity of the small intestine.

The sigmoid flexure is much more frequently
in contact with the abdominal wall than the
preceding portion of the colon. And its
freedom of movement, to which this contact
is partly due, also allows the bowel to deviate
considerably from its ordinary curvature and

rent! y from **, membrum, and ***, atvus (T$ TV*



See Art. PERITONEUM.



366



STOMACH AND INTESTINE.



position. The shape of this bend corresponds
so exactly to its name, that it is scarcely
necessary to bestow any further description
upon it. Its curve is chiefly in the vertical
plane ; though a slight lateral curvature is
almost always present, and is easily exag-
gerated into a much more distinct bend by
the mobility of this segment of the canal.

The use of the sigmoid flexure seems to be
that of forming a receptacle for the faeces :
a receptacle of which the shape and arrange-
ment are such as to spare the rectum and its
sphincter from much of the pressure and
weight against which they would otherwise
constantly have to contend. When full, the
convexity of its lower bend often projects
below the iliac fossa, so as to descend into the
pelvis. Indeed, the whole of the colon is very
liable to displacement from the various po-
sitions just assigned to it: prolonged dis-
tention by its accumulated contents, or the
mechanical force exerted by the pressure of
stays externally, or of tumours internally
being all capable of altering its relations, and
even confusing its different parts* with each
other.

The colon retains the sacculated shape as-
sumed by the cascum. Its size undergoes a
progressive though slight decrease, from its
commencement in the caecum to its termina-
tion in the rectum. Its peritoneal coverings
reach their minimum in the ascending and
descending portions ; where they only cover
about two-thirds of the moderately distended
bowel, and leave its posterior or attached third
quite unoccupied by this membrane, and con-
nected by loose areolar tissue to the subjacent
parts. Hence it is these portions of the bowel
which are selected in the operation for artificial
anus, -f- But, just as great distention can
always increase this uncovered portion, so,
vice versa, excessive contraction may reduce
it to a mere line, or may even develope a kind
of short meso-colon in connection with either
of these parts. The muscular strata which
cause its sacculated shape, also retain the
arrangement existing in the coecum. But on
the transverse colon, the internal longitudinal
band becomes inferior. And on the sigmoid
flexure, this and the posterior band generally
merge into a single one. The latter change
is accompanied by an indistinctness of the
transverse sacculi themselves.

Throughout the whole of the large intestine,
the peritoneum is here and there developed
into peculiar, processes, called the appendices
cplploiccB (t?ri rXoov, omentum). These are
short pouches of the serous membrane, which
generally form flattened duplicatures or folds.
They are prolonged from the peritoneum cover-
ing the surface of the intestine itself; and are
therefore absent from that portion of the
rectum, or terminal segment of the large in-
testine, which does not receive any covering

* See Abnormal Anatomy.

f Other things being equal, the left or descend-
ing portion is preferred : on account of a larger
extent of the canal being thus left to be traversed
by the intestinal contents.



of this membrane. Their number, size, and
arrangement, are liable to great variety. Some-
times they are so numerous, as to form a
single or double row along the free surface
of the bowel. In other instances they are
very few and imperfect. Their size is so
far related to the state of the bowel, that,
like most other processes of peritoneum, they
are enlarged by its contraction, and dimi-
nished by its distention. From their con-
tents, which consist of areolar and adipose
tissue, they would seem to be small reser-
voirs of fatty matter. Hence in cases of re-
markable obesity, their size is much increased.
Indeed they sometimes acquire a length of
one or two inches ; and have even been
known to encircle and strangulate the bowel.

Movement of the large intestine. The exact
nature of the movement which is executed by
the muscular coat of the large intestine can
at most only be conjectured from some of its
attendant circumstances. Like that of the
preceding segment of the canal, though its
general mechanism is obvious, its details remain
unknown.

As regards the investigation of the con-
tents of this intestine in its ordinary situation
during life, all that can be stated is, that,
even in health, they include a quantity of
gaseous matter ; which usually maps out the
ccecum, and more or less of the colon, with
tolerable distinctness, from the less resonant
convolutions of the small intestine.

After death, the quality and quantity of these
contents are so much affected by the nature of
the previous food, the mode of dying, and a va-
riety of kindred causes, that scarcely any gene-
ral proposition can be laid down with respect to