Chapter XIV: Part III (2)

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.

(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.

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).

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.

_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.]

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.)]

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).

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_.