blunt termination. Fig. 395 shows its structure in _Zalophus
gillespiei_, Gillespie's sea-lion, and Fig. 396 in _Phoca vitulina_, the
harbor seal.
[Illustration: FIG. 395.--_Zalophus gillespiei_, Gillespie's sea-lion.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 90.)]
[Illustration: FIG. 396.--_Phoca vitulina_, harbor seal. Ileo-colic
junction and caecum. (Columbia University Museum, No. 762.)]
B. FISSIPEDIA.--The _Cynoidea_, including the dogs, jackals, wolves and
foxes, form a well-marked central group with well-developed convoluted
caeca placed laterally to the ileo-colic junction (Figs. 397-399).
[Illustration: FIG. 397.--_Vulpes fulvus_, red fox. Ileo-colic junction
and caecum; dried preparation. (Columbia University Museum, No. 114.)]
[Illustration: FIG. 398.--_Canis familiaris_, dog. Ileo-colic junction
and caecum, Type I. (Columbia University Museum, No. 1550.)]
[Illustration: FIG. 399.--_Canis familiaris_, dog. Ileo-colic junction
and caecum, Type II. (Columbia University Museum, No. 1551.)]
From this type depart on the one hand the _Ailuroidea_, including the
civets, ichneumons and true cats, with the caecum uniformly present, but
short and markedly pointed, suggesting the degeneration of a formerly
better developed structure (Figs. 400-406), while on the other the
_Arctoidea_, including the bears, weasels and raccoons, constitute a
group united by many common fundamental peculiarities of structure,
among which is the entire absence of a caecal pouch (Figs. 407-415).
[Illustration: FIG. 400.--_Genetta vulgaris_, genet. Ileo-colic junction
and caecum. (Columbia University Museum, No. 1625.)]
[Illustration: FIG. 401.--_Felis concolor_, puma. Ileo-colic junction
and caecum; dried preparation. (Columbia University Museum, No. 119.)]
[Illustration: FIG. 402.--_Felis borealis, var. rufus_, red lynx.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 177.)]
[Illustration: FIG. 403.--_Paradoxurus typus_, paradoxure. Ileo-colic
junction and caecum; dried preparation. (Columbia University Museum, No.
112.)]
[Illustration: FIG. 404.--_Herpestes sp.?_, ichneumon. Ileo-colic
junction and caecum; dried preparation. (Columbia University Museum, No.
120.)]
[Illustration: FIG. 405.--_Herpestes griseus_, mongoose ichneumon.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 149.)]
[Illustration: FIG. 406.--_Proteles lalandii_, aard-wolf. Ileo-colic
junction and caecum. (Columbia University Museum, No. 1520.)]
[Illustration: FIG. 407.--_Nasua rufa_, brown coati-mundi. Ileo-colic
junction. (Columbia University Museum, No. 1089.)]
[Illustration: FIG. 408.--_Nasua rufa_, brown coati-mundi. Ileo-colic
junction, opened, showing pyloric-like ileo-colic valve. (Columbia
University Museum, No. 1581.)]
[Illustration: FIG. 409.--_Bassaris astuta_, raccoon-fox. Ileo-colic
junction; dried preparation. (Columbia University Museum, No. 289.)]
[Illustration: FIG. 410.--_Mustela sp.?_, marten. Ileo-colic junction;
dried preparation. (Columbia University Museum, No. 199.)]
[Illustration: FIG. 411.--_Taxidea americana_, American badger.
Ileo-colic junction; dried preparation. (Columbia University Museum, No.
180.)]
[Illustration: FIG. 412.--_Procyon lotor_, raccoon. Ileo-colic junction;
dried preparation. (Columbia University Museum, No. 230.)]
[Illustration: FIG. 413.--_Cercoleptes caudivolvulus_, kinkajou.
Ileo-colic junction; dried preparation. (Columbia University Museum, No.
295.)]
[Illustration: FIG. 414.--_Ursus americanus_, black bear. Ileo-colic
junction; dried preparation. (Columbia University Museum, No. 226.)]
[Illustration: FIG. 415.--_Ursus maritimus_, polar bear. Ileo-colic
junction. (Columbia University Museum, No. 748.)]
[Illustration: FIG. 416.--_Hyaena striata_, striped hyena. Ileo-colic
junction and caecum; dried preparation. (Columbia University Museum, No.
56.)]
[Illustration: FIG. 417.--_Felis leo_, lion, Ileo-colic junction and
caecum. (Columbia University Museum, No. 1516.)]
Among the ailuroid carnivora, the hyaena and the lion occupy an isolated
position in regard to the caecum. Both of these animals possess a
well-developed long caecal pouch with blunt extremity (Figs. 416 and
417). They probably afford examples of a persistent ancestral common
type from which the remaining carnivorous forms are derived by reduction
of the caecal apparatus in conformity with the food-habits of these
animals. The caecum of both the lion and hyaena resembles very closely the
pouch of the herbivorous marsupials, such as _Halmaturus_ or _Didelphis_
(cf. Figs. 348 and 350, p. 205).
IX. Order: Cheiroptera.
In the bats the alimentary canal is uniformly simple without caecum and
scarcely any differentiation between small and large intestine (Fig.
418).
[Illustration: FIG. 418.--_Pteropus medius_, Indian fruit-bat.
Ileo-colon; dried preparation. (Columbia University Museum, No. 533.)]
X. Order: Insectivora.
In the true Insectivora the caecum is also absent and the alimentary
canal a simple non-differentiated tube.
In certain herbivorous animals included in this group on the other hand,
such as _Galeopithecus_ (Fig. 419), the caecum is present as an enormous
sacculated pouch with spiral convolutions.
[Illustration: FIG. 419.--_Galeopithecus volans_, colugo. Ileo-colic
junction, caecum, and colon. (Columbia University Museum, No. 1844.)]
XI. Order: Primates.
The caecum is uniformly present. In certain of the Lemuroidea the
terminal portion of the pouch is reduced, forming a species of appendix.
A typical vermiform appendix is regularly found in man and in the
anthropoid apes, orang, gibbon, chimpanzee and gorilla.
1. Suborder Lemuroidea.
In the typical lemurs the caecum is long, frequently terminating in a
pointed appendage. The proximal segment of the colon is looped and
coiled, resembling the spiral colon of the Ungulates and Rodents. Fig.
420 shows the caecum of _Nycticebus tardigradus_, the slow lemur, with
the typical appendage, and Fig. 421 shows the spiral arrangement of the
proximal colon immediately beyond the ileo-colic junction in the same
animal. Fig. 422, taken from another specimen of the same animal shows
the caecum, appendix and spiral colon. Figs. 423, 424, 425 illustrate the
structure of the parts in three other members of the group, _Lemur
macaco_, _Lemur mongoz_ and _Otolicnus crassicaudatus_, all showing
terminal reduction of the caecal pouch and tendency to spiral coiling of
the proximal colon. In _Tarsius spectrum_ (Fig. 426) the caecum is
relatively well-developed, but forms a simple pouch of uniform diameter,
without terminal reduction.
[Illustration: FIG. 420.--_Nycticebus tardigradus_, slow lemur.
Ileo-colic junction, caecum, appendix, and colon; dorsal view. (Columbia
University Museum, No. 20/1468.)]
[Illustration: FIG. 421.--_Nycticebus tardigradus_, slow lemur. Same
preparation as Fig. 420; ventral view, showing spiral coiling of
proximal colon. (Columbia University Museum, No. 20/1468.)]
[Illustration: FIG. 422.--_Nycticebus tardigradus_, slow lemur.
Ileo-colic junction, caecum, appendix, and spiral colon. (Columbia
University, Study Collection.)]
[Illustration: FIG. 423.--_Lemur macaco_, lemur. Ileo-colic junction and
caecum. (Drawn from preparation.) (Columbia University Museum, No.
1623.)]
[Illustration: FIG. 424.--_Lemur mongoz_, lemur. Ileo-colic junction and
caecum. (Drawn from preparation.) (Columbia University Museum, No.
1473.)]
[Illustration: FIG. 425.--_Otolicnus crassicaudatus_, galago. Ileo-colic
junction and caecum. (Drawn from preparation.) (Columbia University
Museum, No. 1626.)]
[Illustration: FIG. 426.--_Tarsius spectrum_, spectre lemur. (Drawn from
preparation.) (Columbia University Museum, No. 1521.)]
2. Suborder Anthropoidea.
A. CYNOMORPHA.
=1. Cynocephalus.=--The baboons have a well-developed capacious caecum.
The apex of the pouch is usually blunt and rounded, or only slightly
pointed. The caecum is sacculated, conforming in structure to the rest of
the large intestine. Two low vascular folds or ridges, a ventral and a
dorsal, carry the ventral and dorsal caecal branches of the ileo-colic
artery. The intermediate non-vascular fold is large, frequently fused
with the dorsal vascular fold (cf. p. 264).
Figs. 427-433 show the structures in _Cynocephalus sphinx_, _porcarius_,
_babuin_, _anubis_ and in _Cercopithecus pogonias_, _sabaeus_ and
_campbellii_.
[Illustration: FIG. 427.--_Cynocephalus sphinx_, Guinea baboon.
Ileo-colic junction and caecum. (Columbia University Museum, No. 1082.)]
[Illustration: FIG. 428.--_Cynocephalus porcarius_, Chacma baboon.
Ileo-colic junction and caecum. (Columbia University Museum, No. 1071.)]
[Illustration: FIG. 429.--_Cynocephalus babuin_, yellow baboon; dried
preparation. (Columbia University Museum, No. 89.)]
[Illustration: FIG. 430.--_Cynocephalus anubis_, olive baboon. (Columbia
University Museum, No. 51/1618.)]
[Illustration: FIG. 431.--_Cercopithecus pogonias_, bearded monkey.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 228.)]
[Illustration: FIG. 432.--_Cercopithecus sabaeus_, green monkey.
Ileo-colic junction and caecum. (Drawn from preparation.) (Columbia
University Museum, No. 746.)
1. Ventral ileo-caecal vascular fold.
2. Dorsal ileo-caecal vascular fold.
3. Intermediate ileo-caecal non-vascular fold.
]
[Illustration: FIG. 433.--_Cercopithecus campbellii_, cercopithecus
monkey. Ileo-colic junction and caecum. (Drawn from preparation.)
(Columbia University Museum, No. 55/1542.)]
=2. Macacus.=--The caecum is of large caliber, blunt, or in some forms
slightly pointed at the apex, sacculated like the colon.
The two vascular folds are narrow and low, studded with epiploic
appendages. The intermediate non-vascular fold is large, placed nearer
to the dorsal than to the ventral vascular fold.
Figs. 434-439 show the structures in _Macacus cynomolgus_, _ochreatus_,
_rhesus_ and _pileatus_.
Fig. 439 is from a formaline hardened situs preparation of the abdominal
viscera in _Macacus cynomolgus_, the Kra monkey.
[Illustration: FIG. 434.--_Macacus cynomolgus_, Macaque monkey.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 19.)]
[Illustration: FIG. 435.--_Macacus ochreatus_, ashy-black macaque.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 11.)]
[Illustration: FIG. 436.--_Macacus rhesus_, rhesus monkey. Ileo-colic
junction and caecum. (Columbia University Museum, No. 1126.)]
[Illustration: FIG. 437.--_Macacus pileatus_, macaque. Ileo-colic
junction and caecum. (Columbia University Museum, No. 719.)]
[Illustration: FIG. 438.--_Macacus sinicus_, bonnet macaque. Ileo-colic
junction and caecum. (Columbia University Museum, No. 1072.)]
[Illustration: FIG. 439.--_Macacus cynomolgus_, kra monkey. Abdominal
viscera, hardened _in situ_. (Columbia University Museum, No. 1801.)]
B. ARCTOPITHECINI.
The marmosets have a long crescentic-shaped caecum, turning the concavity
of the curve upwards and to the left, terminating in a blunt point.
Typical forms are shown in Fig. 440, _Hapale jacchus_, Fig. 441, _Midas
ursulus_, and Fig. 442, _Midas geoffrei_.
[Illustration: FIG. 440.--_Hapule jacchus_, common marmoset. Ileo-colic
junction and caecum. (Columbia University Museum, No. 975.)]
[Illustration: FIG. 441.--_Midas ursulus_, negro tamarin. Ileo-colic
junction and caecum; dried preparation. (Columbia University Museum, No.
235.)]
[Illustration: FIG. 442.--_Midas geoffrei_, Geoffrey's marmoset.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 197.)]
C. CEBIDAE.
=1. Ateles= and other howlers have a large caecum, crescentic in shape,
narrowed at the apex, separated from the colon by a sharp and deep
constriction, opposite the wedge-shaped ileo-colic junction.
The ileo-caecal folds are well-developed and symmetrical, two equal
vascular folds, and a free intermediate non-vascular reduplication.
Types: _Ateles ater_ (Figs. 443-445), _Chrysothrix sciureus_ (Fig. 447)
and _Nyctipithecus commersonii_ (Fig. 446). In _Mycetes_ (Figs. 448-450)
the pouch is shorter, less curved, with a slight reduction toward the
less distinctly pointed apex.
[Illustration: FIG. 443.--_Ateles ater_, black-faced coaita. Ileo-colic
junction and caecum; dried preparation. (Columbia University Museum, No.
240.)]
[Illustration: FIG. 444.--_Ateles ater_, black-faced coaita. Ileo-colic
junction and caecum, with ileo-caecal folds. (Columbia University Museum,
No. 720.)]
[Illustration: FIG. 445.--_Ateles ater_, black-faced coaita. Ileo-colic
junction and caecum, with ileo-caecal folds. (Drawn from preparation.)
(Columbia University Museum, No. 300.)
1. Ventral vascular ileo-caecal fold.
2. Intermediate non-vascular ileo-caecal fold.
3. Dorsal vascular ileo-caecal fold.
]
[Illustration: FIG. 446.--_Nyctipithecus commersonii_, Vitoe monkey.
Ileo-colic junction and caecum; dried preparation. (Columbia University
Museum, No. 238.)]
[Illustration: FIG. 447.--_Chrysothrix sciureus_, Viti monkey.
Ileo-colic junction and caecum. (Columbia University Museum, No. 1624.)]
[Illustration: FIG. 448.--_Mycetes cavaya_, black howler. Ileo-colic
junction and caecum. (Columbia University Museum, No. 1136.)]
[Illustration: FIG. 449.--_Mycetes fuscus_, brown howler. Ileo-colic
junction and caecum, with ileo-caecal folds; ventral view. (Columbia
University Museum, No. 674.)
1. Ventral vascular ileo-caecal fold.
3. Intermediate non-vascular ileo-caecal fold.
]
[Illustration: FIG. 450.--Drawn from the same preparation as Fig. 449;
dorsal view.
2. Dorsal vascular ileo-caecal fold.
3. Intermediate non-vascular ileo-caecal fold.
]
[Illustration: FIG. 451.--_Lagothrix humboldtii_, Humboldt's lagothrix.
Ileo-colic junction and caecum. (Columbia University Museum, No. 1511.)]
[Illustration: FIG. 452.--_Pithecia satanas_, black saki monkey.
Ileo-colic junction and caecum. (Columbia University Museum, No. 641.)]
=2. Lagothrix.=--The caecum is very capacious and long, bent at a sharp
angle upwards and to the left toward the ileo-colic junction.
Type: _Lagothrix humboldtii_ (Fig. 451).
=3. Pithecia.=--The caecum resembles in general the type presented by
_Ateles_, but is less curved and less reduced and pointed at the
termination.
Type: _Pithecia satanas_ (Fig. 452).
In general the Arctopithecini and _Ateles_, _Mycetes_, _Lagothrix_ and
_Pithecia_ among the Cebidae form a group containing a series of caecal
transition types which lead up to the anthropomorphous type,
illustrating the following conditions:
(_a_) The inherent crescentic curve of the caecum, with the concavity
directed toward the left, and carrying the apex of the pouch upward
toward the lower border of the ileum and the ileo-colic junction.
(_Hapalidae_, _Ateles_, _Lagothrix_.)
(_b_) The reduction in caliber of the terminal part, foreshadowing by
the pointed and narrow extremity of the pouch the appearance of the
appendix in the anthropomorphous group. (_Hapalidae_, _Ateles_.)
(_c_) The constriction at the level of the ileo-caecal junction, with the
corresponding well-marked differentiation between caecum and colon in the
interior. (_Ateles._)
(_d_) The sharp bend in the pouch as it makes its turn upward and to the
left, repeated in certain types of adult human caeca (cf. p. 247).
(_Lagothrix._)
(_e_) _Pithecia_ forms a transitive type between the blunt sacculated
caeca of the Cynomorpha and the curved pointed pouches of the Cebidae,
partaking of the characters of both.
(_f_) The same character is seen in the caecum of _Mycetes fuscus_ the
brown howler monkey (Figs. 449 and 450).
=4. Cebinae.=--In the typical genus _Cebus_ the caecum is placed laterad
to the small intestine which is in direct linear continuity with the
colon. The pouch is slightly convoluted toward its termination,
resembling in this respect and in its position relative to the lumen of
the intestinal canal, the disposition of the parts in the cynoid
carnivora. Figs. 453 and 454 show the structures in two typical species,
_Cebus monachus_ and _C. leucophaeus_.
[Illustration: FIG. 453.--_Cebus monachus_, capuchin monkey. Ileo-colic
junction and caecum; dried preparation. (Columbia University Museum, No.
26.)]
[Illustration: FIG. 454.--_Cebus leucophaeus_, capuchin monkey.
Ileo-colic junction and caecum. (Columbia University Museum, No. 1467.)]
D. ANTHROPOMORPHA.
The caecum is large, sacculated, provided uniformly with a vermiform
appendix.
The pouch of the four anthropoid apes agrees in curve, direction,
implantation of the appendix and the general arrangement of the vascular
and peritoneal folds with the structure in the human subject.
=1. Hylobates hoolock, Gibbon.=--Figs. 455 and 456 represent
respectively the ileo-caecum of this animal in the ventral view, and from
the left side with the ileum turned forward. The caecum is a globular
rounded pouch of nearly uniform diameter, only slightly enlarged to the
right of the root of the appendix which arises from its lowest part and
is pendent.
[Illustration: FIG. 455.--_Hylobates hoolock_, hoolock gibbon.
Ileo-colic junction and caecum; ventral view. (Drawn from Columbia
University Museum preparation No. 1530.)]
[Illustration: FIG. 456.--Drawn from same preparation as Fig. 455; view
from left side, showing formation of posterior ileo-caecal fossa.]
[Illustration: FIG. 457.--_Gorilla savagei_, gorilla. Ileo-colic
junction and caecum, with ileo-caecal folds. (Drawn from Columbia
University Museum preparation No. 1543.)]
(For arrangement of the ileo-caecal folds and fossae in this form see p.
269.)
=2. Gorilla savagei, Gorilla= (Fig. 457).--The caecum is large,
distinctly sacculated, presenting a decided curve with the concavity
directed toward the left. The appendix is implanted at the center of the
blunt apex of the pouch, the caecal sacculations on each side of the root
of the appendix being of nearly equal size (folds and fossae, cf. p.
269).
[Illustration: FIG. 458.--_Simia satyrus_, orang. Caecum and ileo-colic
junction; ventral view. (Drawn from Columbia University Museum
preparation No. 716.) 1. Appendix. 2. Ventral vascular fold.]
[Illustration: FIG. 459.--_Simia satyrus_, orang. Caecum and ileo-colic
junction; dorsal view. (Drawn from Columbia University Museum
preparation No. 716). 1. Appendix. 2. Intermediate non-vascular fold.]
=3. Simia satyrus, Orang-outang.=--Figs. 458 and 459 represent
respectively the ventral and dorsal views of the caecum and ileo-colon in
a nearly adult male specimen of orang, about 41/2 feet high.
The caecum is funnel-shaped, gradually narrowing to the origin of the
appendix from its apex, which is carried upwards to the left by the
well-marked crescentic curve of the pouch. The sweep of the funnel to
the left and upwards is characterized by the curved course of the
ventral longitudinal muscular band (Fig. 458), whose fibers spread out
over a surface 3 cm. wide. The apex is thus placed behind the terminal
ileum close to its entrance into the large intestine.
At the level of the upper margin of the ileo-colic junction the narrow
pointed termination of the caecum passes gradually into the beginning of
the appendix (Fig. 459).
The appendix measures along its free border 22.6 cm. It follows the
direction of the caecal curve for 2.7 cm., at which point it appears
somewhat constricted and takes an abrupt bend downwards for 4.3 cm.;
curving again upwards for 7.5 cm., it turns downward a second time for
5.4 cm. and terminates in a hook-like extremity 2.7 cm. long (Fig. 459).
[Illustration: FIG. 460.--_Troglodytes niger_, chimpanzee. Ileo-colic
junction and caecum; ventral view. (Drawn from Columbia University Museum
preparation No. 675.) 1. Appendix. 2. Intermediate non-vascular
ileo-caecal fold. 3. Colon.]
[Illustration: FIG. 461.--_Troglodytes niger_, chimpanzee. Dorsal view.
(Drawn from Columbia University Museum preparation No. 675.) 1.
Appendix. 2. Intermediate non-vascular ileo-caecal fold. 3. Dorsal
vascular fold.]
=4. Chimpanzee, Troglodytes niger.=--Figs. 460 and 461 represent the
ventral and dorsal view respectively of the caecum and ileo-colon in a
young specimen.
The caecum is curved to the left and the lowest point of the pouch is
formed by the right lateral and ventral wall of the gut, but the extreme
crescentic bend which carries the origin of the appendix up and to the
left behind the ileo-colic junction is not yet developed in the young
animal; on the other hand this character of the caecum is typically
apparent in Figs. 462 and 463, taken from an adult individual of the
same species.
[Illustration: FIG. 462.--_Troglodytes niger_, chimpanzee. Ileo-colic
junction and caecum; ventral view. (Drawn from Columbia University Museum
preparation No. 1083.) 1. Ventral vascular ileo-caecal fold.]
[Illustration: FIG. 463.--_Troglodytes niger_, chimpanzee. Ileo-colic
junction and caecum; dorsal view. (Drawn from Columbia University Museum
preparation No. 1083.) 1. Appendix.]
[Illustration: FIG. 464.--_Troglodytes niger_, chimpanzee. Ileo-colic
junction and caecum. (Drawn from Columbia University Museum preparation
No. 1525.)]
This extreme curve is well seen in the ventral view in Figs. 462 and
464, the latter taken from a large adult specimen. Seen from behind in
Fig. 463 the sharp bend or kink in the lumen of the caecal pouch produced
by this curve is striking and resembles the arrangement of certain types
of adult human caeca (p. 247).
II. PHYLOGENY OF THE TYPES OF ILEO-COLIC JUNCTION AND CAECUM IN THE
VERTEBRATE SERIES.
The segments of the alimentary canal illustrate very clearly the
adaptation of structure to function. Diversity of kind and quantity of
food habitually taken and variations in the rapidity of tissue
metabolism produce marked morphological modifications in different
forms. This is more especially the case with the junction of the mid-
and hindgut, the site of development of the caecal apparatus and of
structural alterations of the large intestine possessing a similar
physiological significance. No other portion of the visceral tract,
with the possible exception of the stomach, illustrates more completely
the result of physiological demand on the development of anatomical
structure and the morphological possibilities of departure, progressive
and retrograde, from a common primitive type in accordance with varying
conditions of alimentation.
In cooerdinating, from the morphological standpoint, the structural
differences encountered in this segment of the alimentary canal, two
facts become apparent.
1. In the first place the serial study of the ileo-colic junction, as we
can briefly define the region in question by borrowing the terminology
of anthropotomy, reveals a limited number of principal structural types
from which by successive gradations the vast variety of individual forms
may be derived.
[Illustration: FIG. 465.--Schematic table of the vertebrate types of
ileo-colic junction.]
(In the schematic Fig. 465 the fundamental types and their derivatives
are indicated. In the following the individual forms illustrating these
types are referred to this schema in brackets.)
2. The observer will be impressed by the fact that representatives of
all the main types of ileo-colic junction are found within a very
limited zoological range, as within the confines of a single order.
Examples of this are furnished by the Marsupialia and, to a lesser
extent, by the Edentata. The members of these zoological groups, while
united by certain common anatomical characters, such as the reproductive
system and dentition, differ widely in habit and in the kind and
quantity of the food normally taken. These differences in the method of
nutrition have impressed their influence on the structure of the
alimentary canal and have led to the evolution of varying and divergent
types of ileo-colic junction. The study of this segment of the
intestinal tract can therefore elucidate the mutual relationship of the
vertebrate groups only to a limited degree and in special cases. On the
other hand, it renders very clear the fundamental structural ground-plan
common to all vertebrates and accentuates the specialized modifications
of this plan which develop in response to the physiological environment.
Moreover, such a review serves to reveal the significance of
rudimentary and vestigial structures, such as the human vermiform
appendix and the serous and vascular folds connected with the same.
Throughout the entire vertebrate series the alimentary canal is found to
respond with great readiness in its structure to varying grades of
functional demand. This fact becomes still more apparent if the inquiry
is not limited strictly to the region of the ileo-colic junction but
takes into account likewise the structural modifications of similar
physiological significance in other segments of the alimentary tract.
A caecal pouch or diverticulum in some form at the junction of mid- and
hindgut is a very common and widely distributed mammalian character. The
activity of the tissue-changes in warm-blooded animals, and the
consequent necessity for a rapid and complete digestive process, account
for the structural modifications of the alimentary tract so commonly
encountered among these forms. On the other hand, in the lower
cold-blooded vertebrates, notably in fishes and amphibians, the
metabolism is slow and the alimentary canal usually simple.
Specifically, the caecum appears as a pouch or diverticulum in which
food-substances, already partially digested and mixed with the
secretions of the small intestine, are retained until their elaboration
is completed and the nutritive value of the food ingested is secured for
the organism. Consequently the most complicated and highly developed
caecal apparatus is found among mammalia in the Herbivora, such as the
Ungulates and Rodents, whose food contains a comparatively small amount
of nutriment in ratio to its bulk, and hence requires considerable
elaboration before absorption. On the other hand the caecum appears as a
reduced or even rudimentary organ, or defaults entirely, in Carnivora
whose food is concentrated and easily assimilated, containing only a
small amount of non-nutritive material.
The function of the caecal apparatus may be defined as follows:
1. It provides space for the retention of partly digested substances,
and of such as are difficult of digestion, mixed with the secretions of
the preceding intestinal segment, until the digestive elaboration is
completed.
2. It increases the intestinal mucous surface for absorption, and may
develop, in certain cases, special localized areas of lymphoid tissue.
These two functional characters may be shared by other segments of the
intestinal tract, which undergo corresponding structural modifications.
It is only necessary to refer in this connection to the extreme
morphological variations encountered in the stomach. The intestinal
canal proper, however, in many instances exhibits structural
peculiarities which possess the functional significance of the caecal
apparatus. Thus the projection into the lumen of the canal of a series
of mucous folds, or the development of a continuous spiral mucous valve,
evidently serves the double purpose of prolonging the period during
which the intestinal contents are retained, and of increasing the
intestinal mucous surface for absorption.
[Illustration: FIG. 466.--_Squalus acanthius_, dog-fish. Alimentary
tract, spleen, pancreas. (Drawn from Columbia University Museum
preparation No. 1405.)]
[Illustration: FIG. 467.--_Galeus canis_, dog-shark. Alimentary tract
opened, showing spiral intestinal valve. (Drawn from Columbia University
Museum preparation No. 1429.)]
[Illustration: FIG. 468.--_Ceratodus forsteri_, Australian lung-fish.
Intestinal canal with spiral valve. (Columbia University Museum, No.
1645.)]
[Illustration: FIG. 469.--_Python molurus_, Indian python. Mid-gut,
distended and fenestrated to show spiral course of lumen. (Columbia
University Museum, No. 725.)]
This spiral mucous fold is encountered in the straight intestinal canal
of the Cyclostomata (Fig. 465, _IV_, 1, and Fig. 310), Selachians (Figs.
466 and 467) and Dipnoeans (Fig. 468). Phylogenetically it is a very old
structure, for evidences of its existence are found in the fossil
remains of some Elasmobranchs. In the Ostrich (Fig. 341) the enormously
developed caeca possess the same spiral mucous fold in the interior. The
direct combination of the caecum and spiral fold is again seen in certain
mammalia, as in _Lepus_ (Fig. 387). In some Ophidians the same
physiological purpose is served by the manner in which the convolutions
of the long intestine are bound together by a subperitoneal arachnoid
membrane. The lumen of the canal is thus made to assume a spiral course
(Figs. 331 and 469). The mucous folds of the human intestine, both the
valvulae conniventes and the crescentic folds of the large intestine,
represent the same spiral valve, perhaps modified and influenced by the
erect posture of man (Figs. 470-475).
[Illustration: FIG. 470.--Human small intestine, opened to show valvulae
conniventes. (Columbia University Museum, No. 1841.)]
[Illustration: FIG. 471.--Human large intestine, showing colic taenia and
plica. (Columbia University Museum, No. 1848.)]
[Illustration: FIG. 472.--Human large intestine, opened and in section,
showing colic plicae. (Columbia University Museum, No. 1847.)]
[Illustration: FIG. 473.--_Cynocephalus anubis_, olive baboon. Large
intestine, with cross-section showing colic taenia and plicae. (Columbia
University Museum, No. 26/1168.)]
[Illustration: FIG. 474.--Comparison of portion of human transverse
colon with distal segment of rabbit's large intestine, showing same
arrangement of longitudinal muscular bands (colic taenia) and colic
sacculations. (Columbia University Museum, No. 1589.)]
[Illustration: FIG. 475.--_Felis leo_, lion. Large intestine, with
transverse section, showing smooth carnivore lumen, without sacculations
or plicae. (Columbia University Museum, No. 1600.)]
A second modification of the intestinal canal, suggesting the same
physiological interpretation as the ileo-colic caecum, is presented by
the so-called pyloric caeca or appendices of many Teleosts and Ganoids
already referred to (p. 119). While these structures in some forms very
probably have assumed a secretory function (Figs. 476 and 477), they
evidently act in others as diverticula in which material undergoing
digestion is retained, while they increase at the same time the
intestinal mucous secretory and absorbing surface (Figs. 478 and 479).
They thus correspond physiologically to the ileo-colic caecum. In this
connection it is interesting to note that in Ganoids, which possess both
the pyloric appendices and the spiral valve, the two structures develop
in inverse ratio to each other, indicating their functional identity. In
the serial review of the structure and significance of the vertebrate
caecum and ileo-colic junction these functionally allied modifications of
other segments of the intestinal canal deserve notice.
[Illustration: FIG. 476.--Pyloric caeca of _Gadus callarias_, codfish.
(Columbia University Museum, No. 1825.)
_A._ Bound together by connective tissue and blood-vessels.
_B._ Dissected to show confluence of caeca to form a smaller number of
terminal tubes of larger calibre entering the intestine.]
[Illustration: FIG. 477.--Alimentary canal of _Accipenser sturio_,
sturgeon. Numerous pyloric caeca are bound together to form a gland-like
organ.
In the smaller upper figure on the left the stomach, mid-gut, and
pyloric caeca are seen in section, showing the lumen of the latter and
their openings into the mid-gut.
The lower left-hand figure shows the mid- and end-gut in section, the
latter provided with a spiral mucous valve. (Columbia University Museum,
Nos. 1826, 1827, and 1828.)]
[Illustration: FIG. 478.--Stomach, duodenum, and pyloric caeca of
_Lophius piscatorius_, angler. (Columbia University Museum, No. 1824.)]
[Illustration: FIG. 479.--_Pleuronectes maculatus_, window-pane. Stomach
and mid-gut with pyloric caeca and hepatic duct. (Columbia University
Museum, No. 1432.)]
The study of the vertebrate ileo-colic junction proper begins both
ontogenetically and phylogenetically with the consideration of the
primitive type in which the alimentary tube is not differentiated into
successive segments and in which consequently no distinction between
mid- and hindgut is found (Fig. 465). An example of this primitive
condition is presented by the Cyclostomata, in whom the alimentary canal
traverses the coelom cavity as a straight non-differentiated cylindrical
tube. Fig. 310 shows the alimentary canal of the Lamprey, _Petromyzon
marinus_, and it will be observed that the intestine is provided with
the spiral mucous fold above mentioned.
From this fundamental type the following main groups are to be derived:
I. Symmetrical Form of Ileo-colic Junction. Mid- and Endgut in Direct
Linear Continuity. (Fig. 465, I.)
1. _Ileo-colic junction marked externally by an annular constriction,
corresponding to a ring-valve with central circular opening in the
interior_ (Fig. 465, _I_, 1).
This form is encountered in many Teleosts. The projecting annular mucous
fold resembles the pyloro-duodenal valve.
Figs. 311-315 illustrate the structures in representative Teleosts.
Among the higher forms this type of ileo-colic junction is encountered
in the simple alimentary canal of many Amphibians (Figs. 318-320). Among
Reptiles it is found in certain lizards, as in _Heloderma suspectum_,
the gila monster (Fig. 322). This animal lives almost entirely upon
bird's eggs, and its simple and reduced ileo-colic junction contrasts
strongly with the highly developed and complicated caecal apparatus of
the phytophagous lizards, as _Iguana_ (Figs. 326-330), affording one of
the most striking illustrations of the effect which the character of the
food habitually taken has on the structure of the alimentary canal in
forms otherwise closely allied.
The same type of ileo-colic junction, as a reduction form, occurs in the
arctoid group of Carnivora among Mammalia (cf. p. 212).
2. _Differentiation in caliber of large and small intestine.
Funnel-shaped ileo-colic transition._
This type, compared with the preceding, is characterized (Fig. 465, _I_,
2) by the greatly increased caliber of the large intestine, resulting in
a funnel-shaped transition between mid- and hindgut, the small intestine
continuing into the colon at the apex of the funnel.
Examples of this type are presented by several Edentates, _Myrmecophaga
jubata_, the great ant-eater (Fig. 356), and _Choloepus didactylus_, the
two-toed sloth (Fig. 357).
3. _Abrupt demarcation of small and large intestine with caliber
differentiation_ (Fig. 465, _I_, 3).
The small intestine is still central at the ileo-colic junction,
_i. e._, the axis of its lumen is continuous with the central axis of
the colic lumen. In place of the gradual funnel-shaped transition of
the preceding type the demarcation is abrupt.
An example of this form is furnished by another Edentate, _Tatusia
peba_, the nine-banded armadillo (Fig. 358).
Among reptiles a similar well-marked ileo-colic transition is
encountered in _Alligator mississippiensis_ (Fig. 321).
4. _Colic pouch prolonged back on each side of the ileo-colic junction,
producing symmetrical colic caeca_ (Fig. 465, _I_, 4).
A growth of the colic tube cephalad, on each side of the junction with
the midgut, leads to the formation of this type, characterized by the
presence of two symmetrical globular caecal pouches. In its simplest form
this condition is illustrated by the double colic caeca of another
armadillo, _Dasypus sexcinctus_ (Fig. 359).
The bifid caecal apparatus of the American manatee (Fig. 366) belongs to
the same group.
5. _Caecal pouches of the birds_ (Fig. 465, _I_, 5).--A continuation of
the backward extension of the bilateral colic pouches leads to the
production of the typical double avian caeca in a greater or lesser
degree of development. Frequently the caeca differentiate more completely
from the colon, appearing as pouches of varying capacity joined to the
large intestine by a narrower neck.
Figs. 334-341 show the well-developed pouches as they appear in
representative avian types, while Fig. 333 illustrates the reduction of
the caecal apparatus encountered in many carnivorous birds.
6. Among mammalia _Cyclothurus didactylus_ (Fig. 360), the little
ant-eater, furnishes an example of double symmetrical globular caeca,
connected with the colon by a narrow neck (Fig. 465, _I_, 6). Reference
to the schema given in Fig. 465 will show that the types heretofore
examined all have the following common character:
They appear derived from the primitive type by a differentiation in the
caliber of the gut and by the gradual development of _symmetrical
bilateral_ caecal pouches, resulting in central median implantation of
the small intestine and its direct continuity with the colon.
II. Asymmetrical Development of a Single Caecal Pouch, Lateral to the
Ileo-colic Junction, Mid- and Endgut Preserving Their Linear Continuity.
(Fig. 465, II.)
In the second general group the symmetry of the ileo-colic junction is
disturbed. The following types are encountered, forming a series of
successive stages:
1. The increase in the caliber of the large intestine is chiefly marked
along the border opposite to the mesenteric attachment, resulting in a
greater degree of convexity in this part of the intestinal wall (Fig.
465, _II_, 1). Among Reptilia this condition is found in the ileo-colic
junction of some of the pond-turtles, as _Pseudemys elegans_ (Fig. 323),
while a mammalian example is furnished by the three-toed sloth,
_Arctopithecus marmoratus_ (Fig. 363).
2. An increase of this lateral extension of the colon leads to the
formation of a single lateral caecal pouch (Fig. 465, _II_, 2) such as is
seen in another Edentate, _Tamandua bivittata_ (Fig. 364), among
Mammalia, and in certain Ophidians among Reptiles, as in the _Anaconda_
(Figs. 331 and 332).
3. Prolongation of the pouch and reduction in caliber lead to the
formation of the slender lateral caecum found in all the Monotremes
(Figs. 345-347, Fig. 465, _I_, 3). In its general appearance the caecum
of these singular animals bears a close resemblance to the caecal pouches
of many birds.
4. Direct continuity of small and large intestine, with lateral colic
caecum, extending along the convex free border of the terminal ileum and
slightly convoluted at the extremity (Fig. 465, _II_, 4), characterizes
the entire group of the _Cebidae_ among the new-world monkeys. The caecum
in these animals is a comparatively long pouch, nearly equalling in
caliber the remainder of the intestine, occupying a distinctly _lateral_
position, with the terminal portion rounded and slightly recurved (Figs.
453 and 454).
5. The _Cynoid group_ of Carnivora, including the dogs, wolves, jackals
and foxes, presents a similar relative position of small and large
intestine and caecum (Fig. 465, _II_, 5). The caecum, compared with that
of _Cebus_, is longer and more highly convoluted (Fig. 397). Variations
encountered in certain forms indicate reversions to a more primitive
type. Thus Fig. 398 shows the usual form in the dog, while Fig. 399
exhibits an occasional type in the same animal. The caecum here is less
twisted and indicates the probable derivation of the more commonly
encountered type.
III. Rectangular Ileo-colic Junction with Direct Linear Continuity of
Caecum and Colon. (Fig. 465, III.)
The third general group, to which the large majority of Mammalia belong,
is characterized in its typical form by a right-angled entrance of ileum
into large intestine and by the direct caudal prolongation of the colon
into a caecal pouch of nearly uniform caliber with globular termination.
The axes of the small and large intestine are not in the same line as in
the two former groups, but are placed nearly at right angles to each
other. With this change in the direction of the main intestinal segments
the caecum ceases to be a lateral appendage to the canal and appears as a
caudal prolongation of the colon beyond the ileo-colic junction (Fig.
465, _III_). The type-form of this group is encountered among the
herbivorous Marsupialia, such as the kangaroos and opossums. Fig. 350
shows the ileo-colic junction and caecum in the rock wallaby, _Halmaturus
derbyanus_, and Fig. 348 the same structures in our common opossum,
_Didelphis virginiana_. The majority of the remaining mammalian forms
depend upon modifications of this type, either in the direction of
reduction of the caecal apparatus, or of increased development with
concomitant structural changes of similar physiological import in the
proximal portion of the colon.
The following subdivisions of the general group may be established.
A. 1. The caecum is long, markedly curved or uncinate, with the
crescentic medial margin turned toward the free border of the terminal
ileum. The entire pouch usually diminishes gradually in caliber to its
termination (Fig. 465, _III_, _A_, 1). This type is encountered in a
large group of new-world monkeys, including the marmosets and howlers.
Fig. 440 shows the structures in _Hapale jacchus_, one of the marmosets,
and Fig. 443 illustrates the typical caecum of this form in _Ateles
ater_, the black-handed spider monkey.
2. The caecum and appendix of man and of the anthropoid apes can be
regarded as a reduction form of this type (Fig. 465, _III_, _A_, 2).
Arrest of development of the terminal portion converts the distal
segment of the caecal pouch into an appendix whose relation to the apex
of the funnel-shaped proximal segment or caecum proper is seen in its
pure form in the human embryo (Figs. 512 and 525). With the further
development of the caecum the sharper demarcation between it and the
appendix results (Figs. 517 and 518). The displacement of the root of
the appendix cephalad and to the left, toward the lower margin of the
ileo-colic junction, as it is usually seen in adults, is due to the
relatively greater growth of the right terminal sacculation of the caecum
compared with the left (cf. types of caeca, p. 248). Throughout these
changes the initial crescentic curve of the caecum, turning its concavity
upwards and to the left, can be recognized by tracing the course of the
longitudinal colic muscular bands. The caeca and appendices of the
anthropoid apes present the same characters. The structures in the
orang, chimpanzee, gorilla and gibbon are shown in Figs. 455-464.
B. The AEluroid and Arctoid groups of the Carnivora and the Pinnipedia
constitute a very complete and instructive series illustrating the
gradual reduction of the caecum from the capacious pouch of the primitive
type and its final complete elimination from the organism (Fig. 465,
_III_, _B_).
In _Hyaena_ (Fig. 416), the large caecum with undiminished caliber of the
terminal portion persists in its full development, as seen in the
Marsupials furnishing the fundamental type (Fig. 465, _III_). The same
type of caecum is found in the lion (Fig. 417), the only true cat in
which the caecal apparatus has not undergone extensive reduction.
Phylogenetically the presence of a capacious and uniform caecal pouch in
these two animals is exceedingly important and indicates that this type
of caecum represents the ancestral form common to the aeluroid carnivore
group, which, in the remaining living representatives, has become
reduced in response to the influence which the character of the food has
on the structure of this portion of the intestinal canal. The two
instances of persistence of the primal type are all the more important
as exceptions to the rule which is otherwise universal throughout the
group.
1. The first example of this reduction (Fig. 465, _III_, _B_, 1) is
encountered in the Aard-Wolf, _Proteles lalandii_, a near relative of
hyaena (Fig. 406). The caecum in this animal is considerably shortened,
although still of fairly large and uniform caliber.
A similar type of caecal reduction is encountered in the Pinnipede
Carnivora. Fig. 396 shows the ileo-colic junction and the short blunt
caecum of the harbor seal, _Phoca vitulina_.
2. The caecum of the typical Felidae, other than the lion, is short and
the terminal portion much reduced in caliber, constituting in many forms
a species of pointed rudimentary appendix (Fig. 465, _III_, _B_, 2).
Fig. 401 represents the typical feline caecum as seen in the puma, _Felis
concolor_. Among the smaller AEluroid Carnivora related to the true cats,
as the civets and ichneumons, the terminal reduction of the short caecum
is still more marked, as seen for example in _Herpestes griseus_ (Figs.
404 and 405).
3. In the Arctoid group of Carnivora (Fig. 465, _III_, _B_, 3 and 4) the
reduction of the caecal apparatus has been carried to the complete
elimination of the pouch, restoring the primitive type of a straight
intestinal tube without diverticulum as encountered above in some of the
Edentates (Figs. 356 and 357).
In some forms allied to the true bears, such as _Procyon_, _Bassaris_,
_Cercoleptes_, _Taxidea_ and _Nasua_, the ileo-colic junction is marked
externally by a slight constriction and internally by the projection of
an annular pylorus-like valve (Figs. 407-409). The transition from the
thin-walled ileum to the thick muscular walls of the large intestine is
abrupt. The latter is very short and usually increases in caliber as it
approaches the anal orifice. The mucosa of the terminal ileum presents
very commonly one or two large oval areas of agminated follicles near
the ileo-colic junction. The mucous membrane of the large intestine is
thrown into prominent longitudinal folds. Fig. 408 shows the intestine
of the brown coati, _Nasua rufa_, opened on each side of the ileo-colic
transition.
In some of the Arctoidea, as _Procyon_ and _Nasua_, the beginning of the
colon just beyond the ileo-colic valve is bowed out opposite the
mesenteric border indicating the original site of the eliminated caecum,
and recalling the arrangement of the intestine encountered above in
_Arctopithecus_ among the Edentates (Figs. 363, 407, 412, and 465,
_III_, _B_, 3). Moreover, in the same forms rudimentary vascular and
serous folds around the ileo-colic junction, corresponding to similar
structures found in connection with a well-developed caecal apparatus in
other mammalia, point to the former existence of a caecum.
4. In the typical Ursidae even these remnants and traces of a caecal pouch
have disappeared and the intestinal canal preserves a uniform caliber,
without any differentiation of large and small intestine (Figs. 414 and
415, Fig. 465, _III_, _B_, 4).
C. The last subdivision of the third main group contains forms in which
the large uniform pouch of the primal type appears moderately reduced in
length and sacculated, terminating either in a blunt extremity or
carrying a distal constricted and rudimentary segment as an appendage.
1. The first of these types is encountered in the Old World cynomorphous
monkeys. In all of these animals the caecal pouch is wide but
comparatively short, of nearly uniform caliber and sacculated like the
rest of the colon, of which it forms the direct caudal continuation
(Fig. 465, _III_, _C_, 1). The terminal portion of the pouch is usually
blunt, globular and rounded (Figs. 428, 430 and 431), in a comparatively
small number of forms slightly pointed (Figs. 427 and 437).
2. In the second group the terminal reduced portion persists either as a
fairly distinct appendage, or in the form of a tapering pointed
extremity into which the caecal pouch proper is continued (Fig. 465,
_III_, _C_, 2). This type is encountered in certain non-ruminant
Ungulates. An example of the first condition is furnished by the caecal
apparatus of the peccary (_Dicotyles torquatus_) (Fig. 370), while the
structures in _Tapirus americanus_ (Fig. 377) illustrate the second
form.
=IV. Caecal Apparatus Combined with Structural Modifications of the
Proximal Colon of Similar Physiological Significance. (Fig. 465, IV.)=
The fourth general mammalian group comprises forms in which the caecal
pouch is large, with or without terminal appendage, while in addition
the large intestine develops structural modifications which possess the
general functional significance of the caecal apparatus. This highly
developed and complicated structure of the alimentary canal indicates
that the habitual food of these animals is bulky and difficult of
digestion. Accordingly we find the group composed in main of the
majority of the Ungulates and Rodents (with the exception of _Myoxus_),
forms in which the diet under natural conditions is purely herbivorous.
Other mammalian orders, however, also furnish representatives of this
type of caecal apparatus, the conditions as regards character and
quantity of food habitually taken corresponding to those encountered
among the Ungulates and Rodents. Thus the _Phalangers_ among Marsupials
(Fig. 352), _Galeopithecus_ (Fig. 419) as an exceptional form among the
Insectivora, and certain lemurs among Primates (Figs. 420-425) present
examples of a highly developed and specialized type of caecal apparatus.
The intestinal tract of these forms must therefore be considered from
two points of view:
I. The caecum proper.
II. The analogous structural modifications of the proximal segment of
the colon.
=I. CAECUM PROPER.=
The pouch of the Ungulates and Rodents, taking these forms as the
typical representatives of the entire group, is usually of very large
size compared with the rest of the alimentary canal. Two types are
found:
1. Large capacious smooth caecal pouch of uniform caliber (Fig. 465,
_IV_, 2). This form is met with in the Muridae among Rodents and is
illustrated in Fig. 393 showing the caecum of _Mus decumanus_, var.
_albinus_, the white rat. Fig. 392 represents the entire alimentary
canal of the meadow mouse, _Arvicola pennsylvanicus_, and indicates the
proportion which the caecal apparatus bears to the remainder of the
intestinal tract. The typical caecum of the Ungulates is shown in Fig.
371, taken from _Capra aegagrus_, the bezoar goat, and in Fig. 372, taken
from a preparation of _Boselaphus tragocamelus_, the Nilghai.
2. The caecal pouch is large, markedly crescentic in shape, sacculated,
or provided in the interior with a more or less complete spiral valve,
and reduced in caliber in the terminal segment, forming at times a
pointed appendix (Fig. 465, _IV_, 3). This form is encountered typically
among certain Rodents, as in _Castor fiber_, the beaver (Figs. 381 and
382), and _Erethizon dorsatus_, the Canadian porcupine (Figs. 383 and
384), but is not confined to this order. Thus caeca of very similar
structure are found among the Marsupials, as in _Phascolarctos_ and
_Cuscus_ (Fig. 352). In some of these forms the terminal reduction of
the caecum is very marked, resulting in a long narrow segment of the
pouch tapering to a sharp point. It is significant to note in this
connection that in one member of the marsupial order, the wombat
(_Phascolomys_), this tendency to terminal reduction of the pouch has
led to the development of a caecum and appendix identical in structure
and arrangement with the corresponding parts of man and the anthropoid
apes (Fig. 354). This is merely another illustration of the fact,
evidenced throughout the entire vertebrate series, that a primal
type-form of caecal apparatus, in responding to the conditions which
influence the development of structural modifications, will produce
identical specific types in animals otherwise widely separated in the
zoological series.
Thus again the form of caecum under discussion, found in many Rodents and
certain Marsupials, is encountered in the only Insectivore possessing a
caecum (_Galeopithecus_) (Fig. 419), and in several _Lemuroidea_ among
Primates (Figs. 420-425).
II. Structural Modifications of the Proximal Segment of the Colon
Analogous in Their Functional Significance to the Caecal Apparatus.
In these forms, in addition to the caecal apparatus proper, certain
accessory structural modifications of the adjacent large intestine are
developed which possess the physiological significance of the caecal
apparatus in general, since they serve to increase the extent of the
intestinal mucous surface and to prolong the period during which the
contents of the canal are retained for elaboration and absorption. These
modifications, which appear most fully developed in certain Rodents and
Ungulates, are of two kinds.
1. The development of the colic mucous membrane in the form of a
projecting fold or valve usually surrounding the lumen spirally (Fig.
465, _IV_, 1). The significance and phylogeny of this spiral fold has
been considered above (cf. p. 193). Functionally this reduplication must
be regarded as in general equivalent to the caecal apparatus proper, in
producing an increased surface for secretion and absorption and in
retarding the movement of intestinal contents. The caecal pouch evidently
acts as a reservoir in which partly digested substances, mixed with the
secretions of the small intestine, are retained while the slow processes
of digestion and absorption, already inaugurated in the antecedent
segment of the canal, are completed. It is reasonable to suppose that
the system of projecting mucous folds and reduplications encountered in
the colon beyond the caecum have a similar physiological import.
Moreover, in certain forms the caecum itself is provided with a similar
spiral mucous fold, as in the instances already mentioned of _Lepus_
among mammalia (Fig. 381) and of the Ostrich among birds (Fig. 341). We
have seen above (cf. p. 193) that the spiral intestinal valve is
encountered very early in the vertebrate series, in forms in which the
alimentary canal is but slightly, or not at all differentiated, short
and straight in its course. In these forms the evident purpose of the
spiral fold is to retard the movement of the intestinal contents and to
increase the area of the secretory and absorbing surface. As a
structural modification possessing this character we saw the fold in
the Cyclostomata, Selachians and Dipnoeans (Figs. 310, 466, 467 and 468)
and in certain Ophidians (_Python_ and _Anaconda_, Figs. 331 and 469).
Among Mammals it is found in certain Rodentia in two forms:
(_a_) In some of the Muridae, as _Arvicola_ (Fig. 394), the mucous
membrane of the large globular caecal pouch is smooth, but the proximal
segment of the colon, immediately beyond the ileo-colic junction,
develops the spiral fold (Fig. 465, _IV_, 2).
(_b_) In other forms, as in the hares (Fig. 465, _IV_, 3), the greater
part of the caecum carries a typical spiral fold, continued up to the
root of the terminal appendage (Fig. 388), in which segment the mucous
membrane is devoid of folds, but studded thickly with lymphoid
follicles. Beyond the caecum proper the spiral fold is continued in the
opposite direction into the proximal segment of the colon, which is
large and capacious and evidently shares both the physiological and
morphological characters of the caecum proper, forming so to speak an
accessory caecal chamber. Beyond what we thus might term the caecal
division of the colon the large intestine becomes reduced in caliber,
and the previously continuous spiral fold becomes broken up into
separate semilunar haustral plicae, corresponding to the superficial
constrictions between the colic cells. In structure this distal segment
of the rabbit colon closely resembles the human large intestine (Fig.
474).
One of the most marked examples of this secondary modification of the
colon is presented by the intestinal canal of another Rodent, _Lagomys
pusillus_ (Fig. 391).
The caecum of this animal is long, curved, provided with a well-developed
spiral fold. The terminal segment of the pouch is reduced to an
appendix, with smooth mucosa containing adenoid tissue, as in the
rabbit. A second adenoid appendix, representing the globular saccus
lymphaticus of the rabbit, is derived from the caecum at the ileo-colic
junction. The first segment of the colon beyond the ileo-colic junction
is dilated and sacculated, the caecal mucous fold being prolonged into
it. This is succeeded by a narrow smooth-walled second segment. The
third division of the colon is again dilated and sacculated,
followed by a short fourth smooth-walled section. A fifth stretch is
again provided with colic cells, beyond which the terminal segment
continues of uniform caliber and with smooth walls to the vent. The
colon therefore presents three distinct sacculated portions whose
structural modifications suggest that they function in the same sense as
the caecal pouch proper. In man and in other Primates the crescentic
colic plicae are disposed in a more or less evident spiral manner around
the axis of the intestine, and it is not difficult to recognize in them
the modified remnants of the typical spiral valve of lower forms. On the
other hand, in conformity with the general reduction of the caecal
apparatus, the mucous membrane of the large intestine in Carnivora is
smooth and devoid of any trace of the spiral fold (Fig. 475).
2. The second structural modification of the large intestine, associated
in functional significance with the caecal apparatus, depends upon the
increase in the length of the proximal segment of the colon beyond the
ileo-colic junction and the twisting or coiling of this segment in a
more or less complicated definite manner, usually in the form of a
spiral, the individual turns of the coil being held in place by the
peritoneal connections. The proximal colon thus modified is admirably
adapted to retard the movement of contents not yet completely digested
and to increase the absorbing surface of the intestine, and hence is
functionally allied to the caecal apparatus.
This colic modification is found in its highest degree of development in
the ruminant Ungulates, whose caecal pouch proper is also enormously
developed. In these animals the colon immediately beyond the ileo-caecal
junction is arranged in the form of a double spiral, the afferent
(caecal) and efferent (colic) tubes alternating, and continuous with each
other in the center of the coil (Fig. 465, _IV_, 5). Examples of this
type of spiral colon are shown in Fig. 373 (_Bos indicus_), Fig. 374
(_Cervus sika_), Fig. 375 (_Ovis aries_), Fig. 376 (_Oryx leucoryx_).
Ontogenetically the complicated spiral colon of the ruminants starts as
a simple loop of the proximal colon, which, with the further rapid
growth of this segment of the intestine, is bent to produce the turns of
the coil as shown in the schematic Figs. 480-482. Phylogenetically the
same gradual development can be traced in the vertebrate series. Perhaps
the earliest tendency to structurally modify the intestine in the
direction named is found in the manner in which the intestinal coils are
bound together by the subperitoneal arachnoid in many Ophidians (Fig.
331). Further in the Manidae among the Edentates there is no caecal pouch,
but the intestine at the ileo-colic junction is twisted into a figure 8
and held in this position by the peritoneal connections (Figs. 362 and
465, _IV_, 4). In certain Marsupials with well-developed caecal pouches,
such as _Phascolarctos_ and the Vulpine Phalangers (Figs. 351 and 352),
the colon immediately beyond the ileo-colic entrance is sacculated and
bent in the form of a short loop. In the tapir (Fig. 377), the proximal
segment of the colon forms a simple loop, whose afferent and efferent
limbs are closely bound together. The arrangement of the large intestine
in this animal illustrates the early embryonal stage in the development
of the complete ruminant spiral coil (cf. Fig. 480).
[Illustration: FIGS. 480-482.--Schematic representation of three stages
in the development of the ungulate spiral colon.]
The condition encountered in some Rodents presents a more advanced
stage. Thus the large intestine in the agouti (_Dasyprocta agouti_),
shows the development of the spiral coil advanced as far as the second
turn of the original loop (Figs. 389 and 390). It is readily seen that
continued growth of this segment of the intestine leads to the formation
of the complete colic spiral as found in the typical Ungulates.
The same arrangement of the large intestine obtains in certain Lemurs
among the Primates. Thus the proximal colon of the Slow Lemur
(_Nycticebus tardigradus_) is seen in Figs. 421 and 422 to present a
typical spiral coil, and similar conditions are encountered in other
members of the suborder.
=V. Caecal Apparatus and Colon in Hyrax.=
We have left for our final consideration the aberrant and unique
mammalian type found in _Hyrax_ (Fig. 378). In this remarkable little
animal the large intestine develops a typical mammalian sacculated caecum
at the ileo-colic junction, and in addition is provided further on with
two symmetrical pointed lateral colic caeca of large size. It is quite
true that this arrangement is unique among Mammalia, confined entirely
to the members of the suborder formed by the single family of _Hyrax_,
and that no strictly analogous disposition of the alimentary canal is
encountered in the entire vertebrate series. Yet these aberrant
structures are possibly capable of explanation, in regard to the method
of their development, by reference to the caecal apparatus of certain
phytophagous saurians, as _Iguana_ and _Cyclura_. In these forms (Fig.
326-330) the small intestine enters the colon somewhat asymmetrically,
the opening being guarded by a well developed annular valve.
The proximal segment of the large intestine forms an extensive
sacculated pouch. If this is opened (Figs. 328-330) it is seen that the
small intestine leads into a compartment which is separated from the
remainder of the pouch by a valvular diaphragm with central circular
opening. Beyond this primary compartment the colic pouch is incompletely
subdivided by a series of gradually diminishing crescentic folds,
corresponding to the external constrictions between the sacculations.
The entire pouch gradually diminishes in caliber until it passes with a
sharp angular bend into the terminal portion of the endgut. This
terminal segment is differentiated from the elongated colic pouch by the
greater thickness of its muscular walls and by a slight annular
projecting fold in the interior. In considering the intestinal tract of
_Hyrax_ it is conceivable that the unique condition presented by this
animal may be derived from some type conforming in general structure to
the reptilian arrangement of the parts just detailed, as indicated in
the schematic Figs. 483-485. The proximal typical caecal pouch of _Hyrax_
would then correspond to the similar colic pouch of _Iguana_. To explain
the supplementary colic caeca it is necessary to suppose that the
transition of the colic pouch into the terminal hindgut had become well
differentiated, and that on each side of this junction the colic tube
had extended backwards, resulting in the production of the supplementary
bilateral caecal pouches of _Hyrax_.
[Illustration: FIGS. 483-485.--Schematic figures illustrating possible
line of derivation of aberrant mammalian type of alimentary canal
encountered in _Hyrax_.]