MORPHOLOGY OF THE HUMAN CAECUM AND VERMIFORM APPENDIX.
Not only is the anatomy of this portion of the alimentary tract of great
interest in relation to the evolution of the human structure, but in
addition the pathological and surgical importance of the region warrants
a very careful study of the caecum and appendix. This is more especially
the case since a number of variations in the arrangement of the
structures are encountered. These departures from what we consider the
normal human type have an important bearing on the development and
progress of the pathological conditions prone to involve the appendix.
We may consider the subject under the following subdivisions:
I. DEVELOPMENT OF THE CAECUM AND APPENDIX.
Much light is thrown on the adult anatomy of the parts and on the origin
of the variations observed by the study of their embryonic history. In
considering the factors which determine the variations in the position,
size, and shape of the appendix it must be remembered that the
rudimentary character of this structure is responsible for many of the
aberrant conditions encountered.
As a part of the general caecal pouch which persists in an early
developmental stage and which we can regard as destined for further
reduction and ultimate elimination in the course of evolution, the
appendix shares with other vestigial structures a wide range of
variation. Consequently the study of the development of this portion of
the alimentary tract enables us to gain a clearer view of the primary
arrangement of the structures and to trace the causes which are active
in determining the adult conditions most frequently encountered.
[Illustration: FIG. 486.--Alimentary canal and appendages of human
embryo of 12.5 mm. x 12. (Kollmann, after His.)]
[Illustration: FIG. 487.--_A_, schematic representation of alimentary
canal, with umbilical loop and mesenteric attachments in human embryo of
about six weeks. _B_ and _C_, stages in the intestinal rotation.]
At the time when the umbilical loop of the intestine has formed and has
begun to protrude into the cavity of the umbilical cord (fifth to sixth
week), the first indication of the future caecum appears as a
circumscribed thickening of the returning or ascending limb of the
intestinal loop a short distance from the apex (Figs. 486-488). This
rudiment indicates the derivation of the future definite intestinal
segments from the elements of the loop. The descending limb, apex (site
of embryonic vitelline duct, Meckel's diverticulum of adult) and a short
succeeding portion of the ascending limb furnish the ileum and jejunum.
The rest of the ascending limb develops into caecum and appendix,
ascending and transverse colon. The increase in the length of the
intestine is not uniform. The formation of convolutions begins in the
seventh week in the apex and subsequently in the descending limb. By the
eighth week a considerable number of jejuno-ileal coils have resulted
from the growth in length of these parts of the original umbilical loop,
while the growth of the segment which furnishes the colon is at this
time still inconsiderable (Fig. 489). In the meanwhile the thickening of
the tube which forms the first rudiment of the caecum has developed into
a small sac-like enlargement of the gut, budding from the left and
dorsal aspect of the ascending limb, crescentic in shape, turning its
concavity toward the parent tube. In the majority of instances examined
the small outgrowth is packed closely between the incipient ileal
convolutions, lying under cover of the more prominent bulging coils of
the umbilical protrusion, between them and a single coil of larger arc
situated dorsally and belonging to the jejunal or proximal portion of
the small intestine (Fig. 490). Fig. 497, taken from an embryo of 11 mm.
cervico-coccygeal length, represents this stage in the development of
the umbilical loop. The arrangement of the caecum which we can assume as
the typical condition at this stage and which determines in part the
subsequent final arrangement of the structures, is illustrated by this
relation of the caecal bud to the surrounding incipient convolutions of
the small intestine, with the larger part of these coils situated
ventrad of the caecum and only a single coil of larger curve placed
dorsally; the caecal pouch, derived from the ascending limb of the
umbilical loop, is situated between these two divisions, turning its
concave border to the right and embracing the parent tube. At the time
when the human caecum first appears as a distinct structure it forms a
small conical pouch with blunt extremity whose shape is well illustrated
by the caecum of some of the new-world monkeys, as _Mycetes fuscus_, the
brown howler monkey (Figs. 449 and 450). The outgrowth develops rapidly
in length and very soon assumes a distinct crescentic shape, gradually
tapering toward the extremity, a type which is found reproduced in the
caecum of _Ateles ater_, the black-handed spider monkey (Fig. 443). There
is as yet no constriction or demarcation separating the distal segment
(future appendix) from the proximal part (caecum proper) but the entire
pouch gradually narrows funnel-like to its termination.
[Illustration: FIGS. 488-496.--Series of schematic figures illustrating
stages in the rotation of the intestinal canal.]
II. CHANGES IN THE POSITION OF THE CAECUM AND APPENDIX DURING NORMAL
DEVELOPMENT, DEPENDING UPON THE ROTATION OF THE INTESTINE AND THE
SUBSEQUENT DESCENT OF THE CAECUM.
The primary cause leading to the rotation of the intestinal canal and
inaugurating the successive stages which produce the adult disposition
of the tube is to be found in the rapid increase in length of the small
intestine. Numerous convolutions of this tube succeed to the few primary
coils noted in the first stages. This condition is illustrated in Fig.
498, taken from an embryo of 4.4 cm. cervico-coccygeal measure, and the
arrangement of the intestine is indicated in schema, Fig. 491. The caecum
is found nearly in the median line imbedded among the surrounding coils
of the small intestine, which by their rapid increase have pushed the
pouch cephalad nearly into contact with the caudal surface of the liver.
[Illustration: FIG. 497.--Human embryo of 11 mm. cervico-coccygeal
measure. Enlarged view of ventral and left aspect of intestinal canal.
(Columbia University, Study Collection.)]
[Illustration: FIG. 498.--Human embryo of 4.4 cm. cervico-coccygeal
measure. Intestinal canal; Liver removed. (Columbia University, Study
Collection.)]
Three main divisions of the convolutions of the small intestine
can be made out, slightly separated from each other in the figure
to exhibit the caecum between them. The proximal (jejunal) set of
these convolutions occupy the upper and left part of the abdominal
cavity. They are the product of the single larger coil which in the
earlier stage (Fig. 497, schema Fig. 490) appeared dorsad of the
caecal diverticulum. The distal (ileal) division of small intestinal
convolutions has become greatly augmented and lies to the right of the
caecum. The concavity of the pouch is still, as in the earlier stages,
directed to the right and the entrance of ileum into colon takes
place from right to left. The caudal part of the abdominal cavity is
occupied by an intermediate set of transition convolutions which join
the proximal and distal divisions. In the two stages just described
(Figs. 497 and 498, Schema Figs. 490 and 491), the initial step in the
intestinal rotation has been taken, _i. e._, the beginning of the colon
has been displaced cephalad from its original position in the caudal
and left part of the abdominal cavity by the pressure of the rapidly
growing coils of the small intestine and now lies transversely ventrad
of the duodenum, having crossed the duodeno-colic neck or isthmus of
the primitive umbilical loop (cf. Fig. 487, _C_).
At first the distal coils of the small intestine occupy a position
_behind_ as well as to the right of the caecum, forming a dorsal
retro-caecal division connected by intermediate convolutions with the
ventral division occupying the lower and left portion of the abdominal
cavity. The apex of the caecum is frequently imbedded among these
terminal coils of the ileum. With the continued growth of the small
intestines a further displacement of the caecum cephalad and to the right
takes place, while at the same time the terminal ileal coils pass
downwards and to the left, from a retro-caecal into a subcaecal position,
thus permitting a direct apposition of the caecum to the dorsal parietal
(prerenal) peritoneum. The last steps in this process of withdrawal of
the original voluminous dorsal (retro-caecal) division of ileal
convolutions are well seen in the preparation shown in Fig. 499, taken
from an embryo of 6.7 cm. vertex-coccygeal measure, and corresponding to
the schematic stages represented in Figs. 490 and 491.
The caecum in this preparation has not yet completed its rotation and
still turns its concavity upwards and to the right, with the apex
imbedded among the terminal convolutions of the ileum.
The ileo-caecal junction takes place from right to left in a downward
direction. Nearly the entire mass of the small intestine is situated
below and to the left of caecum and colon, but a terminal ileal coil
still occupies, although evidently in the process of withdrawal, the
retro-caecal position, separating the caecum from direct contact with the
dorsal parietal peritoneum. The withdrawal of this terminal coil of the
small intestine is accompanied, or immediately followed, by a further
turn of the colon cephalad and to the right, which brings it into
contact with the caudal surface of the liver and completes the rotation,
producing a change in the relative positions of the terminal ileal coils
and the caecum. In the stages illustrated in Figs. 498 and 499 and shown
schematically in Figs. 490 and 491, the terminal coils of the ileum pass
from right to left behind the caecum to enter the colon, and the
concavity of the caecal pouch is directed upwards and to the right. After
the final rotation has occurred (schema, Fig. 492) the ileum enters the
large intestine from the left and from below, and the concave border of
the caecum is directed caudad and to the left. This change in relative
position has been accomplished by a revolution of the colon and caecum
through an arc of 180 deg. around its own long axis carrying the caecum above
and behind the small intestine and bringing it into contact with the
dorsal prerenal parietal peritoneum. At the same time the terminal coils
of the ileum turn downwards and to the left. If this final step in the
rotation of the large intestine fails to occur, with otherwise normal
development of the parts, the ileum will persist in entering the large
intestine from right to left after the caecum has obtained its final
lodgment in the right iliac fossa. We have had occasion to refer
previously to the significance of these instances of partially arrested
development (cf. p. 61, Figs. 123, 127 and 128).
[Illustration: FIG. 499.--Human embryo of 6.7 cm. vertex-coccygeal
measure. Liver removed. (Columbia University, Study Collection.)]
[Illustration: FIG. 500.--Human embryo of 4.9 cm. vertex-coccygeal
measure. Ventral view of abdominal cavity, with liver partially removed.
(Columbia University, Study Collection.)]
[Illustration: FIG. 501.--The same embryo represented in Fig. 500. The
colic coil further depressed and turned to the left; seen from the right
side.]
In Figs. 500 and 501, taken from an embryo of 4.9 cm. vertex-coccygeal
measure, the final rotation of the caecum from the position occupied in
Fig. 498 has occurred and the concavity of the pouch is directed caudad
and towards the left. At the same time the escape of the terminal ileal
coils from behind the caecum and beginning of the colon has not yet taken
place and hence the colon is still kept by these coils from direct
opposition to the dorsal prerenal parietal peritoneum. The condition
presented by this preparation can be schematically indicated by Figs.
492 and 493. The rotation has carried the beginning of the colon (Fig.
500), with the caecal bud and appendix curved on itself and turning its
concavity to the left, into the subhepatic position. The greater part of
the small intestinal coils lie now below and to the left of the caecum,
but the terminal ileal convolutions (Fig. 500) still occupy a
retro-caecal position, separating the pouch and the colon from the dorsal
parietal peritoneum. In Fig. 501 the right lateral view of the same
embryo is shown with the caecum and colon depressed and turned to the
left. The termination of the ileum reaches the ileo-colic junction by
passing behind the caecum, and the immediately adjacent ileal coils are
still retro-caecal, intervening between the pouch and the dorsal parietal
peritoneum.
In the next succeeding stage (schema, Fig. 494) these coils of the ileum
turn downward and to the left so as to lie below and mesad to the caecum
and colon, thus permitting the direct apposition of the large intestine
to the parietal prerenal peritoneum. The terminal ileum now passes from
below and to the left upwards and to the right to its junction with the
colon. This freeing of the dorsal surface of caecum and colon from
contact with the coils of the small intestines, and the consequent
direct apposition of the same to the dorsal parietal peritoneum
influences to a great extent the subsequent arrangement of the parts,
because it affords the conditions necessary to the fixation of the colon
and mesocolon by adhesion to the parietal peritoneum (cf. p. 81).
Fig. 499, taken from an embryo of 6.7 cm. vertex-coccygeal measure,
illustrates this stage, which is encountered in the majority of
instances and during which the retro-caecal coils of the terminal ileum
are withdrawn (schema, Fig. 493). The convolutions of the small
intestine have greatly increased in size and number. The retro-caecal
ileal coils, compared with Fig. 500, have shifted their position caudad
and to the left, so as to lie below and ventrad of the beginning of the
colon. Only a single coil remains behind the caecum and appendix,
intervening between these structures and the ventral surface of the
right kidney, and this coil is in the process of withdrawal from the
dorsal position as indicated by the superficial and short course of the
coil which connects it with the remaining ventral convolutions. As soon
as the withdrawal of this single remaining dorsal coil is completed the
entire mass of the small intestines will occupy a position ventrad,
caudad and to the left of the caecum and colon (Fig. 494), which will
then rest directly against the dorsal parietal peritoneum investing the
ventral surface of the right kidney.
This stage is illustrated in Fig. 502, taken from an embryo of 6.6 cm.
vertex-coccygeal measure. The caecum and appendix here occupy the
subhepatic position, well to the right of the median line and in the
background of the abdominal cavity. The terminal retro-caecal ileal coils
of the embryo shown in Figs. 500 and 501 have descended caudad and to
the left, thus freeing the dorsal surface of caecum and colon and
permitting direct contact with the prerenal parietal peritoneum.
[Illustration: FIG. 502.--Human embryo, 6.6 cm. vertex-coccygeal
measure. Liver removed. (Columbia University, Study Collection.)]
In the succeeding stages the caecum gradually descends along the
background of the right lumbar region from the subhepatic position to
the right iliac fossa, producing by this descent the ascending colon as
a distinct segment of the large intestine.
It will be observed that in the stage shown in Fig. 502 (schema, Fig.
494) the large intestine passes from the caecum to the splenic flexure
transversely from right to left across the upper part of the abdominal
cavity, caudad and ventrad of the stomach and cephalad of the coils of
the small intestine.
In the following stages the disproportionately large size of the
embryonic liver compels the colon, as the caecum descends, to assume an
oblique position. When the caecal descent is completed the colon
traverses the abdominal cavity in contact with the caudal surface of the
liver passing from the right iliac fossa obliquely cephalad and to the
left to the splenic flexure where it becomes continuous with the
descending colon, which segment has early assumed its definite position
in the background of the abdominal cavity on the left side (Fig. 495).
This oblique position of the colon is seen in Figs. 503 and 504. During
this stage the increase in the length of the colon may lead to the
arrangement seen in Fig. 505, where the future transverse segment of the
large intestine is bent caudad in form of an arch whose summit extends
nearly to the pelvis. This condition at times persists in the adult, in
cases of unusually long large intestine, and recalls the normal
arrangement found in many of the cynomorphous monkeys in whom the
transverse colon forms an extensive V-or U-shaped loop, with the apex
directed caudad toward the pubic symphysis (Fig. 506). In other
instances in the human foetus this part of the large intestine is thrown
into a number of shorter irregular coils (Fig. 507).
[Illustration: FIG. 503.--Human embryo, 7.6 cm. vertex-coccygeal
measure. Liver and small intestine from the duodeno-jejunal to the
ileo-colic junction removed. (Columbia University, Study Collection.)]
[Illustration: FIG. 504.--Human foetus, 10.6 cm. vertex-coccygeal
measure. Liver and greater part of small intestine removed. (Columbia
University, Study Collection.)]
[Illustration: FIG. 505.--Human foetus, 20.4 cm. vertex-coccygeal
measure. (Columbia University, Study Collection.)]
[Illustration: FIG. 506.--Abdominal viscera of _Macacus rhesus_, rhesus
monkey, hardened _in situ_. (Columbia University Museum, No 1817.)]
[Illustration: FIG. 507.--Abdominal viscera of human foetus at term,
hardened _in situ_; hepatic flexure formed, and ascending and transverse
colon differentiated. (Columbia University Museum, No. 1816.)]
[Illustration: FIG. 508.--Human foetus of 10.7 cm. vertex-coccygeal
measure. Liver and small intestine from the duodeno-jejunal to the
ileo-colic junction removed. The colon already presents an ascending,
transverse and descending segment. The appendix is retro-caecal, curved,
with the tip turned down, under cover of the ileo-colic junction and
mesentery. (Columbia University, Study Collection.)]
Normally, however, in the process of further development and with the
relative decrease in the size of the liver, the hepatic flexure (Fig.
505) becomes defined and passes cephalad and to the right, taking up the
slack of the bent segment and establishing the typical ascending and
transverse colon as seen in Fig. 508 (schema, Fig. 496).
III. VARIATIONS OF ADULT CAECUM AND APPENDIX.
The study of the variations of the adult caecum and appendix involves the
consideration of the following points:
(_a_) Shape of caecum and origin of appendix. (_Type of adult caecum._)
(_b_) Position, direction and peritoneal relations of the appendix.
(_c_) Arrangement of the vascular and serous ileo-caecal folds.
The peculiarities encountered in any individual case usually depend upon
the combination of all three of these factors, which together influence
and determine the arrangement of the structures in the adult. Hence the
examination of each case should be made with reference to these three
points, which we will now consider in detail.
A. SHAPE OF CAECUM AND ORIGIN OF APPENDIX. TYPES AND VARIATIONS OF ADULT
CAECUM AND APPENDIX.
The various forms of the adult caecum are all derived by modifications
from the foetal type of the pouch.
In the embryo the caecum is funnel-shaped, narrowing gradually and
symmetrically in caliber to the root of the appendix, at which point the
three colic taenia or longitudinal muscular bands of the large intestine
meet. The appendix arises from the apex of the funnel, the lateral walls
of which are equally and symmetrically developed. The entire pouch is of
a crescentic shape, the concavity of the curve turned to the left and
directed toward the caudal margin of the terminal ileum. Two
subdivisions of the foetal type are found:
I. The crescentic curve of the caecum is only slightly marked; the
appendix arises from the most pendent part of the pouch and hangs
downward (schema, Fig. 509, _I, a_).
This form, which is encountered only occasionally in the foetus and
infant, is illustrated by the preparation shown in Fig. 510, taken from
a foetus at term.
[Illustration: FIG. 509.--Schematic table of types of human caeca.]
II. In the majority of cases the inherent crescentic shape of the caecal
pouch is pronounced and carries the termination of the funnel with the
root of the appendix cephalad and to the left toward the caudal margin
of the ileo-colic junction (schema, Fig. 509, _II, a_).
At birth this typical arrangement of the caecum frequently places the
pouch in a nearly transverse position, with the apex and the root of the
appendix turned to the left, in contact with, or under cover of the
terminal piece of the ileum at its junction with the large intestine.
Figs. 511 and 512 represent the parts in the ventral view in the foetus
at term.
[Illustration: FIG. 510.--Human foetus at term. Caecum and ileo-colic
junction; ventral view. (Columbia University, Study Collection.)
1. Appendix.
2. Reduced intermediate non-vascular fold.
3. Ventral vascular fold.
]
[Illustration: FIG. 511.--Human foetus at term. Caecum and ileo-colic
junction; ventral view. (Columbia University, Study Collection.)
1. Appendix, coiled spirally behind terminal ileum.
2. Non-vascular intermediate fold.
]
[Illustration: FIG. 512.--Human foetus at term (negro). Caecum and
ileo-colic junction; ventral view. (Columbia University Museum, No.
692.)]
[Illustration: FIG. 513.--Human foetus at term. Caecum and ileo-colic
junction; dorsal view. (Columbia University, Study Collection.)]
[Illustration: FIG. 514.--Human foetus at term. Caecum and ileo-colic
junction; dorsal view. (Columbia University Museum, No. 1715.)]
Figs. 513 and 514, also taken from the foetus at term, show the caecum
from the dorsal aspect and illustrate well the sharp character of the
curve which carries the apex of the pouch up and to the left.
All the variations observed in the adult caecum are derived from these
two foetal types by a subsequent and usually asymmetrical enlargement and
dilatation of the pouch.
We can consider the derivatives of each form separately.
=I. Adult Caeca Derived From Type I.= (schema, Fig. 509, _I^a_, Fig.
510).
1. Further development leads to an enlargement of the caecal pouch and a
sharper demarcation between the same and the appendix. The resulting
caecum is symmetrical, with equally developed lateral sacculi, between
which the termination of the longitudinal muscular bands and the root of
the appendix is situated (schema, Fig. 509, _I^b_).
[Illustration: FIG. 515.--Human foetus at term. Caecum and ileo-colic
junction; ventral view. (Columbia University Museum, No. 1510.)]
[Illustration: FIG. 516.--Human foetus at term. Caecum and ileo-colic
junction; ventral view. (Columbia University Museum, No. 1548.)]
In Figs. 515 and 516 two infantile caeca are shown which illustrate this
form. The narrow and pointed apex of the foetal conical caecum is replaced
by the capacious pouch which is differentiated sharply from the
appendix. Among the anthropoid apes the same type is seen in the caecum
of the gibbon (Figs. 455 and 456), and of the young chimpanzee shown in
Fig. 460.
2. An increased development of the caecal pouch in the adult leads to the
protrusion caudad of two symmetrical sacculations on each side of the
root of the appendix which appears between them. The original apex of
the caecal pouch is still marked by the implantation of the appendix and
by the termination of the longitudinal muscular bands, but the lowest
level of the pouch is found on each side of this point at the fundus of
the secondary lateral sacculi (schema, Fig. 509, _I^c_). Treves, to whom
belongs the credit of first accurately describing and classifying the
forms of the adult caecum based on the development, found this type in
three of a series of 100 cases examined.
[Illustration: FIG. 517.--Adult human caecum and ileo-colic junction.
(Columbia University, Study Collection.)]
[Illustration: FIG. 518.--Adult human caecum and ileo-colic junction.
(Columbia University Museum, No. 234.)]
Figs. 517 and 518 illustrate this form of the pouch, which, in our
experience, is frequently associated with the retro-caecal erect
position of the appendix (cf. infra, p. 251). Fig. 472 shows this type
in the adult with pendent appendix.
=II. Adult Caeca Derived from Type II.= (schema, Figs. 509 and
511).--From this more commonly observed type of foetal caecum the
following adult forms are developed:
1. The general shape and trend of the foetal caecum is preserved. The
pouch turns sharply to the left, carrying the apex with the root of the
appendix upward toward the ileum, the appendix itself being frequently
placed under cover of the terminal coil of the small intestine (schema,
Fig. 509, _II^b_).
[Illustration: FIG. 519.--Human adult (Smith's Sound Eskimo). Ileo-colic
junction and caecum. (Columbia University Museum, No. 59/1483.)]
[Illustration: FIG. 520.--Human adult. Ileo-colic junction and caecum.
(Columbia University, Study Collection.)]
[Illustration: FIG. 521.--Human adult. Ileo-colic junction and caecum.
(Columbia University, Study Collection.)]
[Illustration: FIG. 522.--Human adult (Smith's Sound Eskimo). Ileo-colic
junction and caecum. (Columbia University Museum, No. 56/1571.)]
The apex of the caecal pouch is either conical, narrowing gradually
toward the root of the appendix (Figs. 520 and 521), or blunt and more
sharply defined against the appendix (Fig. 522). Mr. Treves encountered
this "persistent foetal type" in two per cent. of his series.
The caecum is frequently sharply bent on itself in making the turn upward
and to the left, resulting in a deep indentation of the concave border
and producing a corresponding projecting fold in the interior of the
pouch (Fig. 523). The ventral longitudinal muscular band follows the
crescentic sweep of the caecum to the root of the appendix.
[Illustration: FIG. 523.--Human adult. Ileo-colic junction and caecum;
dorsal view; dried preparation. (Columbia University Museum, No. 200.)]
[Illustration: FIG. 524.--Human foetus at term. Ileo-colic junction and
caecum; ventral view. (Columbia University Museum, No. 1717.)]
[Illustration: FIG. 525.--Same preparation as Fig. 524; dorsal view.]
Figs. 524_a_ and 525_b_, representing the caecum of a foetus at term in
the ventral and dorsal view respectively, show very clearly the
arrangement of the foetal pouch from which the adult type with sharp
angular bend is derived. This type of adult caecum is found in certain of
the anthropoid apes.
In the orang (Figs. 458 and 459) the caecum turns sharply upward and to
the left, gradually narrowing in caliber to the root of the appendix
which is coiled behind the termination of the ileum.
The same type is seen in Figs. 462 and 463, taken from a preparation of
the adult chimpanzee. Fig. 463 shows especially well the sharp bend
between the caecum and colon by means of which the apex of the pouch is
carried cephalad behind the ileo-colic junction.
Fig. 431, taken from another specimen of the same animal, shows the
characteristic crescentic curve of the caecum and the corresponding
course of the longitudinal muscular band. The apex of the pouch in this
preparation is more rounded and blunt.
The same blunt termination of the caecum of this type, with a
corresponding sharper demarcation of the appendix, is seen in the
gorilla (Fig. 457) recalling the conditions found in certain instances
in the human subject (Fig. 522).
2. In by far the larger proportion of cases (ninety per cent. in Treves'
series) the adult caecum obtains its characteristic form by an unequal
development of the walls of the intestine. The right segment between the
ventral and dorso-lateral muscular bands dilates, forming a sacculation
which projects caudad and constitutes the secondary caput coli, while
the segment between the lower border of the ileum and the original apex,
marked by the origin of the appendix, remains stationary or is further
reduced. This unequal development produces a relative displacement of
the root of the appendix upward and to the left toward the ileo-colic
junction.
In some cases the primitive crescentic curve of the caecum, as indicated
by the direction of the ventral longitudinal muscular band, is still
perceptible.
The right wall of the foetal caecum, forming the most pendent portion of
the pouch, dilates uniformly and thus constitutes the adult caput coli.
The left wall appears as a small sacculation separating the root of the
appendix from the ileo-colic junction (schema, Fig. 509, _II, c_). This
type of the adult caecum is illustrated by the preparations shown in
Figs. 526-528. In other cases part of the right wall of the caecum
between the ventral and dorso-lateral colic taenia, dilates abruptly
forming a very prominent rounded sacculation which carries the lowest
part of the pouch caudad in a sharper curve than in the preceding form
as indicated by its deviation from the direction of the longitudinal
muscular band (schema, Fig. 509, _II, d_).
[Illustration: FIG. 526.--Human adult (Smith's Sound Eskimo). Ileo-colic
junction and caecum; dorsal view. (Columbia University Museum, No.
61/1461.)]
[Illustration: FIG. 527.--Human adult. Ileo-colic junction and caecum;
ventral view. (Columbia University, Study Collection.)]
[Illustration: FIG. 528.--Human adult. Ileo-colic junction and caecum;
ventral view. (Columbia University, Study Collection.)]
Figs. 529-531 afford examples of this type, while Fig. 532, taken from
an infantile preparation, shows that the same may begin to develop at a
very early age.
[Illustration: FIG. 529.--Human juvenile. Ileo-colic junction and caecum;
dorsal view. (Columbia University, Study Collection.)]
[Illustration: FIG. 530.--Human adult. Ileo-colic junction and caecum;
dorsal view. (Columbia University Museum, No. 115.)]
[Illustration: FIG. 531.--Human adult. Ileo-colic junction and caecum;
dorsal view. (Columbia University, Study Collection.)]
3. Finally, in about four per cent. to five per cent., adult caeca, the
reduction of the wall to the left of the root of the appendix, between
this point and the ileo-colic junction, is complete. The entire caecal
pouch is formed by the dilated right wall between the ventral and
dorsolateral muscular bands. The ventral band terminates at the lower
border of the ileo-colic junction, from which the appendix appears to
arise, indicating the original apex of the foetal caecum (schema, Fig.
509, _II^e_).
This type is illustrated in the specimens shown in Figs. 533 and 534.
[Illustration: FIG. 532.--Human infant, Ileo-colic junction and caecum,
with secondary terminal sacculation. (Columbia University Museum, No.
1632.)]
[Illustration: FIG. 533.--Human adult. Ileo-colic junction and caecum;
dorsal view; dried preparation. (Columbia University Museum, No. 124.)]
[Illustration: FIG. 534.--Human adult. Ileo-colic junction and caecum;
ventral view; dried preparation. (Columbia University Museum, No. 14.)]
=III. Adult Caeca in Cases of Absence of the Appendix.=--A few instances
of congenital absence of the appendix have been observed.
A. v. Haller[9] describes the condition in the following words:
"Defuisse visa est in homine appendicula, ut tuberculum minimum
superesset."
[9] A. v. Haller, Elements physiologiae, Tom. 7, Liber 24, Sect. 3.
Fr. Arnold,[10] without describing any individual case, states that
"very rarely the appendix is entirely wanting."
[10] Fr. Arnold, Handbuch der Anat. d. Menschen. 1847. II. Bd., cloth,
p. 84.
E. Zuckerkandl,[11] reports having observed one case of absence of the
appendix.
[11] E. Zuckerkandl, "Ueber die Obliteration des Darmfortsatzes beim
Menschen." Anat. Hefte XI. (Bd. IV., Heft 1), 1894, p. 107.
J. D. Bryant,[12] reports a case in which he operated for appendicitis
but found "absolutely no appendix." "The point of tenderness was found
to be a glandular growth located posterior to the usual site of the
appendix."
[12] N. Y. Med. Journal, Vol. LXIX., No. 14, p. 508.
[Illustration: FIG. 535.--Human adult. Ileo-colic junction and caecum;
absence of appendix. (Columbia University Museum, No. 1077.)]
[Illustration: FIG. 536.--Human adult. Ileo-colic junction and caecum,
hardened _in situ_; absence of appendix. (Columbia University Museum,
No. 715.)]
Two instances of this variation are shown in Figs. 535 and 536, taken
from preparations in the Morphological Museum of Columbia University. In
both careful examination of the external as well as of the mucous
surface of the caecum demonstrated the entire absence of the appendix,
and the subjects from which they were obtained presented no scars or
other evidences of operative removal or of pathological processes. They
are both, therefore, authentic instances of complete congenital absence
of the appendix, not of so-called retro-peritoneal or hidden
appendix.[13]
[13] Cf. Quain.
The two examples differ from each other in some details. In the first
case (Fig. 535, schema, Fig. 509, _III^a_) the caecum is rounded and
globular. The ventral longitudinal muscular band is vertical and
continued to the lowest point of the pouch, which greatly resembles the
caecum of a typical cynomorphous monkey.
In the second case (Fig. 536, schema, Fig. 509, _III^b_) the caecum turns
upwards and to the left, terminating in a sharp point, to which several
lobes of epiploic fat are attached.
We must assume that in these cases the embryonic portion of the caecal
bud was developed just sufficiently to yield the required adult pouch
with nothing to spare, so to speak, which could remain rudimentary in
the form of an appendix.
Instances of exceedingly rudimentary and reduced appendix are also
encountered.
In the case illustrated in Fig. 537 the appendix formed a small conical
elevation without distinct lumen, measuring only 0.5 cm. in length.
[Illustration: FIG. 537.--Human adult. Ileo-colic junction and caecum,
with rudimentary appendix. (Columbia University Museum, No. 1655.)]
=B. Position and Peritoneal Relations of the Appendix.=--Statistical
records of the position of the appendix indicate a wide range of
variation. In general the results obtained by different observers show
that certain positions of the appendix are encountered in a sufficiently
large percentage of the cases to enable us to adopt a classification,
but that a very extensive series of records are required in order to
determine even approximately the preponderant relations of the appendix.
The following are the most frequently observed positions:
1. The appendix is directed upward, inward and to the left, the terminal
portion being frequently coiled under cover of the ileum and mesentery.
This position of the appendix is largely due to the normal crescentic
curve of the caecum, which carries the apex of the pouch and the root of
the appendix upward and to the left. Its production is, moreover,
favored by the tendency of the adult caecum to develop by dilatation of
the ventral and right wall at the expense of the left side of the pouch,
thus relatively shortening the interval between the origin of the
appendix and the ileo-colic junction.
Examples of this commonly encountered position of the appendix are given
in Figs. 512, 513, 514, 520, 521, 523 and 526.
2. The appendix is erected vertically behind the caecum and ascending
colon and closely attached to the dorsal wall of the large intestine. In
some instances the caecum and colon, with the adherent vertical appendix,
possess a free serous dorsal surface, not adherent to the parietal
peritoneum (Figs. 529, 538, 539 and 540). In other cases the ascending
colon is fixed and the greater part of the retro-colic appendix is
buried in the connective tissue which attaches the large intestine to
the abdominal parietes (Fig. 517). Even in these cases, however, the
dorsal surface of the caecum and the root of the appendix retain their
free serous investment.
[Illustration: FIG. 538.--Human juvenile. Caecum _in situ_ lifted up to
show vertical course of appendix, situated behind caecum and ascending
colon. The large intestine has a free peritoneal dorsal surface, and the
appendix is held in position by adhesion to the large intestine.
(Columbia University, Study Collection.)]
[Illustration: FIG. 539.--Human adult. Ileo-colic junction and caecum;
dorsal view. (Columbia University Museum, No. 1594.)]
[Illustration: FIG. 540.--Human adult. Ileo-colic junction and caecum;
dorsal view. (Columbia University Museum, No. 1850.)]
3. The proximal part of the appendix turns upward and to the left in
continuation of the caecal curve, but the distal portion is directed
downward and inward, hanging over the brim of the pelvis (Figs. 505, 541
and 542).
4. The appendix is directed downward, pendent from the lowest point of
the conical caecal pouch, and hangs free over the pelvic brim.
This type is encountered at times in foetal and infantile subjects (Figs.
516 and 543).
[Illustration: FIG. 541.--Human infant. Ileo-colic junction and caecum;
ventral view. (Columbia University, Study Collection.)]
[Illustration: FIG. 542.--Human infant. Ileo-colic junction and caecum;
dorsal view. The dorsal surface of caecum as far as root of the appendix
is adherent to the parietal peritoneum of the iliac fossa. (Columbia
University Museum, No. 394.)]
[Illustration: FIG. 543.--Human foetus at fifth month. Abdominal cavity
and viscera; liver and greater part of small intestine removed. (Drawn
from preparation in Columbia University Museum, No. 1814.)]
[Illustration: FIG. 544.--Human foetus at term. Abdominal cavity and
viscera; greater part of small intestine removed. (Drawn from
preparation in Columbia University Museum, No. 1813.)]
[Illustration: FIG. 545.--Human foetus at term. Ileo-colic junction and
caecum. (Columbia University Museum, No. 998.)]
5. The position of the appendix is variant and abnormal, as _e. g._
placed to the right of caecum and colon (Fig. 544) or turned up ventrad
of the ileo-colic junction (Fig. 545).
These variations in the position of the appendix and the resulting
peritoneal relations of the structure depend upon the following
factors.
1. The influence of peritoneal adhesions established during the descent
of the caecum from the subhepatic position to the iliac fossa.
2. The inherent curve of the caecal pouch.
3. The subsequent alterations in the caliber of the intestine and the
unequal development of the pouch leading to the formation of the types
of adult caeca above considered.
In determining the causes which lead to the establishment of any given
position of the appendix all three of the factors above enumerated must
be taken into account, although their influence is not exerted in every
case to an equal degree.
We have seen that normally, after completed rotation of the intestine,
the caecum with the appendix and the beginning of the colon are lodged in
the upper and right part of the abdomen, below the liver and in contact
with the prerenal parietal peritoneum (schema, Figs. 493, 502). During
the subsequent stages the caecum descends into the right iliac fossa,
thus producing the ascending colon. It is immaterial whether this change
in position is regarded as an actual descent of the pouch over the
ventral surface of the right kidney, which seems more probable, or as a
growing away from the iliac region of the remainder of the abdominal
wall, with a concomitant relative reduction in the size of the liver,
producing a relatively lower position of the caecum, or as a combination
of these processes. In either case during this period the dorsal surface
of the ascending colon and mesocolon normally becomes adherent to the
dorsal parietal peritoneum, connective tissue developing between the
opposed serous areas and leading to the usual fixation of the ascending
colon and obliteration of the free ascending mesocolon. If this process
of adhesion is inaugurated at an early stage, _i. e._, before the
descent of the caecum has been accomplished, it will act as a drag on the
dorsal surface of the colic tube during the subsequent change in
position, which carries the caecum downward into the iliac fossa. This
leads to a backward bend of the caecum and appendix which parts will in
the ventral view appear under cover of the protruding free ventral and
lateral walls of the colon. Hence in many late embryos and foetus at term
the lowest point of the large intestine in the right iliac fossa is
formed by the proximal part of the caecum or by the adjacent segment of
the colon, while the original termination of the pouch, with the root of
the appendix, is turned backward and upward, and, as we have seen, by
reason of the inherent shape of the pouch, also to the left, carrying
the beginning of the appendix frequently behind the terminal ileum and
the ileo-colic junction.
Two of the more common positions of the appendix, viz., backwards,
upwards and inwards behind the ileo-colic junction, and directly
backward, erected vertically behind caecum and colon, can therefore in
part be referred to the mechanical conditions obtaining normally during
the descent of the caecum. Of course the shape of the caecal pouch and the
later development of the adult type of caecum will modify this influence
in individual cases. We have seen that this early adhesion and the
resulting effects on the position of caecum and appendix depend on the
direct apposition of the colic tube and mesocolon to the dorsal parietal
peritoneum. Any condition which will prevent or delay this apposition
will likewise perpetuate the original embryonal condition of the tube,
completely invested by peritoneum and with a free mesocolon.
Such an element is found in the persistence of the dorsal set of ileal
convolutions in the original retro-caecal position beyond the usual
period, as indicated in the schematic Fig. 492, _IV, a_. If the turn
downward and to the left of these coils is for any reason delayed beyond
the usual time the caecal extremity of the colon will descend from the
subhepatic to the iliac position without coming directly into contact
with the dorsal parietal peritoneum, and therefore without the usual
peritoneal adhesion and obliteration of the apposed serous surfaces. The
caecum under these conditions descends without making the backward bend,
and the origin of the appendix is found at the lowest point of the
pendent funnel-shaped pouch, causing it finally to hang downward or
downward and inward over the pelvic brim. The resulting form of the
caecum and the position of the appendix is the one above described as
type _Ia_, _Ib_ and _Ic_ (Fig. 509).
Fig. 510 from a foetus at term, and Figs. 515 and 516 representing
infantile caeca, illustrate this form of the pouch, while the parts are
shown in situ in Fig. 543 taken from a preparation of a five-month
foetus.
[Illustration: FIG. 546.--Human embryo, 6.5 cm. cervico-coccygeal
measure. Abdominal cavity, with liver removed, seen from the right side.
(Columbia University, Study Collection.)]
Fig. 546 exhibits the condition obtaining during the development of this
type in the more exceptional instances of delayed apposition of the
colon to the parietal peritoneum and of increased development of the
terminal ileal coils in the original retro-caecal position. In this
embryo, measuring 6.5 cm. in vertex-coccygeal length, the development
has progressed sufficiently to establish a distinct transverse colon and
to bring the caecum and appendix into the subhepatic position. But in
place of lying in contact with the dorsal parietal peritoneum, as in the
embryo, shown in Fig. 502, over the ventral surface of the right kidney,
the increased mass of the retro-caecal ileal coils keeps the caecum,
already in the process of descent, in contact with the ventral abdominal
wall. When the final rotation of the retro-caecal small intestinal coils
downward and to the left occurs, placing the ileo-colic junction (_C_)
to the left of the large intestine (schema. Fig. 494), the ascending
colon and caecum are not yet fixed by adhesion to the dorsal parietal
peritoneum, and the appendix will present downward and to the left,
affording the necessary conditions for the establishment of the
permanent pendent position of the tube or causing the same to be
directed downward and inward over the brim of the pelvis.
[Illustration: FIG. 547.--Human embryo, 5.9 cm. vertex-coccygeal
measure. (Columbia University, Study Collection.)]
In contrast with the preceding is the condition shown in Fig. 547, taken
from an embryo of 5.9 cm. vertex-coccygeal measure. The transverse colon
in this preparation has already begun to assume an oblique position,
passing down and to the right from the splenic flexure. The caecum and
appendix are in contact with the dorsal prerenal parietal peritoneum.
The escape of the dorsal set of ileal convolutions from the retro-caecal
position, by rotation downwards and to the left, is accomplished. The
caecum and appendix are placed in the position which they would have
occupied in the embryo shown in Fig. 546 if the dorsal ileal coils had
not prevented, in the latter preparation, the apposition of the colon to
the dorsal parietal peritoneum.
In considering the effect of these variant conditions on the adult
arrangement of the structures it is necessary to bear in mind the second
of the above-mentioned factors, namely, the inherent shape of the caecal
pouch and appendix and the resulting direction of its axis.
As previously stated the normal type of the human embryonal caecum is
represented by the pouch of some of the new-world monkeys, as _Ateles_
(Fig. 443) or of certain lemurs, of which _Nycticebus_ (Fig. 420)
furnishes an excellent example. The caecum is distinctly crescentic,
turning its concave margin, after completed intestinal rotation, upwards
and to the left, toward the lower margin of the ileum. The distal
diminished segment of the pouch in _Ateles_ has already assumed the
character of a caecal appendage in _Nycticebus_ and becomes by further
reduction the typical appendix in man and the anthropoid apes, while the
proximal portion develops into the capacious sacculated caecum proper.
Consequently the initial curve of the caecum tends to carry the root of
the appendix upward and to the left toward the ileo-colic junction. This
curve of the pouch, combined with the mechanical effects produced by the
adhesion of the colon during the caecal descent, accounts for the
frequency with which the caecum in the later months of foetal life and at
birth is found curved backward, upward and to the left, placing the root
of the appendix under cover of the terminal ileal convolutions (Fig.
548). We have seen that this disposition of the structures accounts for
the preponderance of that type of adult caecum which results from the
further and unequal development and dilatation of the segment of the
pouch situated to the right of the origin of the appendix.
[Illustration: FIG. 548.--Human foetus at term. Ileo-colic junction and
caecum. Early colic and caecal adhesion with retroverted appendix.
(Columbia University Museum, Study Collection.)]
Bearing in mind the three elements just considered, viz., the effect of
adhesion during the caecal descent, the inherent shape of the pouch and
the unequal alterations in caliber in the development of the adult type,
we can at once take up the resulting variations in the peritoneal
relations of the adult caecum and appendix which have an important
influence on the progress of pathological processes in this region. It
should be remembered that in the following schematic figures the colon,
caecum and appendix are represented in the profile view in a straight
line, without indicating the characteristic turn of the crescentic caecal
pouch upwards and to the left.
Fig. 549 shows the arrangement in unimpeded caecal descent without
adhesion of colon and mesocolon to the parietal peritoneum. This
disposition of the structures, if carried into adult life, would produce
the permanently free ascending colon and mesocolon which we encountered
exceptionally in the human subject (cf. p. 82) and normally in certain
of the cynomorphous monkeys (p. 83). The ascending colon and mesocolon
can, under these conditions, be turned mesad, lifting them away from the
primary parietal peritoneum investing the ventral surface of the right
kidney. Caecum and appendix have, of course, a complete serous
investment.
[Illustration: FIG. 549.]
[Illustration: FIG. 550.]
[Illustration: FIG. 551.]
[Illustration: FIG. 552.]
[Illustration: FIG. 553.]
[Illustration: FIG. 554.]
[Illustration: FIGS. 549-554.--Schematic series illustrating the
variations in the arrangement of the caecal and colic peritoneum.]
Normally, however, in the human subject, even if the obliteration of the
apposed serous surfaces and the resulting fixation of the ascending
colon has been delayed beyond the usual period, as above indicated,
adhesion takes place subsequently, involving the dorsal surface of the
ascending colon between the ileo-colic junction and the hepatic flexure
(schema, Fig. 550). The dorsal surface of the caecum usually retains its
free serous surface in whole or in greater part. The appendix is
pendent, entirely invested by peritoneum and hangs free in the abdominal
cavity, directed toward the pelvic brim, illustrating the effect of
delayed fixation of the colon on the position of the appendix.
Examples of this condition are not frequent, and are confined almost
exclusively to foetal and juvenile subjects. Illustrations are afforded
by Figs. 515 and 516.
We have already noted (p. 246) the resulting foetal type of pendent caecum
(Fig. 510).
More commonly colic adhesion before the caecum obtains its final iliac
position results in imparting a backward turn to the pouch, leading to
the peritoneal disposition shown in schema, Fig. 551, in which the root
of the appendix is involved in the area of obliteration, while the
terminal segment remains free. An example of this condition is furnished
by the embryo shown in Fig. 508 (10.7 cm. vertex-coccygeal measure). The
colon is already segmented into an ascending, transverse and descending
portion. The caecum is retroverted and its apex with the appendix is
placed under cover of the terminal ileum which enters the large
intestine in the direction from below upward and to the right. In the
side-figure the divided end of the ileum is displaced upward to show
caecum and appendix and their relation to the ileal mesentery.
The disposition of the structures illustrated by this example probably
depends upon delayed adhesion of the colic embryonal tube to the dorsal
parietal peritoneum. The caecum and appendix appear to have descended
freely until the final position in the right iliac fossa has been nearly
attained, adhesion and fixation of the colon taking place just before
the descent is completed, and thus producing the backward turn of the
caecal end of the tube. Further development of the caecum to form the
adult caput coli in these cases leads to the unequal and exaggerated
expansion of the ventral and lateral walls of the pouch, as compared
with the fixed and adherent dorsal wall. The former are distended and
pushed downwards, producing a relative recession of the root of the
appendix upward and to the left, until it comes into relation with, or
even under cover of, the ileo-colic junction and of the terminal ileal
coil entering the colon at this point.
The resulting characteristic adult position of the appendix in these
cases is as follows:
The termination of the caecum proper, and the root of the appendix are
under cover of the terminal ileum and frequently adherent to the
parietal peritoneum of the iliac fossa (Fig. 555). The distal portion of
the appendix remains free, either hanging down and in over the brim of
the pelvis (Fig. 542), or turned upwards and to the left and coiled in
several turns (Figs. 504, 555 and 556).
[Illustration: FIG. 555.--Human foetus at term. Ileo-colic junction and
caecum; dorsal view. The area of peritoneal adhesion is seen to involve
the dorsal aspect of the caecum as far as the root of the appendix.
(Columbia University Museum, No. 1549.)]
[Illustration: FIG. 556.--Human infant. Ileo-colic junction and caecum,
with extensive adhesion to parietal peritoneum. (Columbia University
Museum, No. 301.)]
Finally the _erect vertical retro-caecal_ position of the appendix
presents several important variations in the disposition of the
peritoneal investment. In Fig. 503, taken from an embryo of 7.6 cm.
vertex-coccygeal length, the early complete recession of the retro-caecal
ileal convolutions has probably permitted an early apposition and
adhesion of the beginning of the colon to the dorsal prerenal parietal
peritoneum. The subsequent descent into the iliac fossa produces a bend
in the ventral wall of the colic tube, with a marked convexity directed
downwards and forwards, the apex of the bend situated at or near the
level of the ileo-colic junction, while the dorsal colic wall is held by
the adhesion to the parietal peritoneum, thus giving a backward
inclination to the entire caecum and appendix. During the subsequent
descent of the caecum proper this bend in the colon is gradually
diminished and the tube becomes straightened but the apex of the caecum
remains turned back and the appendix is placed in a more or less
vertical erect position behind caecum and ascending colon.
As regards the disposition of the peritoneal membrane in this type of
appendix the following conditions are to be noted:
(_a_) (Schema, Fig. 552.)--The apex of the caecum and the entire appendix
are extraperitoneal, imbedded in the loose connective tissue which
occupies the area of serous obliteration. The line of peritoneal
reflection from the dorsal wall of the secondary caput coli to the
parietal peritoneum of the right iliac fossa is placed transversely
below the true apex of the foetal caecum and the root of the appendix. The
latter tube, imbedded in connective tissue, passes vertically upwards
behind the ascending colon, its tip frequently reaching the ventral
surface of the right kidney. A well-marked example of this arrangement
in the adult is shown in Figs. 557 and 558 (ventral and dorsal view,
with peritoneal reflection and vertical retro-colic appendix).
[Illustration: FIG. 557.--Human adult. Ileo-colic junction and caecum;
ventral view. (Columbia University Museum, No. 1612.)]
[Illustration: FIG. 558.--Same preparation as Fig. 557; dorsal view. The
appendix, erected vertically between caecum and colon, is completely
imbedded in connective tissue.]
(_b_) (Schema, Fig. 553.)--In other cases, with the same position of the
appendix, the entire caecum and greater part of the ascending colon
remains free. The vertically erected appendix is closely attached to the
dorsal surface of the ascending colon, included within the serous
investment of the large intestine. The adhesion of the latter is
confined to a limited area near the hepatic flexure. Consequently caecum
and greater part of ascending colon can be turned up, away from the
parietal peritoneum of the iliac fossa, and the dorsal surface of the
appendix shows the free serous covering of the adjacent large intestine.
We may assume that this type of the peritoneal relations of the appendix
is produced in one of two ways:
1. Either the retro-colic appendix has become early attached to the
adjacent large intestine, whose dorsal surface in large part remains
free, or
2. The arrangement of the peritoneum indicated in schema, Fig. 552, may
be subsequently changed into that shown in schema, Fig. 553, by a
continued downward displacement of the caecum, producing a secondary
serous investment of the dorsal surface of appendix and part of
ascending colon.
Examples of this type are found both in infantile and adult subjects.
In Fig. 538, taken from an infant three years of age, the caecum is
lifted up to show the vertical position of the appendix behind the caecum
and ascending colon, the dorsal surface of the large intestine retaining
its free serous covering. Another illustration of this arrangement in a
juvenile subject is shown in Fig. 529. The same condition in the adult
subject is illustrated in Figs. 539 and 540.
(_c_) (Schema, Fig. 554.)--Occasionally, with the appendix erected
vertically behind the ascending colon, the apex of the caecum and the
proximal portion of the appendix are invested by peritoneum for a short
distance and the tip of the appendix likewise obtains a free serous
investment, while the intermediate greater portion of the appendix and
the corresponding segment of the dorsal surface of the ascending colon
are extraperitoneal, adherent to the abdominal parietes. Examples of
this peritoneal relation of the appendix in an infant are shown in Figs.
559 and 560, while Fig. 509 represents the same arrangement in an adult
specimen. The condition is produced from the arrangement of schema, Fig.
554, by secondary adhesion and obliteration of the serous surfaces over
the intermediate portion of the retroverted appendix and the adjacent
dorsal surface of the ascending colon.
[Illustration: FIG. 559.--Human infant. Ileo-colic junction and caecum;
dorsal view. Retro-colic appendix, adherent to the free dorsal serous
surface of the large intestine, with intermediate extraperitoneal
segment. (Columbia University Museum, No. 1638.)]
[Illustration: FIG. 560.--Human adult. Ileo-colic junction and caecum;
dorsal view. Appendix with intermediate non-peritoneal segment, while
the proximal portion and the tip are covered by serous investment.
(Columbia University Museum, No. 1615.)]
C. ILEO-CAECAL FOLDS AND FOSSAE.
Certain peritoneal folds, either mesenteric in character, _i. e._,
containing blood vessels, or non-vascular, pass between the terminal
ileum and the caecum and appendix, modifying in some instances very
markedly the position and peritoneal relations of the structures.
In considering the influence which these vascular mesenteric and
non-vascular serous folds exert in producing further changes in the
shape, position and relations of the human appendix it is necessary to
remember that in the early embryonal stages these bands and folds of the
peritoneum appear only slightly marked, but that they gain their
importance and influence on the final adult configuration of the caecal
pouch and appendix in the course of the further development of these
structures.
For this reason the comparative study of the corresponding parts in
other vertebrates, especially in certain mammalia, is of the utmost
value, if we seek to explain and understand the derivation, significance
and typical arrangement of these folds. We have seen that the caecum as
found in the large majority of mammalian forms is equivalent to the
caecum and appendix of the human subject and anthropoid apes; that in
other words the vermiform appendix represents the distal segment of a
caecal pouch, originally uniform in caliber, which has remained
undeveloped, while the proximal portion has progressed evenly with the
general development of the alimentary canal to form the caecum proper. We
have seen that this tendency to retain the distal portion of the pouch
in a rudimentary condition, _i. e._, the production of an appendage to
the caecum proper, is encountered in several of the lower forms, as
certain Marsupials, Carnivores, Ungulates and Lemurs. The morphology of
the ileo-caecal folds is hence best understood by considering these
structures as they appear in connection with the various caecal types
presented by the lower mammalia. Their arrangement and significance can
here be readily made out. On the other hand, in studying these
structures in the human appendix we are following lines which are
already becoming indistinct on account of the rudimentary character of
the organ, which we must regard as undergoing an exceedingly slow
process of reduction, with a view to its ultimate elimination from the
body. We have seen that the structural uncertainty impressed on caecum
and appendix by this evolutionary influence finds its expression in the
wide range of variation in size and arrangement which these parts
present. Necessarily, of course, this tendency to variation is shared,
and even exhibited to a more marked degree, by what we can term the
accessory structures connected with caecum and appendix, viz., the
mesenteric vascular and non-vascular serous folds passing to them from
the ileum.
We can most profitably begin our consideration of these folds in a form
in which they are preserved in their entire and original development,
and then successively trace the changes leading up to the normal
disposition in the human subject. Such a type is presented by the caecum
of _Ateles ater_, the black-handed spider monkey (Figs. 444 and 445).
The caecum of this animal presents a uniform crescentic curve, with the
concavity directed upward and to the left, and the gradual diminution in
the caliber of the pouch, from the ileo-colic junction to the apex,
denotes the tendency to retain the distal segment in a rudimentary
condition, foreshadowing the eventual formation of a vermiform appendix.
In the ventral view, with the terminal ileum lifted up, the following
arrangement of folds passing between ileum and caecum is noted (Figs. 444
and 445).
(_a_) _Vascular Mesenteric Folds._--The peritoneal vascular folds,
carrying the blood vessels to supply the caecum, are two in number, a
ventral (1) and dorsal (3). They are of nearly equal size and extent,
passing from the ventral and dorsal aspect of the ileo-colic junction
nearly to the apex of the caecum. Each contains a branch of the
ileo-colic artery, which forks in the ileo-colic mesentery, in the angle
between ileum and large intestine. The ventral branch continues in the
ventral mesenteric fold (Fig. 445) downward across the ventral surface
of the ileo-colic junction to supply the ventral part of the caecum,
while the dorsal branch descends behind the ileo-colic junction,
preserving a similar course in the dorsal mesenteric fold. The dorsal
arterial branch is somewhat larger than the ventral and its distribution
extends a little further down to the actual apex of the caecum.
(_b_) _Non-vascular Ileo-caecal Serous Reduplication._--Between the two
vascular mesenteric folds a third serous reduplication, carrying no
blood vessels, is found passing between the ileum and caecum. This fold
begins, in the preparation from which the figure is taken, on the ileum
opposite the attached mesenteric border, 2.7 cm. from the ileo-colic
junction, and passes for exactly the same distance down on the adjacent
left concave surface of the caecum. It is placed a little nearer to the
dorsal than to the ventral vascular fold, so that it passes, if the
distance between the two vascular folds on the caecum be divided into
three parts, at the junction of the dorsal third with the ventral two
thirds. The production of this intermediate non-vascular ileo-caecal
reduplication, which is of very constant occurrence in the mammalian
series, is to be led back to the development of the caecum. When the
pouch protrudes from the smooth surface of the embryonic intestine
opposite the mesenteric border, it extends backward along the future
small intestine and lifts off the serous investment of the gut in the
form of a small peritoneal plate filling the interval between itself and
the adjacent ileum. A very perfect illustration of this process can be
seen in the instance of Meckel's diverticulum shown in Fig. 561. The
proximal portion of the diverticulum is here still closely connected to
the small intestine along which it extends, both being surrounded by the
common visceral peritoneum. The distal part of the diverticulum has
separated more completely from the intestine, and in so doing has drawn
out the serous investment in the form of the triangular fold which is
seen to pass between the free margin of the intestine and the adjacent
surface of the pouch. The same process can be followed in its different
stages in certain normal mammalian caecal types.
[Illustration: FIG. 561.--Human adult ileum with Meckel's diverticulum.
Ileo-diverticular serous fold and persistent omphalo-mesenteric artery.
(Columbia University Museum, No. 1803.)]
[Illustration: FIG. 562.--Human adult. Ileum with Meckel's diverticulum,
131.5 cm. from ileo-colic junction; a distinct vascular fold is
prolonged from the ileal mesentery to the margin of the diverticulum.
(Columbia University Museum, No. 1849.)]
In this connection it may be noted that the production of the caecal
vascular folds and their relation to the mesentery is also very
perfectly illustrated in some forms of Meckel's diverticulum. Thus in
the preparation shown in Fig. 562, a broad triangular serous fold passes
from the ileal mesentery to the margin of the diverticulum, carrying the
blood vessels which supply the pouch. If the section of the intestine to
the left of the figure is regarded as representing the terminal ileum,
that to the right the colon, and the diverticulum the caecal pouch, the
formation of the fold and its relation to the mesentery, blood vessels
and intestine will correspond closely to the ileo-caecal vascular folds.
Fig. 350 shows the ileo-colic junction and caecum of _Halmaturus
derbyanus_, the rock kangaroo. The caecum here extends backwards along
the free border of the ileum to which it is closely bound by the common
investing visceral peritoneum for the greater part of its extent. In
another marsupial form, a small species of opossum from Trinidad (Fig.
349), the caecum has separated itself more completely from the adjacent
small intestine--thus drawing out the peritoneum into a narrow
connecting fold. Finally, in the Virginia opossum (Fig. 348), the ileum
has attained the usual position at right angles to caecum and colon. The
former pouch is separated from the small intestine by a considerable
interval and the angle between the two is filled out by a well-developed
triangular serous fold, connecting the free margin of the terminal ileum
and the adjacent left border of the caecum.
This is the "intermediate non-vascular" ileo-caecal fold.
Passing now from the condition presented by _Ateles_, with three fully
developed and distinct ileo-caecal folds, to the next stage leading up to
the normal human arrangement, we find the same illustrated in the caecum
of another new-world monkey, _Mycetes fuscus_, the brown howler monkey,
shown in the ventral and dorsal views in Figs. 449 and 450. The ventral
vascular fold (Fig. 449, 1) is still well developed, the contained
ventral branch of the ileo-colic artery descending over the ventral wall
of the ileo-colic junction and caecum and supplying both. The dorsal
vascular fold (Fig. 450, 2), on the other hand, is nearly completely
fused with the intermediate non-vascular reduplication (Figs. 449 and
450, 3), the approximation between these structures exhibited by
_Ateles_ having in _Mycetes_ reached the point of actual union, so that
the larger dorsal branch of the ileo-caecal artery descends to the apex
of the caecum in the following manner: The main post-caecal artery passes
over the dorsal surface of the ileo-colic junction included in a short
serous fold which corresponds to the dorsal vascular fold of _Ateles_.
Beyond the lower border of the ileo-colic junction this fold fuses with
the intermediate non-vascular fold, one arterial branch descending along
the line of attachment of this fold to the caecum, the other distributed
over the dorsal surface of the pouch.
A third type, also taken from the lower Primates, is presented by
the caecum of a cynomorphous monkey, _Cercopithecus sabaeus_, the
African green monkey, shown in Fig. 432, in the ventral and left
aspect with the terminal ileum lifted up. The caecum of this animal
is comparatively short, somewhat conical, terminating in a blunt
apex. The vascular supply is arranged on the same type as in _Ateles_
and _Mycetes_, _i. e._, a trunk of the ileo-colic artery divides
at the ileo-colic notch, one branch descending ventrad, the other
dorsad of the ileo-colic junction. The slightly larger size of the
dorsal vessel, noted in _Ateles_ and _Mycetes_, has been increased in
_Cercopithecus_ until the ventral artery (1) supplies merely the front
of the ileo-colic junction and the upper part of the adjoining ventral
wall of the caecum, while the larger dorsal vessel (2) descends behind
the ileo-colic junction, supplying the same and the entire dorsal and
apical portions of the caecum. The relation of these caecal arteries to
the peritoneum is moreover different from that encountered in _Ateles_.
In place of running in distinct mesenteric folds, as in the latter
species, the vessels pass close to the surface of the intestine, merely
covered and partly surrounded by slightly redundant visceral peritoneum
containing numerous pads of epiploic fat, which bead the course of the
vessels at regular intervals. Between the two arteries the intermediate
non-vascular fold (2) is seen, presenting much the same arrangement as
in _Ateles_ and passing between the left border of the caecum and the
adjacent margin of the ileum, nearer to the dorsal larger than to the
ventral smaller caecal artery.
We have, therefore, in the three types just considered, the following
variations in the arrangement of the vascular and non-vascular folds:
1. (_a_) Ventral and dorsal vascular folds distinct }
and free. Ventral and dorsal caecal }
arteries of nearly equal size. }
(_b_) Intermediate non-vascular fold free on }_Ateles._
both surfaces, placed nearer to the }
dorsal than to the ventral vascular fold. }
2. (_a_) Ventral vascular fold distinct. Ventral }
caecal artery somewhat further reduced }
in size. Dorsal vascular fold distinct }
only over the dorsal surface of the ileo-colic}
junction. At the lower border of }
the ileo-colic junction the dorsal vascular }_Mycetes._
fold fuses with the intermediate non-vascular }
fold. }
(_b_) Intermediate non-vascular fold free only }
on ventral surface, the dorsal surface }
below the ileo-colic junction being fused }
with the dorsal vascular fold. }
3. (_a_) Dorsal and ventral vascular folds reduced. }
Dorsal artery much larger than ventral. }_Cercopithecus._
(_b_) Intermediate non-vascular fold well developed,}
free on both surfaces. }
We may judge from this series that the following factors are capable of
materially modifying the definite arrangement of the structures:
1. The vascular folds are capable of reduction until the vessels run
close to the intestinal surface, merely covered by somewhat redundant
peritoneum containing epiploic appendages. (_Cercopithecus._)
2. The dorsal caecal artery tends to assume in all three forms the
greater share in the caecal vascular supply. This tendency is slightly
developed in _Ateles_, becomes more pronounced in _Mycetes_, and is well
marked in _Cercopithecus_, in which animal the dorsal vessel nearly
replaces the ventral branch, the latter confining itself to the ventral
surface of the ileo-colic junction and the adjacent ventral parts of the
caecal wall.
3. The intermediate non-vascular fold is placed nearer to the dorsal
larger than to the ventral smaller caecal artery. This condition, present
in both _Ateles_ and _Cercopithecus_, foreshadows the fusion of the
intermediate and dorsal vascular folds at the lower border of the
ileo-colic junction, as seen in _Mycetes_.
4. This fusion of the two folds named in _Mycetes_ results in giving
different values to the dorsal vascular fold in its proximal and distal
segments. The proximal segment descends from the ileo-colic notch behind
the ileo-colic junction to its lower border as a distinct fold. Beyond
this point its fusion with the distal (caecal) segment of the
intermediate fold rounds out a fossa, the inferior or posterior
ileo-caecal, which is consequently bounded in front by the intermediate
vascular fold, behind by the proximal segment of the dorsal vascular
fold, to the right side by the inner wall of the caecum, between the
intermediate and dorsal vascular folds, above by the lower border of
ileum and ileo-colic junction, and below by the fusion of the two
folds.
This pocket or fossa which is the most important and constant of the
peritoneal recesses in the neighborhood of the caecum, opens upward and
to the left.
5. A superior or anterior ileo-caecal fossa, formed in cases of
well-developed ventral vascular fold between the same and the ventral
wall of the ileo-colic junction, is of small size and shallow.
The cause of the greater development of the dorsal as compared with the
ventral caecal artery is probably to be sought in the adhesion of the
colon to the dorsal parietal peritoneum. In _Cercopithecus_ the dorsal
surface of the ascending colon is adherent to the parietal peritoneum
down as far as the iliac region and beginning of the caecum, whereas in
_Mycetes_ the entire caecum, as well as the ascending colon, are free and
non-adherent to the abdominal parietes. The influence of this adhesion
on the arrangement of the vascular supply of the lower portion of the
ascending colon and caecum appears to be important. Some of the
departures from the _Ateles_ type presented by _Cercopithecus_ become
still better developed in the human subject, where the adhesion of the
ascending colon and the obliteration of the apposed serous surfaces of
ascending mesocolon and parietal peritoneum is normally complete, even
if the caecum remains entirely free, or only adheres to the iliac
parietal peritoneum in the proximal part of its dorsal surface.
Comparison with forms presenting non-adherent colic and caecal tubes
indicates that the adhesion determines the relative size and arrangement
of the ileo-colic vessels.
Thus the partially adherent colon and caecum of _Cercopithecus_ presents,
compared with the free tube of _Ateles_ and _Mycetes_, a marked
reduction of the ventral and a corresponding enlargement of the dorsal
caecal artery. Further progress in the same direction is noted in the
human subject where normally the ascending colon and at times the
proximal portion of the caecum are adherent to the dorsal parietal
peritoneum.
It appears that in the adhesion of the colic tube to the parietal
peritoneum the dorsal ileo-colic vessels find an element favorable to
their more complete development and extension, replacing in part or
entirely the ventral caecal artery which becomes limited in distribution
to the region of the ileo-colic junction. The adhesion and fixation of
the dorsal wall of the intestine seems to afford an advantage to the
dorsal vessel, whereas the greater mobility and the alternating
conditions of distension and contraction, with variations of intracaecal
pressure, depending upon the contents of the pouch, appear to operate
unfavorably upon the development of the ventral vessel.
This view is borne out by the conditions observed in the exceptional
instances in which in the human subject the ventral artery assumes the
large share in the supply of caecum and appendix (cf. p. 276). In all the
cases observed the type of the caecum indicated delayed or imperfect
colic adhesion, and the ascending mesocolon remained partially free.
If we now compare the conditions above described for _Ateles_,
_Mycetes_, _Cercopithecus_ with those usually found in man and in the
anthropoid apes, we may appreciate the significance of the structures
encountered by beginning the investigation with a type in which the
derivation of the different parts is still quite evident. Such a
condition is presented by the preparation shown in Fig. 563, taken from
a child one year of age. Here the descent of the caecum has evidently
been quite rapid and uniform without dorsal adhesion. The caecum and
ascending colon remain free and can still be lifted away from the
ventral facies of the right kidney and turned toward the median line to
a point somewhat beyond the renal hilus. The caecum hangs downward
vertically and the appendix arises from the funnel-shaped apex of the
pouch.
[Illustration: FIG. 563.--Human; child one year old. Caecum and
ileo-colic junction; ventral view. (Columbia University, Study
Collection.)
1. Ventral caecal artery, surrounded by epiploic appendages.
2. Dorsal vascular fold, forming appendicular mesentery.
3. Intermediate non-vascular fold.]
The ventral caecal branch of the ileo-colic artery is slightly developed,
(1) as a small vessel descending in an epiploic fold over the ventral
surface of the ileo-colic junction as far as the root of the appendix.
The intermediate non-vascular fold (3) is well marked, measuring 2.9 cm.
in length, extending from the free border of the terminal ileum to the
caecum and appendix and crossing over the well-developed dorsal vascular
fold (2), which descends, as the appendicular mesenterolium, to the tip
of the appendix, carrying the dorsal artery. In studying the conditions
presented by this specimen, it is not difficult to trace the analogous
structures in the caeca of _Cercopithecus_, _Ateles_ and _Mycetes_. The
same vascular and non-vascular serous reduplications are found passing
between the ileum and caecum. In accordance with the type presented by
_Cercopithecus_ the ventral artery is much reduced and runs in a short
serous fold loaded with epiploic appendages. The dorsal artery, on the
other hand, is well developed and the intermediate non-vascular fold is
distinct. In their relative arrangement these folds follow the _Ateles_
type. The dorsal vascular fold forms the true mesentery of the appendix,
and, although close to and crossed by the intermediate non-vascular
reduplication, remains still quite separable and distinct from the same;
consequently the lower limit of the usual posterior ileo-caecal fossa,
produced by the fusion of the dorsal vascular and the intermediate
non-vascular fold, is absent.
A very perfect illustration of this type of the human ileo-caecal fold is
presented by the preparation of _Gorilla savagei_ shown in Fig. 457. The
ventral fold and artery appear reduced in this animal. The dorsal
vascular fold forms a broad triangular plate of serous membrane carrying
the dorsal artery in its free border and extending to the tip of the
appendix.
The intermediate non-vascular fold is narrow but distinct, continued for
a considerable distance along the ileum, opposite to the attached
border, but only for a short extent along the left border of the caecum
below the ileo-colic junction. It crosses the ventral surface of the
broad dorsal vascular fold in passing to the caecum, but remains entirely
free and is not adherent to the same.
Consequently here again the dorsal or posterior ileo-caecal fossa loses
its distal limitation. The usual arrangement of the parts, as found in
the human subject and derived from the preceding, is well illustrated by
another anthropoid ape, _Hylobates hoolock_. Fig. 455 shows the
ileo-caecum of this animal in the ventral view and the homologous parts,
as compared with _Gorilla_, are readily recognized. On turning the
terminal ileum ventrad and cephalad (Fig. 456), it is, however, seen
that the intermediate non-vascular fold does not merely cross the dorsal
vascular reduplication, as in _Gorilla_, but that it has begun to adhere
to the same at the point of intersection. Consequently a well-marked and
clearly limited posterior or dorsal ileo-caecal fossa is formed, bounded
ventrally by the intermediate fold at its accession to the caecum,
dorsally by the proximal part of the dorsal vascular fold, to the right
by the left wall of the caecum, behind by the attachment of the
intermediate fold, below by the confluence of the two folds, and above
by the lower border of ileum and ileo-colic junction.
The open mouth of the fossa looks to the left. Fig. 464, taken from an
adult specimen of the chimpanzee, _Troglodytes niger_, shows the extent
of the dorsal vascular fold and of its connection with the mesentery of
the terminal ileum.
The intermediate non-vascular fold extends from the ileum downwards
along the entire left border of the caecum to the root of the appendix,
fusing with the dorsal vascular fold and rounding out a deep posterior
ileo-caecal fossa.
The typical arrangement, as encountered in the human subject,
corresponds closely to the conditions presented by these anthropoid
apes.
[Illustration: FIG. 564.--Human adult. Caecum and ileo-colic junction.
(Drawn from preparation in Columbia University, Study Collection.)
1. Dorsal vascular fold at the beginning of the distal free portion,
forming the appendicular mesentery.
2. Proximal segment of dorsal vascular fold, fusing with
3. Intermediate non-vascular fold.
4. Rounded edge of union of dorsal vascular and intermediate folds
bounding the ileo-caecal fossa caudad.
5. Point of accession to appendix of proximal branch of appendicular
artery derived from posterior ileo-caecal artery.]
In Fig. 564, taken from an adult male human subject, the dorsal surface
of the ascending colon and of the ileo-colic junction is adherent to the
parietal peritoneum. The distention of the caecum is nearly uniform, the
right sacculation being only slightly larger than the left. The
appendix, measuring 18.4 cm. in length, arises from the dorsal surface
of the caput coli, 1.7 cm. from the point where the ventral longitudinal
muscular band turns around the caudal end of the pouch between the two
sacculations, and 3.7 cm. below the caudal margin of the ileo-colic
junction.
The dorsal vascular fold (2), forming the broad appendicular mesentery
(1), is well developed and free in its distal portion, extending, with
gradually diminishing width, to the apex of the appendix. The proximal
segment of this fold (between 1 and 2) descends over the dorsal surface
of the ileo-colic junction and meets (at 4) the intermediate
non-vascular fold (3) which extends between the ileum and caecum,
rounding out a crescentic ridge (4) which bounds the entrance into the
posterior ileo-caecal fossa (between 2 and 3). The influence of the folds
and of the blood vessels on the position and curves of the appendix is
quite apparent in this preparation.
The dorsal larger branch of the ileo-colic artery, supplying caecum and
appendix, passes over the dorsal surface of the ileo-colic junction (2)
where the same, as well as the adjacent dorsal surface of the colon, is
adherent to the parietal peritoneum. At the point where the dorsal
vascular fold intersects and fuses with the intermediate non-vascular
fold (4) the artery divides into a proximal and distal branch. The
former proceeds to the caecum and root of the appendix, reaching this
tube at the point marked 5. The latter continues (from 1 on) in the free
border of the appendicular mesentery to the beginning of the distal
third of the appendix, from which point on the fold extends as a narrow
reduplication to the tip of the tube. The segment of the appendix
situated between these two main arterial branches is thrown into several
coils, the expression of the continued growth between two points
relatively fixed by the accession of the two arterial branches. The
pathological significance of these bends is apparent when we consider
the effect which the kinking of the tube would have on catarrhal and
other inflammations accompanied by distension of the appendix.
Typical examples of the posterior ileo-caecal fossa and of the mutual
relationship of the limiting folds are seen in Figs. 565 and 566, both
taken from adult human subjects.
[Illustration: FIG. 565.--Human adult, Caecum and ileo-colic junction
with large intermediate non-vascular fold and deep posterior ileo-caecal
fossa. (Columbia University Museum, No. 1546.)]
[Illustration: FIG. 566.--Human adult. Ileo-colic junction and caecum.
(Columbia University Museum, No. 1659.)]
The significance and mutual relations of the folds seen in the
preparations just considered--which illustrate the typical adult human
arrangement of the structures--will perhaps be best understood by
comparison with an adult caecum in which the infantile condition, as seen
in Fig. 563, has become further developed.
[Illustration: FIG. 567.--Human adult. Ileo-colic junction and caecum;
dorsal view. (Drawn from preparation in Columbia University, Study
Collection.)
1. Dorsal vascular fold, carrying the distal appendicular branch of the
dorsal caecal artery in the mesentery of the appendix.
2. Proximal branch of the same vessel, turning downward to caecum and
root of appendix.
3. Intermediate non-vascular fold.]
Fig. 567 shows the dorsal view of such a preparation. The caecum is
funnel-shaped with the apex, carrying the root of the appendix, turned
upward and to the left, the sacculation to the right of the ventral
muscular band being somewhat dilated. The appendix--7.2 cm. long--turns
sharply upward and to the left, closely applied to the left caecal
sacculation, passes dorsad to the ileo-colic junction and lies in its
terminal part under cover of the ileo-colic mesentery. The ventral
branch of the ileo-colic artery descends over the ileo-colic junction,
supplying the ventral wall of the caecum. The intermediate non-vascular
fold (3) is 3.9 cm. long and entirely free.
The dorsal vascular fold contains the large dorsal branch of the
ileo-colic artery, dividing into two main branches. The first of these
(1) passes distally in the free edge of the fold to the terminal part of
the appendix. The other proximal branch (2) turns downward to the root
of the appendix and the adjacent wall of the caecum, aiding materially in
holding the proximal upturned segment of the appendix in contact with
the left caecal sacculation.
The intermediate fold, short in its caecal attachment, does not meet the
dorsal vascular fold at any point, consequently the ileo-caecal fossa is
not limited caudad toward the root of the appendix. The conditions
presented by this specimen correspond exactly to those found in the
gorilla (Fig. 457) and in the human infantile preparation (Fig. 563).
In comparing Figs. 564 and 567 it will be noticed that the line of
fusion between the intermediate fold and the dorsal vascular fold (Fig.
564, 4) corresponds to the point where the dorsal ileo-caecal artery
divides into its proximal and distal branches (Fig. 567, angle between 1
and 2). Fig. 567 shows that the proximal arterial twig, even without
fusion with the intermediate fold, suffices to influence to a
considerable degree the curves and position of the appendix, inasmuch as
it serves to hold the proximal segment of the tube closely applied in
the erected position to the surface of the left caecal sacculation. The
intermediate segment of the appendix, between the points of accession of
the two arterial branches, is most prone to develop spiral
twists and bends, especially when the usual fusion of the two folds
takes place and still further fixes the parts, while the distal segment,
carrying the narrow crescentic terminal appendicular mesentery, remains
free.
[Illustration: FIG. 568.--Human adult. Ileo-colic junction and caecum.
(Drawn from preparation in Columbia University, Study Collection.)
1. Distal and
2. Proximal branch of dorsal ileo-caecal artery running in dorsal
vascular fold.
3. Intermediate non-vascular fold fusing with 2 and forming a narrow
caudal limit to the posterior ileo-caecal fossa.]
Finally, in a certain number of cases, an intermediate condition between
the types presented by Figs. 564 and 567 is encountered. In Fig. 568 the
general arrangement of the parts corresponds pretty accurately to that
seen in Fig. 566, but the transition from a completely free intermediate
non-vascular fold to one which has begun to fuse with the dorsal
vascular fold is evident. The caecum is bent upward and to the left, the
caput coli being formed by the right sacculation. The appendix, 7.8 cm.
long, takes a wide {~WREATH PRODUCT~}-shaped curve. The convexity of the proximal curve
corresponds to the point where the proximal appendicular artery (2)
passes to the tube. The non-vascular intermediate fold (3), measuring
2.2 cm., fuses with the dorsal vascular fold at this point.
The three preparations illustrate serially the share which the
peritoneal folds take in the formation of the posterior ileo-caecal
fossa.
In Fig. 566 the failure of the intermediate fold to meet and fuse with
the dorsal vascular fold has left the caudal boundary of the fossa
(between 2 and 3) incomplete, the ventral and dorsal walls being formed
by the folds in question. Fig. 568, in which fusion between the
non-vascular and the dorsal vascular folds has commenced, shows the
shallow form of the complete fossa under these conditions, while in Fig.
567, with extensive union of the folds, the fossa has correspondingly
increased in depth.
[Illustration: FIG. 569.--Human adult. Ileo-colic junction and caecum.
(Columbia University Museum, No. 1610.)]
[Illustration: FIG. 570.--Human infant, four days old. Ileo-colic
junction and caecum. (Columbia University Museum, No. 879.)]
[Illustration: FIG. 571.--Human infant. Ileo-colic junction and caecum.
(Columbia University, Study Collection.)]
A similar series is shown in Figs. 569, 570 and 571. In Fig. 569, taken
from an adult subject, the intermediate non-vascular fold is entirely
free, the dorsal branch of the ileo-caecal artery passes to caecum and
appendix in an area of adhesion between parietal peritoneum and the
intestine which includes the dorsal vascular fold. There is consequently
no caudal boundary to the ileo-caecal fossa. Figs. 570 and 571 are both
taken from infantile preparations.
In Fig. 570 the dorsal vascular and the intermediate folds nearly meet
at the root of the appendix. They serve to outline the fossa, which
appears completed in Fig. 571 by the actual meeting and fusion of the
folds.
_The Ileo-caecal Folds in the Anthropoid Apes.--(1) Chimpanzee,
Troglodytes niger._
The structures in a juvenile specimen of this animal are shown in Figs.
460 and 461.
The ventral vascular fold (Fig. 460, 3), containing epiploic fat,
descends over the ileo-colic junction nearly to the level of the lower
ileal margin. The intermediate non-vascular fold (Figs. 460 and 461, 2),
derived from the ileum opposite to the mesenteric border, passes to the
ventral and left aspects of the caecum and meets, near the root of the
appendix, the dorsal vascular fold (Fig. 461, 3) carrying the dorsal
caecal branch of the ileo-colic artery, which ramifies over the caecum and
supplies the appendix.
The appendix measures 12.3 cm. and presents a terminal hook, slightly
dilated.
The appendicular mesentery terminates within the concavity of this hook
and measures 1.5 cm. in width at the broadest part, about 4.5 cm. from
the root of the appendix.
Figs. 462 and 463 show the caecum of the adult chimpanzee in the ventral
and dorsal view. The ventral vascular fold (Fig. 462, 1) is well
developed, heavily fringed with epiploic appendages.
The non-vascular fold is extremely short and tense, fusing with the
short appendicular mesentery near the point where in the dorsal view
(Fig. 463) the appendix is seen bent at its origin sharply to the right.
Fig. 464, also taken from an adult specimen of the same animal, shows a
very well-developed dorsal vascular fold, which fuses with the
intermediate fold to limit a distinct ileo-caecal recess.
The chimpanzee, therefore, agrees closely with the human subject in the
arrangement of the folds.
(2) _Orang, Simia satyrus._
In Figs. 458 and 459 the arrangement of the folds in an adult specimen
of the orang is shown.
The ventral caecal artery (Fig. 458) is well developed, forming with the
peritoneal fold and epiploic appendages surrounding it, a sharp
sickle-shaped edge which descends over the ventral surface of the
ileo-colic junction following the curve of the left caecal margin, and
turning its concavity to the left toward the entering ileum.
The ventral caecal artery follows the left margin of the caecum below the
ileo-caecal junction and passes for 0.5 cm. upon the portion of the pouch
which turns up behind the terminal ileum.
The dorsal caecal artery is a vessel of large size, supplying branches to
the narrow appendicular mesentery which extends, with many epiploic
appendages, to within 9 mm. of the blunt apex of the appendix. 2.5 cm.
beyond the first bend in the appendix the fold is narrowed to a fringe
not more than 0.75 cm. wide. Up to this point the dorsal vascular fold
measures 1.5 cm. in width, and just where it narrows it is joined by the
intermediate non-vascular fold (Fig. 459), which forms a membranous
band, 3.3 cm. wide in the middle, spread out in the angle between the
lower and dorsal surfaces of the ileum and the dorsal surface of the
caecum which turns up behind the ileo-colic junction. Between this fold
and the dorsal vascular fold is seen the deep recess of the posterior
ileo-caecal fossa--which by reason of the sharp curve of the caecum looks
not only to the left but also upward and backward.
Direct comparison of the preparations of these two anthropoid apes just
described with the conditions found in many adult human caeca shows the
close correspondence in the arrangement of these folds and of their
influence on the configuration of the parts.
Figs. 572 and 573--taken from an adult human subject--show a caecum and
appendix which almost reproduces that of the chimpanzee illustrated in
Figs. 462 and 463 and closely resembles that of the orang.
[Illustration: FIG. 572. Human adult. Ileo-colic junction and caecum;
ventral view. (Drawn from preparation in Columbia University, Study
Collection.)
1. Ventral vascular fold.]
[Illustration: FIG. 573.--Dorsal view of the same preparation.
1. Appendix.
2. Intermediate non-vascular fold.]
Fig. 572, giving the ventral view, shows, by the course of the ventral
longitudinal muscular band, the turn of the caecum upwards and to the
left. The ventral caecal artery runs in a fold (1) loaded with epiploic
appendages.
The non-vascular intermediate fold (Fig. 573, 2) passes to the root of
the appendix, joining the proximal segment of the dorsal vascular fold
in which the dorsal branch of the ileo-colic artery runs to the tip of
the appendix. The distal two thirds of the appendicular mesentery are
free.
3. _Gibbon, Hylobates hoolock_ (Figs. 455 and 456).--In the gibbon the
folds appear well developed. The intermediate and dorsal vascular folds
are quite distinct structures, although fusion (Fig. 456) has begun at
one point, thus limiting a typical posterior ileo-caecal fossa.
4. _Gorilla, Gorilla savagei_ (Fig. 457).--Finally in the gorilla all
three folds appear quite distinct and separate from each other, the
dorsal vascular fold being especially well developed.
_Unusual and Aberrant Types of Ileo-caecal Folds and Fossae.--(A) Ventral
caecal artery larger than the dorsal, supplying the greater part of the
caecum and the appendix._
This condition is occasionally encountered. Dr. Martin, in a recent
examination of the vascular supply of caecum and appendix in one hundred
subjects, found it to obtain in six instances.
Apparently the dorsal wall of the caecum and of the proximal segment of
the ascending colon remains free in these cases and does not become
adherent to the parietal peritoneum. The shape of the pouch, moreover,
indicates a free and unimpeded embryonal caecal descent. The normal
relative size of the two vascular folds is reversed. A good example of
this variation, in the caecum of an infant, is seen in Fig. 516. The same
arrangement in an adult specimen is seen in Fig. 574.
[Illustration: FIG. 574.--Human adult. Caecum and ileo-colic junction;
well-developed ventral vascular fold, carrying appendicular artery.
(Columbia University Museum, No. 1613.)]
In the Slow Lemur (_Nycticebus tardigradus_) (Fig. 420) the ventral
artery is normally the larger of the two, extending in the ventral fold
to the tip of the reduced appendix of the caecal pouch.
(_B_) _Fusion of ventral vascular fold with the intermediate fold,
resulting in the production of a well-defined superior or
ventral ileo-caecal fossa._
Normally the reduced ventral artery crosses the ileo-colic junction in a
slightly developed ventral vascular fold, closely adherent to the
intestine, with a very narrow free margin. The superior or ventral
ileo-caecal fossa in these cases is very shallow and confined (Fig. 574)
to the ventral surface of the ileo-colic junction. Occasionally the fold
is better developed and fuses with the intermediate non-vascular fold,
producing a fossa of greater extent, which is bounded dorsad by the
ileum, ventrad and cephalad by the ventral fold, caudad by the fusion of
this fold with the intermediate reduplication, and to the right by the
left wall of the caecum. Figs. 576, 577, 578 and 579 show this aberrant
disposition of the structures in a series of adult human caeca.
[Illustration: FIG. 575.--Human foetus at term. Ileo-colic junction and
caecum; ventral view. (Columbia University Museum, No. 1715.)]
[Illustration: FIG. 576.--Human adult. Ileo-colic junction and caecum;
ventral appendicular artery and ileo-caecal fossa. (Columbia University
Museum, No. 1614.)]
[Illustration: FIG. 577.--Human adult. Ileo-colic junction and caecum;
ventral appendicular artery and ileo-caecal fossa. (Columbia University
Museum, No. 1657.)]
[Illustration: FIG. 578.--Human adult. Ileo-colic junction and caecum;
ventral appendicular artery and ileo-caecal fossa. (Columbia University
Museum, No. 1856.)]
[Illustration: FIG. 579.--Human adult. Ileo-colic junction and caecum;
ventral appendicular artery and ileo-caecal fossa. (Columbia University
Museum, Study Collection.)]
A corresponding arrangement is noted in the preparation of the caecum of
_Cercopithecus campbellii_ (Fig. 433). The large intermediate fold is
joined by the ventral vascular fold, thus defining the lower boundary of
ventral ileo-caecal fossa.
(_C_) _Union of both vascular folds with the intermediate non-vascular
fold._
I have encountered one instance of this arrangement in an infant, whose
caecum and ileo-colic junction is shown in Fig. 580. Both the ventral and
dorsal arteries in this case were equally developed, and shared equally
in the supply of caecum and appendix. Both vascular folds fused with the
intermediate fold, thus producing two typical ileo-caecal fossae, one
ventral, the other dorsal.
[Illustration: FIG. 580.--Human infant. Ileo-colic junction and caecum;
fusion of ventral and dorsal vascular folds, with intermediate fold.
(Columbia University Museum, No. 1663.)]
(_D_) _Abnormal positions of the appendix due to variations in the
arrangement and tension of the intermediate fold._