of the umbilical loop (Figs. 104 and 130) this vessel passes between
the layers of the dorsal mesentery through the narrow duodeno-colic neck
to reach the two limbs and the apex of the intestinal loop. In its
course it gives off successively branches to the gut from each side.
Those from the right side of the main vessel pass to the duodenum,
pancreas, jejunum and ileum. Those from the left side of the main vessel
accede in succession to the colic angle of the isthmus, the proximal
portion of the colon, the caecum and the ileo-colic junction. The
terminal portion of the superior mesenteric artery supplies the ileum
near the ileo-colic entrance. After rotation it will be found that the
turn has occurred at the point _X_ (Fig. 130), _i. e._, in that part of
the vessel which occupies the duodeno-colic isthmus. Hence it will be
found that the first branches derived from the right side of the
primitive superior mesenteric artery, supplying the duodenum and
pancreas (Art. pancreatico-duodenalis inferior) still arise after
rotation from the right side. They are succeeded, beyond the point _X_,
by the original highest _left_ branches passing to colon, caecum and
ileo-colic junction, while all the original right-sided vessels, except
the inferior pancreatico-duodenal, appear now as branches from the left
side of the main artery, supplying the coils of the jejuno-ileum. Hence
in the adult (Fig. 133) the succession of branches derived from the
right or concave side of the superior mesenteric artery is as follows:
1. Arteria pancreatico-duodenalis inferior.
2. Arteria colica media.
3. Arteria colica dextra.
4. Arteria ileo-colica.
On the other hand, the first branches from what has now become the left
or convex side of the vessel are the original lower right-hand vessels
to the small intestine developed from the descending limb of the loop.
Hence in the adult the left side of the superior mesenteric vessel gives
rise to the vasa intestini tenuis.
3. The caudal intestinal arterial branch derived from the aorta is the
inferior mesenteric artery supplying parts of the transverse colon, the
descending colon, sigmoid flexure and rectum (Figs. 130, 131, 132, and
133).
On the other hand in the cases of non-rotation of the intestine as above
described in Figs. 118-122, the embryonic type of the intestinal
arterial supply persists, as indicated schematically in Fig. 134. Not
only the pancreatico-duodenalis inferior, but all the remaining branches
to the small intestine are derived from the right side of the superior
mesenteric artery. The terminal branches of the main artery supply the
ileo-colic junction, while the arterial supply of the large intestine,
A. colica dextra and media, are given off from the left side of the
parent vessel.
[Illustration: FIG. 134.--Schematic representation of intestinal
arterial supply from superior mesenteric artery in cases of arrested
rotation of the intestine.]
II. =Demonstration of Intestinal Rotation in the Cat.=--The changes in
the relative position of the different intestinal segments and the final
disposition of the mesenteries and blood vessels can best be understood
by the direct examination of the abdominal contents in an animal whose
permanent adult arrangement corresponds to one of the early embryonal
human stages, and in which the necessary manipulations can readily be
carried out and their results noted.
It is doubtful if the above detailed developmental stages in man can
ever be clearly comprehended unless the student will for himself examine
the conditions and perform the manipulations in one of the lower
mammals.
The necessity of keeping the three dimensions of space in mind and the
fact that certain structures during and after rotation cover and obscure
each other, make diagrams and drawings unsatisfactory unless the actual
examination of the object itself is combined with their study.
Fortunately, among the common domestic animals of convenient size easily
obtained the cat answers every purpose of this study admirably. The
student is earnestly urged to pursue his study of the development and
adult arrangement of the human abdominal viscera and peritoneum in the
light which the anatomy of this animal can shed on the complicated and
obscure conditions encountered in the human subject. The plan of having
the opened abdominal cavity of the cat directly side by side with the
human subject, while the arrangement of the abdominal viscera and
peritoneum is considered, cannot be recommended too highly.
=Directions.=--After killing the animal with chloroform the abdominal
cavity is to be freely opened by a cruciform incision and the skin flaps
turned well back and secured in this position. It is well to select a
male animal or an unimpregnated female, as the size of the pregnant
uterus in the later stages renders the examination of the abdominal
viscera and peritoneum more difficult.
For purposes of careful study and comparison of the vascular relations
of the abdomen, it is highly desirable to inject the animal with
differently colored gelatine, starch or plaster of Paris mass. The
arterial injection can be made through the carotid artery, the systemic
venous injection through the femoral vein, and the portal circulation
can be filled after opening the abdomen, by injection through the
superior mesenteric or splenic veins. Animals prepared in this manner
are especially useful for the study of the upper portion of the
abdominal cavity and of the peritoneal relations of liver, stomach,
spleen, pancreas and duodenum. They may be kept for permanent reference
in a 5 per cent. solution of formaline or 50 per cent. alcohol.
After opening the abdominal cavity turn the great omentum up over the
ventral surface of the thorax and secure it in this position, thus
exposing the underlying intestines completely (Fig. 135). Trace in the
first place the entire course of the intestinal tube from the pyloric
extremity of the stomach down. It will be noticed that the first portion
of the small intestine (duodenum) is freely movable, completely invested
by peritoneum and attached to the dorsal midline by a mesoduodenum
between the layers of which a portion of the pancreas is seen.
[Illustration: FIG. 135.--Abdominal viscera of cat; great omentum
raised; intestines turned down and to left. (From a fresh dissection.)]
Following the duodenum caudad it will be observed that the gut can be
traced directly continuous with the remaining coils of the small
intestine. The ileo-colic junction and the beginning of the large
intestine are marked by a short pointed caecum. The large intestine is
short, as it is in all carnivore mammals, and passes from the caecum
almost directly down into the pelvis.
Take the caecum and the first portion of the large intestine and turn
them caudad and over to the left side as far as the peritoneal
connections will permit.
Spread out the coils of the small intestine in the opposite direction,
_i. e._, over to the right side.
The arrangement of the intestinal tract after these manipulations should
appear as shown in Figs. 136 and 137.
[Illustration: FIG. 136.--Abdominal viscera of cat, hardened; omentum
removed to display derivation of intestines from umbilical loop and the
relation of the superior mesenteric artery and common dorsal mesentery
to the small and large intestines. (Columbia University Museum, No
728.)]
[Illustration: FIG. 137.--Abdominal cavity of cat. (From a fresh
dissection.)]
It will be seen that all the essential features described for the
corresponding stage in the human embryo (Fig. 104, _A_) exist here. The
proximal portion of the small intestine (duodenum) retains its freedom
and mobility, being attached to the ventral surface of the vertebral
column by the portion of the primitive mesentery which now constitutes
the mesoduodenum. The gut itself forms a bend with the convexity turned
to the right.
Observe in the next place that the point (Fig. 136, _X_), where small
intestine and colon approach each other closely, marks the situation of
the foetal duodeno-colic isthmus. The small intestine at this point
corresponds to the future duodeno-jejunal angle as will be seen after
rotation has been accomplished.
Recalling the development of the jejuno-ileum it will not be difficult
to recognize in the numerous coils of small intestine which succeed to
the duodeno-colic isthmus the results of the increase in length of the
descending or efferent limb of the human embryonal umbilical loop.
Tracing these coils it will be found that the terminal portions of the
ileum correspond to the apex and to the proximal part of the ascending
or recurrent limb of the primitive loop, while the remainder of this
limb furnishes the caecum and the next succeeding segment of the large
intestine. Following the tube up to this point the colic boundary of the
duodeno-colic isthmus will be reached; from here the short large
intestine of the carnivore descends straight into the pelvis, attached
to the ventral surface of the vertebral column by a mesocolon which
corresponds to the distal part of the original primitive dorsal
mesentery.
Now with the parts still in this position examine carefully the
arrangement of the mesentery and of the intestinal blood vessels.
Starting with the duodenum it will be seen that the primitive sagittal
mesentery of this portion of the intestine has followed the gut in its
turn to the right, so that the original right layer of the sagittal
membrane is now directed dorsad and lies in contact with the parietal
peritoneum which invests the background of the abdominal cavity in the
right lumbar region below the liver and covers the ventral surface of
the right kidney. Beneath this parietal peritoneum the inferior vena
cava is seen, receiving the right renal vein and ascending to enter the
dorso-caudal aspect of the right lobe of the liver. If now we assume
that in the cat the opposed serous surfaces of the original right leaf
of the mesoduodenum, now directed dorsad, and of the parietal peritoneum
adhere to each other, and that the visceral peritoneum covering the
dorsal surface of the descending duodenum likewise becomes obliterated
by adhesion to the subjacent parietal peritoneum, we will obtain the
arrangement found in the adult human subject, in which the descending
duodenum is fixed by adhesion below the right lobe of the liver and
ventrad of the medial portion of right kidney, right renal vein and
inferior vena cava. During this process of anchoring the head of the
pancreas, which is found between the two layers of the free mesoduodenum
of the cat, would also become fixed to the abdominal background by
adhesion of the original right leaf of the mesoduodenum, investing what
has now become the dorsal surface of the pancreas, to the parietal
peritoneum. The original left layer of the primitive mesoduodenum would
then appear as _secondary_ parietal peritoneum covering what has now
become the ventral surface of the transversely disposed head of the
gland. The stages may be represented schematically in Figs. 138-140.
[Illustration: FIGS. 138-140.--Diagrammatic representation of three
stages in the development of the mesoduodenum, duodenum, and pancreas
leading to the secondary "retroperitoneal" position of these viscera.]
[Illustration: FIG. 138.--Free mesoduodenum in sagittal plane, including
head of pancreas between right and left layers.]
[Illustration: FIG. 139.--Mesoduodenum folded to right; left leaf has
become ventral; right dorsal, directed toward primitive prerenal
parietal peritoneum.]
[Illustration: FIG. 140.--Fixation of head of pancreas and duodenum
under cover of secondary parietal peritoneum by adhesion of apposed
surfaces of mesoduodenum and primitive parietal peritoneum.]
Figs. 138 and 139 shows the arrangement in the cat where a free
duodenum and mesoduodenum exists, with the pancreas included between its
layers.[2]
[2] The student should not be confused by the fact that a considerable
portion of the pancreatic gland in the cat will be found included
between the layers of the great omentum, extending over to the left side
of the abdomen. This circumstance will be found of importance in
studying the development of the dorsal mesogastrium and of the
structures connected with it. For the present attention should only be
given to the right extremity or head of the pancreas, situated close to
the duodenum and included between the layers of the mesoduodenum.
It will be noticed that the duodenum in the cat can be carried over to
the median line (Fig. 138) exposing the entire ventral aspect of the
right kidney and the inferior vena cava beneath the primary lumbar
parietal peritoneum. This manipulation will also expose the dorsal
surface of the head of the pancreas, covered by what originally was the
right leaf of the mesoduodenum.
Fig. 140 indicates the results of adhesion of the duodenum, pancreas and
mesoduodenum to the parietal peritoneum as it normally occurs in the
human subject. It will be seen that the primary parietal peritoneum can
be traced mesad over the ventral surface of the right kidney as far as
the point _X_, and that from here on to the median line the peritoneum
is _secondary_ parietal peritoneum, consisting of the visceral
peritoneal investment of the ventral surface of the duodenum and of the
original left leaf of the mesoduodenum, beneath which the ventral
surface of the pancreas is seen. Pancreas and duodenum occupy in the
adult secondarily a "retro-peritoneal" position, _i. e._, the peritoneum
now covering the ventral surface of these viscera appears as a
continuation of the parietal peritoneum, the transition between primary
and secondary parietal peritoneum occurring along the line marked _X_ in
Fig. 140. The opposed peritoneal surfaces indicated by the dotted lines
have become adherent and converted into loose connective tissue in which
the pancreas and duodenum lie imbedded. In the human embryo this process
of adhesion begins in the eighth week, starting at the duodeno-jejunal
flexure and ascending gradually toward the pylorus. At the end of the
fourth month the union is complete.
Proceeding caudad it will next be observed that the peritoneum of the
mesentery occupies the narrow neck of the duodeno-colic isthmus, and
that large vessels (the superior mesenteric) pass between its two layers
at this point to supply the segments of the intestine forming the loop.
In conformity with the greatly increased length of the intestine it will
be found that the mesentery expands from the narrow pedicle at the neck
in a fan-shaped manner in order to develop a sufficiently long margin
for attachment to the intestine. The following points should be
carefully borne in mind in studying the mesentery with the intestines in
this position:
1. The mesentery presents two free surfaces, right and left. With the
coils of the small intestine turned over to the right, the left leaf of
the mesentery is turned toward the observer.
2. Inasmuch as the descending limb of the embryonic loop has developed
the greater part of the small intestine, while a portion of the large
intestine (caecum and colon up to the isthmus) is the result of
differentiation within the ascending or returning limb of the loop, it
will be at once apparent that the double peritoneal layer which extends
between the duodeno-colic isthmus and the attached border of the gut is
partly mesentery of the small intestine, partly mesocolon passing to the
large intestine (caecum and proximal colon). This condition may be
indicated schematically in Fig. 141.
[Illustration: FIG. 141.--Schematic representation of mesentery of
umbilical loop, common to small intestine and proximal portion of large
intestine.]
The curved line _A_ may be taken as an arbitrary division between the
portion of the membrane which on the right of the figure passes to the
small intestine, and the portion which proceeds to the left to be
attached to the large intestine. In other words the line will
schematically separate the true mesenteric from the mesocolic segment of
the primitive membrane.
With the parts in their present position this line might be assumed to
indicate a strip along which the opposed serous surfaces of the parietal
peritoneum and the right leaf of the primitive mesentery became
adherent. In that case an actual division into a mesenteric and
mesocolic segment would have been effected.
Ventrad and to the right of this line of adhesion we would trace that
portion of the primitive membrane which now passes to the coils of the
small intestine as the true mesentery, having an apparent origin in the
background of the abdomen to the dotted line of adhesion. In the same
manner the peritoneal layers passing to the left to reach the caecum and
beginning of the colon would appear as a free mesocolon with the same
line of apparent origin from the background of the abdomen. (cf. p. 80.)
These considerations should be followed out in the dissection of the cat
in order to become familiar with the principle of _secondary lines of
origin_ for peritoneal layers. As we will see later this factor is of
importance in correctly estimating the value of the human adult
conditions.
3. A brief consideration of the mechanical conditions and comparison
with the earlier stages will show why the peritoneal layers which occupy
the bight of the fully developed umbilical loop are especially prone to
develop secondary lines and areas of adhesion to other serous surfaces.
If we compare the dorsal mesentery in its primitive condition, before
the straight intestinal tube has become differentiated into the
subsequent segments, and before the umbilical loop has been formed (Fig.
142), with the later stages represented by the intestines of the cat as
now arranged (Figs. 143 and 144), it will be seen that the vertical line
of attachment to the ventral surface of the vertebral column, between
the points _a_ and _b_ corresponds in the advanced stages to the
interval _ab_ separating the two points of the duodeno-colic isthmus;
also that the entire mesenteric peritoneal surface beyond the isthmus is
the result of drawing out and lengthening the intestinal tract.
Consequently folding or overlapping of this extensive membrane affords
opportunities for adhesions between its own serous surfaces or between
it and the remaining visceral and parietal peritoneum of the abdomen.
[Illustration: FIGS. 142-144.--Schematic representation of three stages
in the development of the mesentery of the umbilical intestinal loop.]
[Illustration: FIG. 142.--Early stage before differentiation of
intestinal canal.]
[Illustration: FIG. 143.--Stage of umbilical loop. Differentiation of
common dorsal mesentery of earlier stage into dorsal mesogastrium,
mesoduodenum, primitive mesentery of umbilical loop, and descending
mesocolon.]
[Illustration: FIG. 144.--Final stage. With complete differentiation of
large and small intestine, the primitive mesentery of the umbilical loop
contains not only the mesentery of the future jejuno-ileum, but also the
mesocola and the ascending and transverse colon, developed from the
ascending or afferent limb of the umbilical loop.]
Moreover, it will be appreciated that the entire extensive coil of
intestines extending between the two boundaries of the duodeno-colic
isthmus (_a_, _b_) is suspended from the back part of the abdomen by a
narrow pedicle and that consequently rotation will readily occur around
the axis drawn through the neck of the isthmus.
Now proceed to illustrate on the cat the result of the rotation as it
occurs normally during the development of the primate intestinal tract.
Take the caecum and commencement of the colon and draw the same over to
the right across the duodeno-colic isthmus and the duodenum. Twist or
rotate the entire mass of small intestines around the isthmic pedicle,
so that the original left leaf of the mesentery will look to the right
and _vice versa_ (Fig. 145). The conditions thus established will be
found to correspond to the schemata shown in Figs. 114 and 115. The main
features of the intestinal tract in the rearranged position will be as
follows:
1. The two points, _a_ and _b_, of the duodeno-colic isthmus (Fig. 145)
are still close together, but reversed in position, _b_ is in front and
to the right, _a_ behind and to the left, whereas before the rotation
_b_ was situated below and to the left, _a_ above and to the right (Fig.
135).
[Illustration: FIG. 145.--Abdominal viscera of cat, with intestines
rotated to correspond to the stage in the development of the human canal
in which the caecum has reached the subhepatic position, but before the
establishment of the ascending colon. (From a fresh dissection.)]
2. The direction of the ileo-colic entrance is reversed, the ileum now
entering the large intestine from below and the left upwards and to the
right, instead of from right to left.
3. The descending duodenum is now situated dorsad to the colon.
4. The original left leaf of the mesentery has become the right, and
_vice versa_.
5. The superior mesenteric artery crosses over the transverse portion of
the duodenum, and with the exception of the inferior
pancreatico-duodenal artery the original right-sided branches now arise
from the left side of the vessel and _vice versa_.
It is now time to compare the conditions established in the cat by the
manipulations just detailed with the arrangement of the adult human
intestinal tract and peritoneum below the level of the transverse colon
and mesocolon.
I. The shortness of the large intestine in the cat will require careful
manipulation in order to produce a disposition in conformity with the
arrangement of this portion of the human intestinal tract. By
stretching the gut somewhat and pulling it well out of the pelvis
sufficient length will be obtained to establish an ascending, transverse
and descending colon. Move the caecum from the subhepatic position which
it occupies immediately after rotation (Fig. 145) down to the lower and
right-hand corner of the abdomen. Pull the distal portion of the large
intestine well out of the pelvis and obtain thus sufficient length to
establish an ascending, transverse and descending division each provided
with a free mesocolon (Fig. 146). In the formation of the three definite
main segments of the human large intestine, ascending, transverse and
descending colon, the following stages may be recognized:
[Illustration: FIG. 146.--Abdominal viscera of cat, with the intestines
rotated to correspond to the adult human disposition, with ascending,
transverse, and descending segments of the colon. (From a fresh
dissection.)]
1. Immediately after rotation the large intestine lies transversely
along the greater curvature of the stomach, with the caecum on the right
side in front of the duodenum and closely applied to the caudal surface
of the right lobe of the liver (Fig. 147).
[Illustration: FIG. 147.--Human foetus, 6.6 cm., vertex-coccygeal
measure; liver removed. (Columbia University Museum, Study Collection.)
x 4.]
PERSISTENCE OF SUBHEPATIC POSITION OF CAECUM IN ADULT.--The period at
which the caecum descends into the iliac fossa is liable to a
considerable range of variation.
Treves found in two foetus, measuring respectively 41/2" and 51/2", the
caecum on a level with the caudal end of the right kidney, while in
several individuals at full term the caput coli was still placed
immediately below the liver, with no large intestine in the place of the
ascending colon. This condition is well illustrated in the foetus shown
in Fig. 124.
The caecum may remain undescended throughout life. Treves, in an
examination of 100 bodies, found this condition in two subjects, both
females, one 41, the other 74 years of age. Both cases presented an
identical disposition. There was no large intestine in the place of the
ascending colon. The caecum was placed on the right side, immediately
underneath the liver, just to the right of the gall-bladder; it was
quite horizontal in position, continuing the long axis of the transverse
colon and included between the layers of the transverse mesocolon. From
the extremity of the caecum a horizontal fold was continued to the
abdominal parietes and upon it the edge of the liver rested. In one of
these instances the colon from the tip of the caecum to the splenic
flexure measured 38". The great omentum was attached only to the left
half of this portion. The descending colon was very long, measuring 15".
In the other case the distance from the tip of the caecum to the splenic
flexure was 27", the great omentum commencing 5" from the former point.
The descending colon was of normal length.
In both bodies the remaining viscera were normal.
2. The caecum next descends ventrad of right kidney to the iliac fossa.
The future ascending colon is at this time placed very obliquely on
account of the large size of the foetal liver, and passes without a
marked angle into the transverse segment. Thus in Fig. 148, from a foetus
5" in length, the descending colon is vertical and the splenic flexure
well marked, forming the highest point of the colic arch. There is no
hepatic flexure, and no ascending and transverse colon, but instead of
these an oblique segment passing upwards and to the left between caecum
and splenic flexure.
[Illustration: FIG. 148.--Abdominal viscera of human foetus of 12.5 cm.,
vertex-coccygeal measure, hardened _in situ_; transverse and ascending
colon not yet differentiated. (Columbia University Museum, No. 1815.)
Natural size.]
This disposition, due to the large size of the liver, is still marked at
times in the foetus at term, and occasionally even in children up to 2 or
3 years of age.
3. The ascending colon is subsequently differentiated from the
transverse segment and the hepatic flexure formed consequent upon the
diminution of the relative size of the liver, which permits the foetal
oblique segment of the colon extending in the earlier stages between the
right iliac fossa and the spleen to become divided by a right-angled
(hepatic) bend or flexure into an ascending and a transverse segment
(Fig. 149).
[Illustration: FIG. 149.--Abdominal viscera of human foetus at term,
hardened _in situ_; hepatic flexure formed and ascending and transverse
colon differentiated. (Columbia University Museum, No. 1816.)]
4. The splenic flexure develops early and is well marked. It indicates
the point of transition of the original ascending limb of the umbilical
loop into the remaining vertical median segment of the large intestine,
from which the descending colon is formed.
In the adult the ascending and descending portions of the colon are
vertical. The transverse colon is not quite horizontal since the
splenic flexure is higher and placed more dorsally than the hepatic
flexure. In the embryo the rapidly-growing coils of the small intestine
push the descending colon to the left and dorsad into close contact with
the dorsal abdominal wall.
A small bend which appears about the middle of the third month in the
left iliac fossa indicates the rudiment of the future sigmoid flexure or
omega loop.
The rest of the endgut follows the body wall in a well-marked curve,
whose termination lies within the concavity of the caudal portion of the
embryo (Fig. 150). From this terminal part the rectum develops after the
division of the cloaca and the union of the proctodaeum with the
entodermal intestinal pouch has taken place as detailed above.
[Illustration: FIG. 150.--Caudal portion of human embryo of 5 mm., with
the end- and caudal gut at the highest stage of its development. x 25.
(Reconstruction after His.)]
The early position of the colon produced by the large size of the foetal
liver, and before the descent of the caecum has occurred, is shown in
Fig. 124. In Fig. 123, where the liver has regained its normal
proportions with reference to the abdominal cavity and viscera, and the
caecum has descended into the right iliac fossa, the hepatic flexure is
well marked and the first segment of the colon has acquired the vertical
position on the right side, the single obliquely transverse segment of
Fig. 124, having become divided into an ascending and a transverse
colon.
[Fig. 124. Early stage. Liver relatively large. Proximal portion of the
colon extends obliquely between the right lumbar region and the spleen.
The caecum has not yet descended.
Fig. 123. Later stage. The caecum occupies the right iliac fossa.
Relative reduction in the size of the liver allows the colic segment to
be divided by the hepatic flexure into an ascending colon and a
transverse colon.]
At times the transverse colon, whose normal average length in the adult
is 20", greatly exceeds this measurement and forms an arch which hangs
down or makes a well-marked V-shaped bend with the apex directed toward
the pubes. This is the normal arrangement of this portion of the large
intestine in many of the lower primates. Fig. 151 shows the abdominal
viscera of _Macacus rhesus_, hardened _in situ_, seen from the front
and the right side, with the omentum turned up over the stomach. The
transverse colon forms an extensive V-shaped bend, whose apex reaches to
the pubes, from which point the large intestine turns again cephalad and
dorsad to form the splenic flexure and then descends to the pelvis.
[Illustration: FIG. 151.--Abdominal viscera of _Macacus rhesus_, rhesus
monkey, hardened in situ. (Columbia University Museum, No. 1817.)]
The average length of the ascending colon in the adult, measured from
the tip of the caecum to the hepatic flexure, was found by Treves in his
series of 100 bodies to be 8", while the descending colon, from the
splenic flexure to the beginning of the sigmoid loop, measured 81/2".
The descending colon may at times be much longer, up to 15", and become
convoluted.
II. In the next place, in order to understand the arrangement of the
peritoneum in this lower larger compartment of the abdomen, disregard
for the present the peritoneal connections of the stomach, liver,
pancreas and spleen, and the folds of the great omentum entirely. This
latter membrane is adherent in the adult human subject by its dorsal
surface to the upper margin of the transverse colon, so that in turning
the omentum up over the ventral chest wall the transverse colon will be
carried with the omentum and the lower layer of the transverse mesocolon
will be put upon the stretch. This membrane forms in adult man by its
transverse attachment to the abdominal background the cephalic limit of
the larger lower compartment of the abdomen, which is framed laterally
by ascending and descending colon, continuous below with the pelvic
cavity and occupied chiefly by the freely movable coils of the
jejuno-ileum.
Remember that the duodenum starting from the pyloric extremity of the
stomach first turns cephalad and dorsad in contact with the caudal
surface of the right lobe of the liver, forming the first portion or
hepatic angle of the duodenum; that in the next place the second or
descending portion of the duodenum passes down in front of the medial
part of the ventral surface of the right kidney and the inferior vena
cava, but _behind_ the right extremity (hepatic flexure) of what after
rotation and formation of the ascending colon appears as the transverse
colon; that consequently the descending duodenum is divided by its
intersection with the transverse colon into a cephalic supra-colic and a
caudal infra-colic segment.
Also remember that the second angle of the duodenum (transition between
the descending and transverse portions) is consequently situated to the
right of the vertebral column below the level of the transverse colon
and the secondary attachment presently to be considered of the
transverse mesocolon to the background of the abdominal cavity.
The third portion of the duodenum extends from this point more or less
transversely--depending upon the type--to the left, across the vertebral
column and aorta. This transverse portion, after the rotation of the
primitive loop at the duodeno-colic angle, is crossed in the direction
caudad and ventrad by the superior mesenteric vessels, which hence
divide this portion of the intestine into a right and left segment.
The latter turns cephalad and ventrad on the left side of the vertebral
column (4th or ascending portion) to become continuous at the
duodeno-jejunal angle with the free or floating small intestine
(jejunum).
If we imagine in the cat the duodenum anchored or fixed by adhesion of
the dorsal (originally right) leaf of the mesoduodenum and of its own
dorsal visceral peritoneum to the abdominal parietal peritoneum in the
manner above indicated (p. 70) as far as the duodeno-jejunal angle we
will have conditions established which correspond to those found in the
human adult abdominal cavity.
III. It is next necessary to study carefully the disposition of the
primitive dorsal mesentery connected after rotation with the different
segments of the intestinal tube, ascending, transverse and descending
colon and free small intestine.
In order to obtain in the cat a cephalic limit to the region now under
consideration which will correspond to the arrangement of the adult
human peritoneum, we will begin with the peritoneal membrane attached to
the portion of the colon which in the rearranged intestinal tract
represents the human transverse colon. This transverse segment of the
large intestine is now made to extend directly across the abdomen from
the liver to the spleen. The two layers composing the transverse
mesocolon are an upper or cephalic and a lower or caudal layer.
Now it will be seen in the cat that the upper or cephalic layer of the
transverse mesocolon thus established is continuous on each side with
the dorsal (originally right) leaf of the ascending and with the dorsal
(originally left) leaf of the descending mesocolon, which peritoneal
layers are in direct opposition to the parietal lumbar and prerenal
peritoneum. On the other hand, the inferior or ventral layer of the
transverse mesocolon is continuous on each side of the median line with
the ventral (originally respectively left and right) leaves of the same
mesocola, while at the site of the duodeno-colic isthmus the two layers
of the transverse mesocolon are continuous as originally with the two
layers of the mesentery of the jejuno-ileum (Fig. 146).
Now fix the transverse mesocolon firmly against the background of the
abdomen and place the ascending and descending colon as far as possible
over to the right and left side respectively. We will assume a line of
secondary adhesion between the transverse mesocolon and the parietal
peritoneum investing the dorsal abdominal wall. Along this line the
upper or cephalic surface of the transverse mesocolon would become
continuous with the dorsal parietal peritoneum, while the lower or
caudal layer would still be continuous with the left leaf of the
ascending and the right leaf of the descending mesocolon. We have
already seen that the duodenum and mesoduodenum become anchored in the
subhepatic region and that the visceral ventral peritoneum of the gut
and the original left leaf of the mesoduodenum appear then as secondary
parietal peritoneum. Hence a sagittal section through the right lumbar
region, right kidney and descending duodenum would, immediately after
rotation and establishment of the transverse mesocolon, show
the peritoneal arrangement indicated in Fig. 153. After adhesion of the
transverse mesocolon continuity would be established between its upper
or cephalic layer and the secondary parietal peritoneum investing the
supra-colic portion of the descending duodenum (Fig. 154) while its
caudal layer becomes continuous with the secondary parietal peritoneum
covering the infra-colic segment of the duodenum and the lower portion
of the ventral surface of the right kidney.
[Illustration: FIGS. 152-154.--Schematic representation of peritoneum in
fixation of descending duodenum and formation of transverse colon and
mesocolon.]
[Illustration: FIG. 152.--Sagittal section through right kidney and
descending duodenum before adhesion of latter to parietal peritoneum.]
[Illustration: FIG. 153.--Adhesion of descending duodenum to primitive
parietal peritoneum. Colon and mesocolon after rotation of the
intestine, but before adhesion.]
[Illustration: FIG. 154.--Adhesion of mesocolon to duodenum and
primitive parietal peritoneum, resulting in formation of root of
transverse mesocolon.]
Reference to the schematic Figs. 152, 153 and 154, will show that the
adult duodenum becomes fixed to the posterior parietes of the abdomen by
adhesion of its visceral serous covering and of the dorsal layer of the
mesoduodenum to the primitive parietal peritoneum. The supra-colic
segment of the adult descending duodenum lies under cover of a single
peritoneal layer, derived from its own visceral investment and appearing
as secondary parietal peritoneum by continuity laterad along the line of
adhesion with the primitive parietal peritoneum covering the upper part
of ventral surface of right kidney, while mesad, the layer covering this
segment of the duodenum, is continued into the secondary parietal
peritoneum derived from the left or ventral leaf of the mesoduodenum and
covering the ventral surface of the pancreas (cf. Figs. 138-140).
On the other hand, the infra-colic segment of the descending duodenum,
as well as the lower and mesal angle of the ventral surface of right
kidney, between ascending and transverse colon, is covered by a layer of
secondary parietal peritoneum derived from the ventral layer of the
ascending mesocolon and continuous with the caudal layer of the
transverse mesocolon. Beneath this secondary parietal peritoneum are two
obliterated layers, on the one hand the dorsal layer of the mesocolon,
on the other the visceral infra-colic duodenal serosa and the primitive
prerenal parietal peritoneum.
In the further development of the adult human arrangement the changes
below the level of the transverse colon and mesocolon result in the
fixation of the ascending and descending colon to the background of the
right and left lumbar regions. The opposed serous surfaces of the
ascending and descending mesocola and of the dorsal parietal peritoneum
adhere and the process also usually involves the dorsal visceral
peritoneum of the ascending and descending colon, so that these portions
of the gut obtain a fixed position.
Adhesion of the mesocolon to the dorsal body wall (parietal peritoneum)
does not occur at all points at the same time. Usually adhesion proceeds
from the midline laterad. The fixation of the ascending colon in the
human embryo begins about the fourth month.
In the descending segment by the same time adhesion has usually
proceeded nearly up to the descending colon, but the intestine itself is
as yet free. In the fifth month the descending colon has usually become
fixed between the splenic flexure and the beginning of the sigmoidea. In
the latter region a free mesocolon usually persists throughout life.
Differences in the rate of growth between the length of the body wall
and the length of the mesocolon may play an important part in the
production of peritoneal _fossae_, small pouches which in some regions of
the abdomen may assume considerable proportions. Such fossae are found
around the duodeno-jejuneal angle, the caecum and appendix, and the
sigmoid flexure. They will be considered more in detail with these
respective regions, especially in reference to their relation to
retro-peritoneal hernia.
In a certain proportion of cases adhesion between the parietal
peritoneum and the ascending and descending mesocolon is incomplete or
entirely wanting, resulting in the formation of a more or less
completely free ascending and descending mesocolon. Treves, in an
examination of 100 bodies, obtained the following figures:
In 52 subjects there was neither an ascending nor a descending
mesocolon, the intestine being fixed in the manner which is regarded as
normal.
In 22 there was a descending, but no trace of an ascending mesocolon.
In 14 a mesocolon was found in both the ascending and descending
segments of the large intestine.
In 12 there was an ascending mesocolon, but no corresponding fold on the
left side. Hence from this series a mesocolon may be expected on the
left side in 36 per cent., on the right side in 26 per cent.
Both development and comparative anatomy would lead us to expect that
the descending mesocolon would be found more frequently than the
ascending.
In the lower animals the descending mesocolon is always an extensive and
conspicuous membrane. It is well developed in all monkeys and the
anthropoidea, as the remains of the primary vertical fold of the dorsal
mesentery, while the ascending mesocolon is a secondary production,
acquired during the development of the bowel by rotation.
In most of the lower monkeys the ascending mesocolon is also largely or
entirely free. The descending mesocolon can always in these animals be
reflected to the median line (cf. Fig. 155).
[Illustration: FIG. 155.--Abdominal viscera of _Macacus cynomolgus_, Kra
monkey, hardened _in situ_. (Columbia University Museum, No. 1801.)]
The line of attachment in man of the descending mesocolon is usually
along the lateral border of the left kidney and vertical, while the line
of attachment of the ascending mesocolon is usually less vertical,
crossing the caudal end of the right kidney obliquely from right to left
and with an upward direction (Fig. 156).
[Illustration: FIG. 156.--Schema of visceral and peritoneal relations of
ventral surface of right kidney.]
In like manner when both the ascending and descending mesocola are
absent as free membranes the left or descending colon is adherent along
the lateral border of the kidney to the abdominal parietes, while the
ascending colon is fixed at the hepatic flexure a little obliquely
across the ventral surface of the caudal end of the corresponding gland
ascending toward the medial margin.
Treves found in the cases of persistent ascending mesocolon in the adult
that the membrane varied in breadth from 1" to 2" while the persistent
fold on the left side varied between 2" and 3" in breadth.
In the foetus, up to 5"-6" in length, the descending mesocolon is usually
an extensive fold. Its attachment is vertical, but nearer to the median
line than in the adult, usually along the medial border of the left
kidney. It is at times found attached along this line in the adult.
An ascending mesocolon is rare even in the foetus. The caecum and
beginning of the ascending colon are completely invested by peritoneum,
but above the parts so invested the colon is usually adherent along an
oblique line to the ventral and medial aspect of the right kidney.
In the foetus at full term, if the caecum is still undescended and in
contact with the liver, it is not uncommon to find the cephalic portion
of the descending colon provided with a mesocolon, while the caudal part
of the descending colon is fixed by adhesion to the ventral surface and
lateral border of the left kidney. This free membrane is then really a
part of the transverse mesocolon. Where the caecum descends to the iliac
fossa the portion of the foetal descending colon so invested is drawn
over to the right and incorporated in the transverse colon.
Treves in two out of 100 bodies found the caecum in the right iliac
region, but both it and the whole of the ascending colon were entirely
free from any peritoneal connections with the dorsal parietes of the
abdomen.
The gut from the tip of the caecum to the hepatic flexure was entirely
invested by peritoneum continuous with the mesentery. The ascending
colon was covered in the same manner and by the same fold as the small
intestine. The segment of large intestine thus free measured 8" in both
instances.
The mesentery lacked the usual attachment to the dorsal abdominal wall
and its root was represented by the interval between the duodenum and
the transverse colon. The membrane had no other than its original
primary attachment, and small intestine and ascending colon formed
together a loop that practically represented the condition of the great
primary intestinal loop. (Compare p. 73.)
The arrangement presented in these two subjects corresponds to that met
in many animals, such as the cat.
A cross-section of the cat's abdomen arranged as above would show the
following disposition of the peritoneum, corresponding to the stage in
the human development preceding the fixation of the two vertical colic
segments (Fig. 157). It will be seen that the right and left mesocola
can be reflected to the median line where they become continuous ventrad
of the vertebral column and aorta with the mesentery of the small
intestine. The ventral surfaces of both kidneys are seen to be covered
by the primitive parietal peritoneum of the abdominal cavity.
[Illustration: FIGS. 157, 158.--Schema showing peritoneal arrangement in
transection of infra-colic compartment of abdomen before and after
fixation of ascending and descending colon.]
Fig. 158 shows the adult human arrangement of the same parts, after
fixation of the vertical colic segments by adhesion of the opposed
surfaces of their mesocola and the primitive parietal peritoneum. The
background of the abdomen is now seen to be covered by a layer of
secondary parietal peritoneum, _viz._, the original left leaf of the
ascending and right leaf of the descending mesocolon, continuous above
with the lower or caudal layer of the transverse mesocolon.
This adhesion is so complete that the original condition is disregarded
in adult descriptive anatomy. The layer which has adhered to the
parietal peritoneum can no longer be recognized and the other has
assumed the role of parietal peritoneum.
The connection of the transverse mesocolon with the dorsal lamella of
the great omentum will be considered below.
The course of the vessels in the ascending and descending mesocola is
not altered by the secondary adhesions. These vessels are in the adult
situated behind the secondary parietal peritoneum derived from the
mesocola.
The origin of the transverse mesocolon obtains by the fixation of the
hepatic and splenic flexures high up in the abdomen a transverse course,
and the transverse growth of the abdomen holds the membrane in this
position cephalad of the duodeno-jejunal flexure, so that on elevating
the transverse colon the mesocolon appears as separating the upper from
the lower abdominal compartment. This posterior line of attachment or
so-called "root of the transverse mesocolon," is nothing more than the
upper limit of the area of adhesion between the primitive parietal
peritoneum and the opposed surfaces of the ascending and descending
mesocola. Reference to the abdominal cavity of the cat after complete
rotation (Fig. 146) will show the original continuity of the three
mesocola very clearly. A secondary connection is established along the
lateral border of ascending and descending colon (Fig. 158), between the
primitive parietal peritoneum and the ventral visceral peritoneal
investment of the large intestine. Both of the vertical segments of the
colon now appear fixed. Their dorsal surfaces are uncovered by
peritoneum and can be reached in the lumbar region, laterad of the
kidney, without opening the peritoneal cavity (lumbar colotomy).
The caudal portions of both kidneys are covered, beneath the secondary
parietal peritoneum, by a layer of loose connective tissue representing
the result of obliteration by adhesion of the first and second of the
original three layers of prerenal peritoneum, _viz._, the primitive
parietal (1) and the two layers of the mesocola (2 and 3).
LINE OF ATTACHMENT OF THE MESENTERY OF THE JEJUNO-ILEUM.--Examination of
the caudal surface of the transverse mesocolon in the cat, with the
parts in the above outlined position, will show how and why in the adult
human abdomen the duodeno-jejunal angle appears to dip out from beneath
the transverse mesocolon, becoming gradually more and more free until
complete transition to the mobile jejunum is obtained. From this point,
situated to the left of the second lumbar vertebra, the dorsal
attachment of the adult human mesentery of the jejuno-ileum extends
somewhat obliquely caudad and to the right to terminate in the right
iliac fossa at the ileo-colic junction.
Returning to the conditions presented by the cat's intestines to obtain
an explanation of this line of fixation we must recall the fact that in
the peritoneum included within the limits of the umbilical loop, after
differentiation of small and large intestine, but before rotation, we
have both the elements of the mesentery of the small intestines and of
the ascending and transverse mesocolon combined (Fig. 141). For it will
be seen that this membrane carries at this time vessels both to the
jejuno-ileum and to the segments of the large intestine (caecum,
ascending and transverse colon). This fact will be at once recognized if
the cat's intestines are arranged to correspond to the primitive
condition (Fig. 136) and the mesentery examined.
After rotation and differentiation of the colic segments and after the
adhesion of the ascending and descending colon in man, the course of the
main trunk of the superior mesenteric artery passes, after crossing the
third portion of the duodenum, down and to the right to terminate near
the ileo-colic junction by anastomosis with its ileo-colic branch. The
adhesion of the right and left mesocola to the dorsal parietal
peritoneum proceeds mesad as far as this line, leaving free the
mesentery of the small intestines, which contains the vasa intestini
tenuis derived from the left side of the main vessel. The secondary line
of attachment of the mesentery to the abdominal background is therefore
along this line. To obtain a clear idea of these processes of
development in man assume that in the cat, after rotation and
establishment of the three divisions of the colon, the two vertical
(ascending and descending) mesocola become adherent to the dorsal
parietal peritoneum, leaving the mesentery of the small intestine free.
Fig. 159 illustrates schematically the area of mesocolic adhesion in the
human subject after complete rotation, and the line of the mesentery of
jejuno-ileum.
[Illustration: FIG. 159.--Schematic figure to show lines of mesocolic
adhesion, formation of root of transverse mesocolon and root of
mesentery of jejuno-ileum in human subject.]
Fixation of the ascending and descending cola and of their mesocola
proceeds cephalad as far as the line _AB_, which thereby constitutes the
root of the free transverse mesocolon.
The secondary parietal peritoneum derived from the ventral layer of the
ascending mesocolon covers the lower and inner portion of the ventral
surface of the right kidney, the infra-colic division of the descending
and the dextro-mesenteric segment of the transverse duodenum, while
along the root of the jejuno-ileal mesentery it becomes continuous with
the right layer of that membrane. The secondary parietal peritoneum
derived from the ventral layer of the descending colon covers the lower
part of the ventral surface of the left kidney and the
sinistro-mesenteric segment of the transverse duodenum and becomes
continuous along the mesenteric radix with the left layer of the
jejuno-ileal mesentery.
Caudad the adhesion of the descending colon and mesocolon to the
parietal peritoneum proceeds only to the point _C_, following the dotted
line mesad and resulting in the formation of the free mesocolon of the
sigmoid flexure.
=Resume of the Adult Arrangement of the Human Peritoneum in the Lower
Compartment of the Abdomen, Below the Level of the Transverse Colon and
Mesocolon.=--We should now consider the arrangement of the human
peritoneum in the adult below the dorsal attachment of the transverse
mesocolon in the light of the embryological and comparative anatomical
facts just stated. In doing this it will be advisable to study both the
actual conditions encountered and their significance in the sense of
determining the derivation of the peritoneal layers from the primitive
dorsal mesentery. Open the abdominal cavity in the usual manner by a
cruciform incision.
Turn the great omentum up on the chest wall, exposing the underlying
intestines. This manipulation, as already stated, will cause the omentum
to carry the transverse colon with it, on account of the adhesion, in
the adult, of the gut to the dorsal layer of the omentum. Hence the
cephalic or upper layer of the transverse mesocolon will not be seen at
this stage because the omental adhesion just referred to prevents us
from passing between the greater curvature of the stomach and the
transverse colon without tearing peritoneal layers. It will, however, be
possible to trace on the right side the duodenum from the pylorus down
ventrad of the right kidney until the descending portion disappears
behind the hepatic flexure of the colon. With the omentum and transverse
mesocolon turned up, as stated, and the transverse mesocolon put upon
the stretch, it will be seen that the abdominal space now overlooked is
bounded cephalad by the lower layer of the transverse mesocolon and its
attachment to the dorsal abdominal wall. The lateral limits of the space
are given by the ascending and descending colon respectively. The
attachment of the mesentery of the small intestine to the oblique line
extending from the left of the vertebral column at about the level of
the second lumbar vertebra to the right iliac fossa subdivides the
entire space into a secondary right and left compartment.
Begin by following the caudal layer of the transverse mesocolon dorsad
on the right side. In the angle between ascending and transverse colon
(hepatic flexure) pressure will locate the caudal portion of the ventral
surface of the right kidney. Remember that the peritoneum touched in
these procedures appears in the adult as parietal prerenal peritoneum,
but that in reality it is the left leaf of the originally free ascending
mesocolon, whereas the original right leaf of this membrane and the
primitive parietal peritoneum have, by adhesion of their serous
surfaces, been converted into the loose subserous connective tissue
covering the ventral aspect of the kidney beneath what now appears as
parietal peritoneum.
Mesad of the resistance offered to the finger by the right kidney the
caudal (infra-colic) portion of the descending duodenum and the angle of
transition between it and the third or transverse portion will be found,
invested in the same way by secondary (mesocolic) parietal peritoneum.
It will be seen, especially if the duodenum is injected or inflated,
that the hepatic flexure of the colon lies ventrad of the vertical
descending second portion of the duodenum, so that one part of this
intestine is situated cephalad the other caudad of the colon. (Supra-
and infra-colic segments of descending duodenum.)
Individual differences are observed in the area of colic attachment to
the duodenum. Usually the two intestines are in contact with each other
and adherent over a considerable surface. Exceptionally the transverse
mesocolon extends across to the right so as to include the hepatic
flexure. In this latter case the uncovered non-peritoneal surface of the
descending duodenum is small, represented by the interval between the
layers of the transverse mesocolon, and the hepatic flexure is then not
directly adherent to the gut.
If we now trace the transverse duodenum from right to left we will
encounter the right layer of the root of the jejuno-ileal mesentery. The
caudal layer of the transverse mesocolon, the right leaf of the
mesentery and the secondary parietal peritoneum investing the ventral
surface of the transverse duodenum all meet at this point. Surround the
mesentery of the free small intestine with the fingers of one hand so
that the entire mass of intestinal coils can be swung alternately from
side to side.
Turning them over to the left, as already stated, the proximal portion
of the transverse duodenum can be traced from right to left as far as
the root of the mesentery. Here the peritoneum investing the ventral
surface of the duodenum becomes continuous with the right leaf of the
mesentery. Now swing the whole mass of small intestines over to the
right, exposing the parietal peritoneum in the space to the left of the
vertebral column, between the attachment of the mesentery to the median
side, the root of transverse mesocolon cephalad and the descending colon
to the left. Remember that the same significance attaches to this
secondary parietal peritoneum as on the right side. It appears in the
adult as parietal peritoneum, but is in its derivation the original
right leaf of the descending mesocolon. Close to the root of the
mesentery the continuation from the right side of the transverse
duodenum will be seen, crossing the median line from right to left
ventrad of aorta and vertebral column and usually turning cephalad on
the left side of the lumbar vertebrae, as the fourth or ascending
duodenum, to reach the caudal surface of the transverse mesocolon near
its attachment, where the gut turns ventrad to form the duodeno-jejunal
angle and become continuous with the free small intestine.
From the fact that the transverse duodenum is thus seen on each side of
the root of the mesentery it will be recalled that after rotation of the
primitive intestine the superior mesenteric artery crosses the
transverse portion of the duodenum to reach its distribution between
the leaves of the mesentery. Hence this portion of the small intestine
consists of a dextro- and sinistro-mesenteric segment. This intersection
of mesentery and duodenum marks the site of the primitive duodeno-colic
isthmus through which the superior mesenteric artery passed to reach its
distribution to the gut composing the embryonic umbilical loop.
To the left of the ascending duodenum a portion of the caudal surface of
the pancreas will be seen, covered by the continuation of the caudal
leaf of the transverse mesocolon into the parietal peritoneum. The
consideration of this relation of peritoneum and pancreas will
profitably be deferred until we have studied the developmental changes
in the region of the dorsal mesogastrium and great omentum.
In the angle between termination of the transverse colon and proximal
part of descending colon (splenic flexure) the caudal part of the
ventral surface of the left kidney will be felt. The disposition of the
peritoneum and its significance is the same as on the right side.
Inasmuch as we have already seen that the secondary parietal peritoneum
covering the dorsal abdominal wall on each side of the small intestine's
mesenteric attachment is derived from the primitive ascending and
descending mesocolon, it will be readily understood why the blood
vessels supplying the ascending and descending colon (arteria
ileo-colica, a. colica dextra, a. colica sinistra) are placed _behind_
the parietal peritoneum, while the colica media, supplying the
transverse colon, runs between the layers of the transverse mesocolon.
Originally the same condition obtained for the two vertical colic
segments, but with the anchoring of these portions of the large
intestine and the adhesion of their mesocola to the parietal peritoneum
the blood vessels which formerly ran between the two layers of the
membrane, as long as it remained free, now appear as retroperitoneal
vessels placed beneath the parietal peritoneum derived secondarily from
the mesocola.
This fact must be borne in mind in studying the arrangement of certain
folds and fossae of the parietal peritoneum which are now to be
considered.
=Duodenal Fossae. Fossa of Treitz and Retro-peritoneal Hernia.=--The
peritoneal cavity of the cat can be used to great advantage in order to
obtain a clear idea of the formation of these folds and fossae, whose
relation to the so-called "retro-peritoneal hernia" has led to an
exaggerated elaboration of minute detail and a somewhat puzzling
terminology in human descriptive anatomy.
=Directions for Examining the Folds and the Formation of the
Duodeno-jejunal Fossa in the Cat.=--Turn the omentum and the coils of
the small intestine cephalad out of the abdomen until they rest upon the
ventral thoracic wall. Press the large intestine over to the left side,
putting the mesocolon on the stretch until the parts are arranged as
shown in Fig. 160. The loop of the duodenum with the head portion of the
pancreas will be seen caudad of the liver and ventrad of the right
kidney. A well-marked peritoneal fold, somewhat sickle-shaped, with the
concavity of the free edge directed caudad and to the right, will be
seen extending from the convex border of the duodenum, directly opposite
the mesenteric or attached margin, to the right leaf of the mesocolon.
This fold indicates the beginning adhesion of the duodenum to the
mesocolic peritoneum, the first step toward the subsequent complete
fixation of the gut as it is found in man.
[Illustration: FIG. 160.--Abdominal cavity of cat, with intestines
everted and elevated to show duodenal fold. (From a fresh dissection.)]
[Illustration: FIG. 161.--Abdominal viscera of _Nasua rufa_, brown
coaiti. (From a fresh dissection.)]
Fig. 161 shows the abdominal cavity of Nasua rufa, the brown
Coati-mundi, a South American arctoid carnivore, with the intestines
everted and turned to the left side. In this animal the large intestine
is very short, there is no caecum, the ileo-colic junction is only marked
on the surface by a pyloric-like constriction of the tube and in the
interior by the projection of a ring-valve (Fig. 408).
The duodenal fold is very well developed, passing between the convex
surface of the duodenal loop and the adjacent right leaf of the short
mesocolon.
In Primates, in which complete rotation of the intestine, on the plan of
the human development, takes place, still further and more extensive
agglutination of the serous surface of the duodenum to the peritoneum of
the mesocolon occurs. Fig. 162 shows the condition in _Hapale
vulgaris_, one of the marmosets. The ascending and descending mesocola
and the mesoduodenum of this animal are still free, but the surface of
the duodenum has become fastened to the opposed mesocolon. With fixation
of the hepatic flexure and adhesion of the ascending colon, such as
occurs in man, the duodenum is carried dorsad against the ventral
surface of the right kidney, and now anchoring of the duodenum, by
obliteration of the mesoduodenum and adhesion to the prerenal parietal
peritoneum, takes place as already detailed above. To return now to the
formation of the duodeno-jejunal fossa by means of this fold, as
illustrated in the cat. Perform the manipulations already described in
rotation of the intestine. The appearance of the parts then will be as
shown in Fig. 163. The large intestine is drawn over so as to represent
the human ascending and transverse colon in one segment, the descending
colon in the other, and the mesocolon appears correspondingly as
transverse and descending. In other words the cat's intestines as
arranged in the figure would represent the stage in the human
development in which caecum and beginning of large intestine are still
subhepatic in position ventrad of the right kidney, before
differentiation of ascending and transverse colon by descent of caecum
into right iliac fossa.
[Illustration: FIG. 162.--Abdominal viscera of _Hapale vulgaris_, the
marmoset. (Columbia University Museum, No. 1818.)]
[Illustration: FIG. 163.--Abdominal viscera of cat; intestines rotated
and turned to the right to show duodenal fold. (From a fresh
dissection.)]
In the human subject, as we have seen, the transverse mesocolon obtains
a secondary attachment to the background of the abdominal cavity, its
caudal surface remaining free.
The descending mesocolon turns its original right leaf ventrad, its left
leaf dorsad, and the latter adheres to the primitive parietal peritoneum
covering the left lumbar region and ventral surface of left kidney. This
area of adhesion extends up to and usually involves the dorsal surface
of the descending colon, anchoring the same in the left lumbar region,
down to the point where the sigmoid flexure begins and where the
original mesocolon again appears free.
In the cat, therefore, with the intestines arranged to correspond to the
course of the human large intestine after rotation has been
accomplished, the lines representing the peritoneal human adhesions
should be fixed, as shown in the schema, Fig. 159: AB, line of secondary
attachment after rotation resulting in the formation of the "root" of a
free transverse mesocolon. BC, line of limit of secondary adhesion to
the original parietal peritoneum involving the entire left (now dorsal)
layer of the descending mesocolon and the dorsal surface of the
descending colon, resulting in the fixation of the latter part of the
large intestine.
This establishes, as already stated, a secondary parietal peritoneal
surface in the left lumbar region derived from the original right leaf
of the descending mesocolon. Inasmuch as the inferior mesenteric vessels
originally passed to the descending colon between the layers of the
mesocolon they will now apparently be placed beneath the (secondary)
parietal peritoneum of the left lumbar region.
If now the duodenal fold in the cat be examined after rotation of the
intestine it will be found presenting the original relations (Figs. 160
and 163), viz., passing from the convex margin of that portion of the
duodenal loop which would correspond to the human fourth or ascending
portion, to the original right layer of the mesocolon, which in man
becomes secondarily converted into the parietal peritoneum of the left
lumbar region. Hence the connections of the fold are as follows:
_On the right_: ventral surface of the ascending duodenum.
_On the left_: right layer of mesocolon (secondary lumbar parietal
peritoneum in the adult human subject).
_Cephalad_ it abuts against the caudal layer of the transverse mesocolon
along the line which would correspond to the root of the mesocolon in
the adult human subject.
The concave _caudal_ edge is free and bounds the entrance into a fossa,
the "superior duodenal fossa" of anthropotomy. This fossa opens caudad
and extends cephalad to the root of the transverse mesocolon. The
ventral and left wall of the fossa is formed by the fold in question,
its background by the mesocolon (right leaf); to the right the left
circumference of the ascending duodenum enters into the formation of
the fossa, and its fundus is formed by the confluence of the fold and of
the caudal layer of the transverse mesocolon. The inferior mesenteric
vessels are found near the left margin of the entrance into the fossa.
Fig. 164 shows the appearance of the fold in _Nasua rufa_ after rotation
of the intestine. The short course of the large intestine in this
animal, and the consequent reduction of the mesocolon, brings the fold
much below the level which it occupies in the cat.
[Illustration: FIG. 164.--Abdominal viscera of _Nasua rufa_, the brown
coaiti, showing the position of the duodenal fold after rotation of the
intestine. (From a fresh dissection.)]
If we now look for the corresponding structures in man we will find
certain modifications depending chiefly upon still closer adhesion
between duodenum and the mesocolon which is destined to become the left
parietal peritoneum after anchoring of the descending colon. We have
already encountered an example of such closer connection in the marmoset
shown in Fig. 162.
In all cases the "superior duodenal" fold, corresponding to the fold
just encountered in the cat, is the original condition, and the duodenal
fossa consequently opens caudad. In many instances this will be the only
fold and fossa encountered in the adult human subject. In other
instances more extensive duodeno-mesocolic adhesions result in the
addition of an "inferior fold," bounding a fossa the entrance into which
is directed cephalad toward the transverse mesocolon. Such a condition
is seen in Fig. 165 taken from a foetus at term. The duodenal fossa in
this case is bounded by an "upper" and "lower" duodenal fold continuous
with each other on the left side, but separated on the right at their
attachment to the duodenum. It will be seen that the inferior mesenteric
vein runs in the left margin of the fold, following along the left
border of the entrance into the fossa. A segment of the colica sinistra
artery may occupy the same position. This position of the vein, or
artery, or of both vessels, is not the cause leading to the formation of
the duodenal fossa, but is more or less accidental and variable. In many
cases the vessels run at some distance from the folds bounding the
fossa.
[Illustration: FIG. 165.--Abdominal viscera of human foetus at term,
arranged to show duodenal folds and fossa. The jejuno-ileum, ascending
and transverse colon have been removed. (Columbia University Museum, No.
1819.)]
In some subjects the "inferior" fold is the only one found, and the only
duodenal fossa then encountered looks cephalad. This condition, when
associated with the course of the inferior mesenteric vessels in the
free edge of the fold, constituted the classical "fossa
duodeno-jejunalis" of Treitz, and is described as "Treitz's fossa."
Fig. 166 shows the condition in which only a small inferior fold
attaches itself to the termination of the transverse duodenum. There is
practically an entire absence of duodenal or duodeno-jejunal folds and
fossae. The inferior mesenteric vessels course under cover of the
mesocolic secondary parietal peritoneum, but do not produce a fold.
[Illustration: FIG. 166.--Abdominal viscera of human foetus at term.
(Columbia University Museum, No. 1820.)]
[Illustration: FIG. 167.--Abdominal viscera of adult human subject,
showing duodenal folds and fossa. (From a fresh dissection.)]
Fig. 167, from an adult human subject, illustrates the further
development of the fossa from the foetal conditions shown in Fig. 165.
The well-marked duodenal fossa is bounded by a superior and inferior
duodenal fold, uniting laterally in a crescentic margin containing a
segment of the inferior mesenteric vein and colica sinistra artery. The
lower division of the peritoneal recess thus produced corresponds to the
typical (vascular) "fossa of Treitz." Mesally the projection of the
fourth portion of the duodenum bounds the fossa.
In Fig. 168, also taken from an adult human subject, an extensive
duodenal recess is bounded in the same way by a superior and inferior
duodenal fold. In the interior of the fossa a third duodenal
reduplication of the peritoneum ("intermediate duodenal fold") is seen,
as is also the trunk of the inferior mesenteric vein, while the main
trunk of the colica sinistra artery courses laterally behind the
secondary mesocolic parietal peritoneum near the margin of the
descending colon.
[Illustration: FIG. 168.--Abdominal viscera of adult human subject,
showing duodenal folds and fossa. (From a fresh dissection.)]
It will be seen that the freedom of the ascending or fourth portion of
the duodenum depends largely upon the disposition and extent of these
folds. Inasmuch as they are the product of varying degrees of adhesion
of this segment of the intestine they are subject to great individual
variations and have given rise to an unnecessary and complicated
classification of the duodenal folds and fossae. The close relation
maintained between the duodeno-jejunal angle and the caudal layer of the
transverse mesocolon near its root at times leads to the production of
a peritoneal fold connecting this membrane with the duodeno-jejunal
knuckle of intestine (duodeno-jejunal or mesocolic fold) and may result
in the formation of a duodeno-jejunal or mesocolic fossa of the
peritoneum. An instance of this fold is seen in Fig. 168.
The importance of the duodenal fossae, and of similar peritoneal recesses
in other parts of the abdominal cavity, is founded on the fact that by
gradual enlargement they may lodge the greater part of the movable small
intestine in their interior, leading to the formation of intra- or
retro-peritoneal herniae.[3]
[3] For full details of the anatomical and pathological conditions
involved consult B. G. A. Moynihan "On Retro-peritoneal Hernia"--London,
1899.
=Fossa Intersigmoidea.=--A second peritoneal pocket or fossa is
encountered in the region of the sigmoid flexure and its mesocolon. The
formation of this fossa is closely associated with the adult disposition
of the sigmoid mesocolon as part of the original primitive vertical
dorsal mesentery. In the typical arrangement of the parts the sigmoid or
omega loop of the large intestine has a free mesocolon. The adhesion of
the descending mesocolon to the parietal peritoneum usually ceases along
a line drawn horizontally from the lateral margin of the left psoas at a
level with the crest of the ilium to the medial side of the iliac
vessels. This line, along which the mesocolon ceases to be adherent to
the parietal peritoneum, joins the attachment of the distal portion of
the sigmoid mesocolon, which partially retains its primitive vertical
origin to the dorsal midline, at a right angle. This angle is the site
of the _intersigmoid fossa_, the entrance into which is seen usually as
a round opening of variable size on elevating the sigmoid flexure and
putting its mesocolon on the stretch. Fig. 159 shows the area of
adhesion between the primitive descending mesocolon and the parietal
peritoneum (from C mesad) which results in the formation of a free
mesocolon for the sigmoid flexure. Frequently in the angle formed by the
horizontal and vertical line of attachment of the sigmoid mesocolon a
non-adherent strip of the primitive mesocolon roofs in a more or less
extensive intersigmoid fossa, whose fundus is directed upwards and
inwards.
=Caecum, Appendix and Ileo-colic Junction.=--Several peritoneal fossae and
folds are found in the ileo-colic region in connection with the caecum,
appendix and termination of the ileum. The practical importance of this
portion of the intestinal tract and the great morphological interest
which attaches to the same make it worth while to consider its anatomy
in a separate chapter.