Chapter XVI: Part IV (2)

(_b_) _Non-vascular Ileo-caecal Serous Reduplication._--Between the two vascular mesenteric folds a third serous reduplication, carrying no blood vessels, is found passing between the ileum and caecum. This fold begins, in the preparation from which the figure is taken, on the ileum opposite the attached mesenteric border, 2.7 cm. from the ileo-colic junction, and passes for exactly the same distance down on the adjacent left concave surface of the caecum. It is placed a little nearer to the dorsal than to the ventral vascular fold, so that it passes, if the distance between the two vascular folds on the caecum be divided into three parts, at the junction of the dorsal third with the ventral two thirds. The production of this intermediate non-vascular ileo-caecal reduplication, which is of very constant occurrence in the mammalian series, is to be led back to the development of the caecum. When the pouch protrudes from the smooth surface of the embryonic intestine opposite the mesenteric border, it extends backward along the future small intestine and lifts off the serous investment of the gut in the form of a small peritoneal plate filling the interval between itself and the adjacent ileum. A very perfect illustration of this process can be seen in the instance of Meckel's diverticulum shown in Fig. 561. The proximal portion of the diverticulum is here still closely connected to the small intestine along which it extends, both being surrounded by the common visceral peritoneum. The distal part of the diverticulum has separated more completely from the intestine, and in so doing has drawn out the serous investment in the form of the triangular fold which is seen to pass between the free margin of the intestine and the adjacent surface of the pouch. The same process can be followed in its different stages in certain normal mammalian caecal types.

In this connection it may be noted that the production of the caecal vascular folds and their relation to the mesentery is also very perfectly illustrated in some forms of Meckel's diverticulum. Thus in the preparation shown in Fig. 562, a broad triangular serous fold passes from the ileal mesentery to the margin of the diverticulum, carrying the blood vessels which supply the pouch. If the section of the intestine to the left of the figure is regarded as representing the terminal ileum, that to the right the colon, and the diverticulum the caecal pouch, the formation of the fold and its relation to the mesentery, blood vessels and intestine will correspond closely to the ileo-caecal vascular folds.

Fig. 350 shows the ileo-colic junction and caecum of _Halmaturus derbyanus_, the rock kangaroo. The caecum here extends backwards along the free border of the ileum to which it is closely bound by the common investing visceral peritoneum for the greater part of its extent. In another marsupial form, a small species of opossum from Trinidad (Fig. 349), the caecum has separated itself more completely from the adjacent small intestine--thus drawing out the peritoneum into a narrow connecting fold. Finally, in the Virginia opossum (Fig. 348), the ileum has attained the usual position at right angles to caecum and colon. The former pouch is separated from the small intestine by a considerable interval and the angle between the two is filled out by a well-developed triangular serous fold, connecting the free margin of the terminal ileum and the adjacent left border of the caecum.

This is the "intermediate non-vascular" ileo-caecal fold.

Passing now from the condition presented by _Ateles_, with three fully developed and distinct ileo-caecal folds, to the next stage leading up to the normal human arrangement, we find the same illustrated in the caecum of another new-world monkey, _Mycetes fuscus_, the brown howler monkey, shown in the ventral and dorsal views in Figs. 449 and 450. The ventral vascular fold (Fig. 449, 1) is still well developed, the contained ventral branch of the ileo-colic artery descending over the ventral wall of the ileo-colic junction and caecum and supplying both. The dorsal vascular fold (Fig. 450, 2), on the other hand, is nearly completely fused with the intermediate non-vascular reduplication (Figs. 449 and 450, 3), the approximation between these structures exhibited by _Ateles_ having in _Mycetes_ reached the point of actual union, so that the larger dorsal branch of the ileo-caecal artery descends to the apex of the caecum in the following manner: The main post-caecal artery passes over the dorsal surface of the ileo-colic junction included in a short serous fold which corresponds to the dorsal vascular fold of _Ateles_. Beyond the lower border of the ileo-colic junction this fold fuses with the intermediate non-vascular fold, one arterial branch descending along the line of attachment of this fold to the caecum, the other distributed over the dorsal surface of the pouch.

A third type, also taken from the lower Primates, is presented by the caecum of a cynomorphous monkey, _Cercopithecus sabaeus_, the African green monkey, shown in Fig. 432, in the ventral and left aspect with the terminal ileum lifted up. The caecum of this animal is comparatively short, somewhat conical, terminating in a blunt apex. The vascular supply is arranged on the same type as in _Ateles_ and _Mycetes_, _i. e._, a trunk of the ileo-colic artery divides at the ileo-colic notch, one branch descending ventrad, the other dorsad of the ileo-colic junction. The slightly larger size of the dorsal vessel, noted in _Ateles_ and _Mycetes_, has been increased in _Cercopithecus_ until the ventral artery (1) supplies merely the front of the ileo-colic junction and the upper part of the adjoining ventral wall of the caecum, while the larger dorsal vessel (2) descends behind the ileo-colic junction, supplying the same and the entire dorsal and apical portions of the caecum. The relation of these caecal arteries to the peritoneum is moreover different from that encountered in _Ateles_. In place of running in distinct mesenteric folds, as in the latter species, the vessels pass close to the surface of the intestine, merely covered and partly surrounded by slightly redundant visceral peritoneum containing numerous pads of epiploic fat, which bead the course of the vessels at regular intervals. Between the two arteries the intermediate non-vascular fold (2) is seen, presenting much the same arrangement as in _Ateles_ and passing between the left border of the caecum and the adjacent margin of the ileum, nearer to the dorsal larger than to the ventral smaller caecal artery.

We have, therefore, in the three types just considered, the following variations in the arrangement of the vascular and non-vascular folds:

1. (_a_) Ventral and dorsal vascular folds distinct }
and free. Ventral and dorsal caecal }
arteries of nearly equal size. }
(_b_) Intermediate non-vascular fold free on }_Ateles._
both surfaces, placed nearer to the }
dorsal than to the ventral vascular fold. }
2. (_a_) Ventral vascular fold distinct. Ventral }
caecal artery somewhat further reduced }
in size. Dorsal vascular fold distinct }
only over the dorsal surface of the ileo-colic}
junction. At the lower border of }
the ileo-colic junction the dorsal vascular }_Mycetes._
fold fuses with the intermediate non-vascular }
fold. }
(_b_) Intermediate non-vascular fold free only }
on ventral surface, the dorsal surface }
below the ileo-colic junction being fused }
with the dorsal vascular fold. }
3. (_a_) Dorsal and ventral vascular folds reduced. }
Dorsal artery much larger than ventral. }_Cercopithecus._
(_b_) Intermediate non-vascular fold well developed,}
free on both surfaces. }

We may judge from this series that the following factors are capable of materially modifying the definite arrangement of the structures:

1. The vascular folds are capable of reduction until the vessels run close to the intestinal surface, merely covered by somewhat redundant peritoneum containing epiploic appendages. (_Cercopithecus._)

2. The dorsal caecal artery tends to assume in all three forms the greater share in the caecal vascular supply. This tendency is slightly developed in _Ateles_, becomes more pronounced in _Mycetes_, and is well marked in _Cercopithecus_, in which animal the dorsal vessel nearly replaces the ventral branch, the latter confining itself to the ventral surface of the ileo-colic junction and the adjacent ventral parts of the caecal wall.

3. The intermediate non-vascular fold is placed nearer to the dorsal larger than to the ventral smaller caecal artery. This condition, present in both _Ateles_ and _Cercopithecus_, foreshadows the fusion of the intermediate and dorsal vascular folds at the lower border of the ileo-colic junction, as seen in _Mycetes_.

4. This fusion of the two folds named in _Mycetes_ results in giving different values to the dorsal vascular fold in its proximal and distal segments. The proximal segment descends from the ileo-colic notch behind the ileo-colic junction to its lower border as a distinct fold. Beyond this point its fusion with the distal (caecal) segment of the intermediate fold rounds out a fossa, the inferior or posterior ileo-caecal, which is consequently bounded in front by the intermediate vascular fold, behind by the proximal segment of the dorsal vascular fold, to the right side by the inner wall of the caecum, between the intermediate and dorsal vascular folds, above by the lower border of ileum and ileo-colic junction, and below by the fusion of the two folds.

This pocket or fossa which is the most important and constant of the peritoneal recesses in the neighborhood of the caecum, opens upward and to the left.

5. A superior or anterior ileo-caecal fossa, formed in cases of well-developed ventral vascular fold between the same and the ventral wall of the ileo-colic junction, is of small size and shallow.

The cause of the greater development of the dorsal as compared with the ventral caecal artery is probably to be sought in the adhesion of the colon to the dorsal parietal peritoneum. In _Cercopithecus_ the dorsal surface of the ascending colon is adherent to the parietal peritoneum down as far as the iliac region and beginning of the caecum, whereas in _Mycetes_ the entire caecum, as well as the ascending colon, are free and non-adherent to the abdominal parietes. The influence of this adhesion on the arrangement of the vascular supply of the lower portion of the ascending colon and caecum appears to be important. Some of the departures from the _Ateles_ type presented by _Cercopithecus_ become still better developed in the human subject, where the adhesion of the ascending colon and the obliteration of the apposed serous surfaces of ascending mesocolon and parietal peritoneum is normally complete, even if the caecum remains entirely free, or only adheres to the iliac parietal peritoneum in the proximal part of its dorsal surface. Comparison with forms presenting non-adherent colic and caecal tubes indicates that the adhesion determines the relative size and arrangement of the ileo-colic vessels.

Thus the partially adherent colon and caecum of _Cercopithecus_ presents, compared with the free tube of _Ateles_ and _Mycetes_, a marked reduction of the ventral and a corresponding enlargement of the dorsal caecal artery. Further progress in the same direction is noted in the human subject where normally the ascending colon and at times the proximal portion of the caecum are adherent to the dorsal parietal peritoneum.

It appears that in the adhesion of the colic tube to the parietal peritoneum the dorsal ileo-colic vessels find an element favorable to their more complete development and extension, replacing in part or entirely the ventral caecal artery which becomes limited in distribution to the region of the ileo-colic junction. The adhesion and fixation of the dorsal wall of the intestine seems to afford an advantage to the dorsal vessel, whereas the greater mobility and the alternating conditions of distension and contraction, with variations of intracaecal pressure, depending upon the contents of the pouch, appear to operate unfavorably upon the development of the ventral vessel.

This view is borne out by the conditions observed in the exceptional instances in which in the human subject the ventral artery assumes the large share in the supply of caecum and appendix (cf. p. 276). In all the cases observed the type of the caecum indicated delayed or imperfect colic adhesion, and the ascending mesocolon remained partially free.

If we now compare the conditions above described for _Ateles_, _Mycetes_, _Cercopithecus_ with those usually found in man and in the anthropoid apes, we may appreciate the significance of the structures encountered by beginning the investigation with a type in which the derivation of the different parts is still quite evident. Such a condition is presented by the preparation shown in Fig. 563, taken from a child one year of age. Here the descent of the caecum has evidently been quite rapid and uniform without dorsal adhesion. The caecum and ascending colon remain free and can still be lifted away from the ventral facies of the right kidney and turned toward the median line to a point somewhat beyond the renal hilus. The caecum hangs downward vertically and the appendix arises from the funnel-shaped apex of the pouch.

1. Ventral caecal artery, surrounded by epiploic appendages.

2. Dorsal vascular fold, forming appendicular mesentery.

3. Intermediate non-vascular fold.]

The ventral caecal branch of the ileo-colic artery is slightly developed, (1) as a small vessel descending in an epiploic fold over the ventral surface of the ileo-colic junction as far as the root of the appendix. The intermediate non-vascular fold (3) is well marked, measuring 2.9 cm. in length, extending from the free border of the terminal ileum to the caecum and appendix and crossing over the well-developed dorsal vascular fold (2), which descends, as the appendicular mesenterolium, to the tip of the appendix, carrying the dorsal artery. In studying the conditions presented by this specimen, it is not difficult to trace the analogous structures in the caeca of _Cercopithecus_, _Ateles_ and _Mycetes_. The same vascular and non-vascular serous reduplications are found passing between the ileum and caecum. In accordance with the type presented by _Cercopithecus_ the ventral artery is much reduced and runs in a short serous fold loaded with epiploic appendages. The dorsal artery, on the other hand, is well developed and the intermediate non-vascular fold is distinct. In their relative arrangement these folds follow the _Ateles_ type. The dorsal vascular fold forms the true mesentery of the appendix, and, although close to and crossed by the intermediate non-vascular reduplication, remains still quite separable and distinct from the same; consequently the lower limit of the usual posterior ileo-caecal fossa, produced by the fusion of the dorsal vascular and the intermediate non-vascular fold, is absent.

A very perfect illustration of this type of the human ileo-caecal fold is presented by the preparation of _Gorilla savagei_ shown in Fig. 457. The ventral fold and artery appear reduced in this animal. The dorsal vascular fold forms a broad triangular plate of serous membrane carrying the dorsal artery in its free border and extending to the tip of the appendix.

The intermediate non-vascular fold is narrow but distinct, continued for a considerable distance along the ileum, opposite to the attached border, but only for a short extent along the left border of the caecum below the ileo-colic junction. It crosses the ventral surface of the broad dorsal vascular fold in passing to the caecum, but remains entirely free and is not adherent to the same.

Consequently here again the dorsal or posterior ileo-caecal fossa loses its distal limitation. The usual arrangement of the parts, as found in the human subject and derived from the preceding, is well illustrated by another anthropoid ape, _Hylobates hoolock_. Fig. 455 shows the ileo-caecum of this animal in the ventral view and the homologous parts, as compared with _Gorilla_, are readily recognized. On turning the terminal ileum ventrad and cephalad (Fig. 456), it is, however, seen that the intermediate non-vascular fold does not merely cross the dorsal vascular reduplication, as in _Gorilla_, but that it has begun to adhere to the same at the point of intersection. Consequently a well-marked and clearly limited posterior or dorsal ileo-caecal fossa is formed, bounded ventrally by the intermediate fold at its accession to the caecum, dorsally by the proximal part of the dorsal vascular fold, to the right by the left wall of the caecum, behind by the attachment of the intermediate fold, below by the confluence of the two folds, and above by the lower border of ileum and ileo-colic junction.

The open mouth of the fossa looks to the left. Fig. 464, taken from an adult specimen of the chimpanzee, _Troglodytes niger_, shows the extent of the dorsal vascular fold and of its connection with the mesentery of the terminal ileum.

The intermediate non-vascular fold extends from the ileum downwards along the entire left border of the caecum to the root of the appendix, fusing with the dorsal vascular fold and rounding out a deep posterior ileo-caecal fossa.

The typical arrangement, as encountered in the human subject, corresponds closely to the conditions presented by these anthropoid apes.

1. Dorsal vascular fold at the beginning of the distal free portion, forming the appendicular mesentery.

2. Proximal segment of dorsal vascular fold, fusing with

3. Intermediate non-vascular fold.

4. Rounded edge of union of dorsal vascular and intermediate folds bounding the ileo-caecal fossa caudad.

5. Point of accession to appendix of proximal branch of appendicular artery derived from posterior ileo-caecal artery.]

In Fig. 564, taken from an adult male human subject, the dorsal surface of the ascending colon and of the ileo-colic junction is adherent to the parietal peritoneum. The distention of the caecum is nearly uniform, the right sacculation being only slightly larger than the left. The appendix, measuring 18.4 cm. in length, arises from the dorsal surface of the caput coli, 1.7 cm. from the point where the ventral longitudinal muscular band turns around the caudal end of the pouch between the two sacculations, and 3.7 cm. below the caudal margin of the ileo-colic junction.

The dorsal vascular fold (2), forming the broad appendicular mesentery (1), is well developed and free in its distal portion, extending, with gradually diminishing width, to the apex of the appendix. The proximal segment of this fold (between 1 and 2) descends over the dorsal surface of the ileo-colic junction and meets (at 4) the intermediate non-vascular fold (3) which extends between the ileum and caecum, rounding out a crescentic ridge (4) which bounds the entrance into the posterior ileo-caecal fossa (between 2 and 3). The influence of the folds and of the blood vessels on the position and curves of the appendix is quite apparent in this preparation.

The dorsal larger branch of the ileo-colic artery, supplying caecum and appendix, passes over the dorsal surface of the ileo-colic junction (2) where the same, as well as the adjacent dorsal surface of the colon, is adherent to the parietal peritoneum. At the point where the dorsal vascular fold intersects and fuses with the intermediate non-vascular fold (4) the artery divides into a proximal and distal branch. The former proceeds to the caecum and root of the appendix, reaching this tube at the point marked 5. The latter continues (from 1 on) in the free border of the appendicular mesentery to the beginning of the distal third of the appendix, from which point on the fold extends as a narrow reduplication to the tip of the tube. The segment of the appendix situated between these two main arterial branches is thrown into several coils, the expression of the continued growth between two points relatively fixed by the accession of the two arterial branches. The pathological significance of these bends is apparent when we consider the effect which the kinking of the tube would have on catarrhal and other inflammations accompanied by distension of the appendix.

Typical examples of the posterior ileo-caecal fossa and of the mutual relationship of the limiting folds are seen in Figs. 565 and 566, both taken from adult human subjects.

The significance and mutual relations of the folds seen in the preparations just considered--which illustrate the typical adult human arrangement of the structures--will perhaps be best understood by comparison with an adult caecum in which the infantile condition, as seen in Fig. 563, has become further developed.

1. Dorsal vascular fold, carrying the distal appendicular branch of the dorsal caecal artery in the mesentery of the appendix.

2. Proximal branch of the same vessel, turning downward to caecum and root of appendix.

3. Intermediate non-vascular fold.]

Fig. 567 shows the dorsal view of such a preparation. The caecum is funnel-shaped with the apex, carrying the root of the appendix, turned upward and to the left, the sacculation to the right of the ventral muscular band being somewhat dilated. The appendix--7.2 cm. long--turns sharply upward and to the left, closely applied to the left caecal sacculation, passes dorsad to the ileo-colic junction and lies in its terminal part under cover of the ileo-colic mesentery. The ventral branch of the ileo-colic artery descends over the ileo-colic junction, supplying the ventral wall of the caecum. The intermediate non-vascular fold (3) is 3.9 cm. long and entirely free.

The dorsal vascular fold contains the large dorsal branch of the ileo-colic artery, dividing into two main branches. The first of these (1) passes distally in the free edge of the fold to the terminal part of the appendix. The other proximal branch (2) turns downward to the root of the appendix and the adjacent wall of the caecum, aiding materially in holding the proximal upturned segment of the appendix in contact with the left caecal sacculation.

The intermediate fold, short in its caecal attachment, does not meet the dorsal vascular fold at any point, consequently the ileo-caecal fossa is not limited caudad toward the root of the appendix. The conditions presented by this specimen correspond exactly to those found in the gorilla (Fig. 457) and in the human infantile preparation (Fig. 563).

In comparing Figs. 564 and 567 it will be noticed that the line of fusion between the intermediate fold and the dorsal vascular fold (Fig. 564, 4) corresponds to the point where the dorsal ileo-caecal artery divides into its proximal and distal branches (Fig. 567, angle between 1 and 2). Fig. 567 shows that the proximal arterial twig, even without fusion with the intermediate fold, suffices to influence to a considerable degree the curves and position of the appendix, inasmuch as it serves to hold the proximal segment of the tube closely applied in the erected position to the surface of the left caecal sacculation. The intermediate segment of the appendix, between the points of accession of the two arterial branches, is most prone to develop spiral twists and bends, especially when the usual fusion of the two folds takes place and still further fixes the parts, while the distal segment, carrying the narrow crescentic terminal appendicular mesentery, remains free.

1. Distal and

2. Proximal branch of dorsal ileo-caecal artery running in dorsal vascular fold.

3. Intermediate non-vascular fold fusing with 2 and forming a narrow caudal limit to the posterior ileo-caecal fossa.]

Finally, in a certain number of cases, an intermediate condition between the types presented by Figs. 564 and 567 is encountered. In Fig. 568 the general arrangement of the parts corresponds pretty accurately to that seen in Fig. 566, but the transition from a completely free intermediate non-vascular fold to one which has begun to fuse with the dorsal vascular fold is evident. The caecum is bent upward and to the left, the caput coli being formed by the right sacculation. The appendix, 7.8 cm. long, takes a wide {~WREATH PRODUCT~}-shaped curve. The convexity of the proximal curve corresponds to the point where the proximal appendicular artery (2) passes to the tube. The non-vascular intermediate fold (3), measuring 2.2 cm., fuses with the dorsal vascular fold at this point.

The three preparations illustrate serially the share which the peritoneal folds take in the formation of the posterior ileo-caecal fossa.

In Fig. 566 the failure of the intermediate fold to meet and fuse with the dorsal vascular fold has left the caudal boundary of the fossa (between 2 and 3) incomplete, the ventral and dorsal walls being formed by the folds in question. Fig. 568, in which fusion between the non-vascular and the dorsal vascular folds has commenced, shows the shallow form of the complete fossa under these conditions, while in Fig. 567, with extensive union of the folds, the fossa has correspondingly increased in depth.

A similar series is shown in Figs. 569, 570 and 571. In Fig. 569, taken from an adult subject, the intermediate non-vascular fold is entirely free, the dorsal branch of the ileo-caecal artery passes to caecum and appendix in an area of adhesion between parietal peritoneum and the intestine which includes the dorsal vascular fold. There is consequently no caudal boundary to the ileo-caecal fossa. Figs. 570 and 571 are both taken from infantile preparations.

In Fig. 570 the dorsal vascular and the intermediate folds nearly meet at the root of the appendix. They serve to outline the fossa, which appears completed in Fig. 571 by the actual meeting and fusion of the folds.

_The Ileo-caecal Folds in the Anthropoid Apes.--(1) Chimpanzee, Troglodytes niger._

The structures in a juvenile specimen of this animal are shown in Figs. 460 and 461.

The ventral vascular fold (Fig. 460, 3), containing epiploic fat, descends over the ileo-colic junction nearly to the level of the lower ileal margin. The intermediate non-vascular fold (Figs. 460 and 461, 2), derived from the ileum opposite to the mesenteric border, passes to the ventral and left aspects of the caecum and meets, near the root of the appendix, the dorsal vascular fold (Fig. 461, 3) carrying the dorsal caecal branch of the ileo-colic artery, which ramifies over the caecum and supplies the appendix.

The appendix measures 12.3 cm. and presents a terminal hook, slightly dilated.

The appendicular mesentery terminates within the concavity of this hook and measures 1.5 cm. in width at the broadest part, about 4.5 cm. from the root of the appendix.

Figs. 462 and 463 show the caecum of the adult chimpanzee in the ventral and dorsal view. The ventral vascular fold (Fig. 462, 1) is well developed, heavily fringed with epiploic appendages.

The non-vascular fold is extremely short and tense, fusing with the short appendicular mesentery near the point where in the dorsal view (Fig. 463) the appendix is seen bent at its origin sharply to the right.

Fig. 464, also taken from an adult specimen of the same animal, shows a very well-developed dorsal vascular fold, which fuses with the intermediate fold to limit a distinct ileo-caecal recess.

The chimpanzee, therefore, agrees closely with the human subject in the arrangement of the folds.

(2) _Orang, Simia satyrus._

In Figs. 458 and 459 the arrangement of the folds in an adult specimen of the orang is shown.

The ventral caecal artery (Fig. 458) is well developed, forming with the peritoneal fold and epiploic appendages surrounding it, a sharp sickle-shaped edge which descends over the ventral surface of the ileo-colic junction following the curve of the left caecal margin, and turning its concavity to the left toward the entering ileum.

The ventral caecal artery follows the left margin of the caecum below the ileo-caecal junction and passes for 0.5 cm. upon the portion of the pouch which turns up behind the terminal ileum.

The dorsal caecal artery is a vessel of large size, supplying branches to the narrow appendicular mesentery which extends, with many epiploic appendages, to within 9 mm. of the blunt apex of the appendix. 2.5 cm. beyond the first bend in the appendix the fold is narrowed to a fringe not more than 0.75 cm. wide. Up to this point the dorsal vascular fold measures 1.5 cm. in width, and just where it narrows it is joined by the intermediate non-vascular fold (Fig. 459), which forms a membranous band, 3.3 cm. wide in the middle, spread out in the angle between the lower and dorsal surfaces of the ileum and the dorsal surface of the caecum which turns up behind the ileo-colic junction. Between this fold and the dorsal vascular fold is seen the deep recess of the posterior ileo-caecal fossa--which by reason of the sharp curve of the caecum looks not only to the left but also upward and backward.

Direct comparison of the preparations of these two anthropoid apes just described with the conditions found in many adult human caeca shows the close correspondence in the arrangement of these folds and of their influence on the configuration of the parts.

Figs. 572 and 573--taken from an adult human subject--show a caecum and appendix which almost reproduces that of the chimpanzee illustrated in Figs. 462 and 463 and closely resembles that of the orang.

1. Ventral vascular fold.]

1. Appendix.

2. Intermediate non-vascular fold.]

Fig. 572, giving the ventral view, shows, by the course of the ventral longitudinal muscular band, the turn of the caecum upwards and to the left. The ventral caecal artery runs in a fold (1) loaded with epiploic appendages.

The non-vascular intermediate fold (Fig. 573, 2) passes to the root of the appendix, joining the proximal segment of the dorsal vascular fold in which the dorsal branch of the ileo-colic artery runs to the tip of the appendix. The distal two thirds of the appendicular mesentery are free.

3. _Gibbon, Hylobates hoolock_ (Figs. 455 and 456).--In the gibbon the folds appear well developed. The intermediate and dorsal vascular folds are quite distinct structures, although fusion (Fig. 456) has begun at one point, thus limiting a typical posterior ileo-caecal fossa.

4. _Gorilla, Gorilla savagei_ (Fig. 457).--Finally in the gorilla all three folds appear quite distinct and separate from each other, the dorsal vascular fold being especially well developed.

_Unusual and Aberrant Types of Ileo-caecal Folds and Fossae.--(A) Ventral caecal artery larger than the dorsal, supplying the greater part of the caecum and the appendix._

This condition is occasionally encountered. Dr. Martin, in a recent examination of the vascular supply of caecum and appendix in one hundred subjects, found it to obtain in six instances.

Apparently the dorsal wall of the caecum and of the proximal segment of the ascending colon remains free in these cases and does not become adherent to the parietal peritoneum. The shape of the pouch, moreover, indicates a free and unimpeded embryonal caecal descent. The normal relative size of the two vascular folds is reversed. A good example of this variation, in the caecum of an infant, is seen in Fig. 516. The same arrangement in an adult specimen is seen in Fig. 574.

In the Slow Lemur (_Nycticebus tardigradus_) (Fig. 420) the ventral artery is normally the larger of the two, extending in the ventral fold to the tip of the reduced appendix of the caecal pouch.

(_B_) _Fusion of ventral vascular fold with the intermediate fold, resulting in the production of a well-defined superior or ventral ileo-caecal fossa._

Normally the reduced ventral artery crosses the ileo-colic junction in a slightly developed ventral vascular fold, closely adherent to the intestine, with a very narrow free margin. The superior or ventral ileo-caecal fossa in these cases is very shallow and confined (Fig. 574) to the ventral surface of the ileo-colic junction. Occasionally the fold is better developed and fuses with the intermediate non-vascular fold, producing a fossa of greater extent, which is bounded dorsad by the ileum, ventrad and cephalad by the ventral fold, caudad by the fusion of this fold with the intermediate reduplication, and to the right by the left wall of the caecum. Figs. 576, 577, 578 and 579 show this aberrant disposition of the structures in a series of adult human caeca.

A corresponding arrangement is noted in the preparation of the caecum of _Cercopithecus campbellii_ (Fig. 433). The large intermediate fold is joined by the ventral vascular fold, thus defining the lower boundary of ventral ileo-caecal fossa.

(_C_) _Union of both vascular folds with the intermediate non-vascular fold._

I have encountered one instance of this arrangement in an infant, whose caecum and ileo-colic junction is shown in Fig. 580. Both the ventral and dorsal arteries in this case were equally developed, and shared equally in the supply of caecum and appendix. Both vascular folds fused with the intermediate fold, thus producing two typical ileo-caecal fossae, one ventral, the other dorsal.

(_D_) _Abnormal positions of the appendix due to variations in the arrangement and tension of the intermediate fold._

Fig. 510 shows a foetal caecum in the ventral view. The ventral vascular fold (3) is well developed. The non-vascular fold is short, arising from the ventral surface of the ileum, instead of from the free border of the intestine opposite to the mesenteric attachment. It fuses with the ventral vascular fold a short distance below the ileo-colic junction, thus limiting a small ventral ileo-caecal fossa. The dorsal caecal artery in this specimen was large, but the fold carrying it extremely narrow.

The preparation illustrates the type resulting from the reduction in size and extent of the non-vascular and mesenteric folds. The intermediate fold is reduced to a short and narrow band. Compared with the usual infantile type the caecum lacks the characteristic turn upwards and to the left, possibly in consequence of the slight traction caused by the rudimentary intermediate fold. The pouch occupies a nearly vertical pendent position, which the appendix, arising from the lowest point of the caecal funnel, shares. The appendix is not drawn into the retro-ileal position by the dorsal vascular fold, which is much reduced.

In Fig. 511, representing the caecum and appendix of a foetus at term, the effect of the tense non-vascular intermediate fold (2) is seen in the sharp turn to the left which it imparts to the nearly transversely directed funnel-shaped caecum. The appendix (1) is coiled spirally for 13/4 turns behind the ileo-colic junction, with the tip directed upward behind the mesentery of the terminal ileum. The non-vascular intermediate fold (2) extends to the rest of the appendix. It appears short in its caecal attachment, on account of the turn of the caecum backwards and to the left and the close connection between the adjacent margins of the ileum and caecum.

1. Appendix, terminal portion turned ventrad of ileo-colic junction.

2. Intermediate non-vascular fold.]

In Fig. 581--a foetal preparation at term--the caecum is turned to the left, below and behind the terminal ileum. The non-vascular fold (2) is well developed as regards _length_ of _ileal_ attachment, but is very narrow and tense, passing between ileum and the proximal curve of the caecum behind the ileo-colic junction, where it merges with the dorsal vascular fold. The appendix takes a sudden turn caudad at this point and then continues up _ventrad_ to the ileo-colic junction, the proximal portion being kept firmly in contact with the dorsal and caudal circumference of the ileum by the tension of the non-vascular band. It is quite evident that this peculiar turn of the appendix is directly due to the confining influence of the non-vascular band--which passes from its ileal attachment almost directly dorsad to the point of fusion with the dorsal vascular fold, causing the sharp downward and forward turn of the proximal segment of the appendix. Similar cases with ventral position of the appendix are shown in Figs. 545 and 582.

INDEX.

Abdominal vein in Anure Amphibian, 158
in Reptilia, 167
in _Iguana_, 160
in Urodele Amphibian, 157
viscera of _Macacus rhesus_, 77

Abnormal positions of appendix, 277

Abomasum, 49

_Accipenser sturio_, biliary ducts in, 145
pyloric appendices in, 120
ileo-colic junction of, 212

Ailuroidea, ileo-colic junction of, 212

Alimentary canal of _Ammocoetes_, 42
of _Amphioxus_, 42
of _Belone_, 40
of _Chelydra_, 58
of Cyclostomata, 40, 42
derivation of epithelium, 30
of muscular and connective tissue, 30
differentiation from body-cavity, 21, 29
divisions of, 38
early developmental stages, 21, 29
mammalian embryonal stages, 40
of _Esox_, 40
of _Echelus conger_, 54
of _Necturus_, 40
of _Petromyzon_, 200
of _Proteus_, 40
primitive type, 40, 42
of _Pseudemys elegans_, 55
of _Rana_, 55
separation from yolk-sac, 22
tract of _Necturus maculatus_, 52
of _Tamandua_, 56

Allantois in Amniota, 36
arteries of, 63, 146
derivation from alimentary canal, 35
function of, 36
relation to placenta, 36
to primitive intestine, 24
to urinary bladder, 24

_Alligator mississippiensis_, ileo-colic junction of, 201
stomach of, 51

_Ammocoetes_, alimentary canal of, 42
pancreas in, 117

_Ammodytes_, pyloric appendix in, 120

Amnion, definition of, 36

Amniota, development of liver in, 143

Amphibia, development of pancreas, 115
folds of intestine in, 196
ileo-colic junction of, 201

Amphibians, biliary ducts in, 145
intestinal canal of, 191

_Amphioxus_, alimentary canal of, 42
hepatic diverticulum of, 43
intestinal canal of, 191

Anthropoid apes, ileo-caecal folds of, 274

Anthropoidea, ileo-colic junction of, 213

Anthropomorpha, ileo-colic junction of, 216

_Anguilla anguilla_, stomach of, 47

Anure Amphibian, abdominal vein of, 158
cardiac vein in, 158
musculo-cutaneous vein in, 158
pelvic vein in, 158
post-cava in, 158
pre-cava in, 158
venous system in, 158

Aorta, early condition of intestinal branches, 32

Aortal arterial system, development of, 63

Aplacentalia, definition of, 36

Appendix, abnormal positions of, 277
absence of, 249
influence of dorsal vascular fold on shape of, 271
origin of, and shape of caecum, 245
position and peritoneal relations of, 250
variations of peritoneal relations, 258

Arctoidea, ileo-colic junction of, 212

_Arctopithecus marmoratus_, ileo-colic junction of, 208

Arctopithecini, ileo-colic junction of, 214

Arrest of development before intestinal rotation, 60

Arteries, of allantois, 64, 146

Artery, caudal, 64
ileo-colica, 66
colica dextra, 66
media, 66
coronary, 181
external iliac, 64
gastro-epiploica sinistra, 108
hepatic, 65, 179
ileo-colic, 262
inferior mesenteric, 67
internal iliac, 64
pancreatico-duodenalis inferior, 66
omphalo-mesenteric, 64, 146
sacralis media, 64
splenic, 65, 108
superior mesenteric, 64, 65
umbilical, 64
vitelline, 64, 146

Artiodactyla, ileo-colic junction of, 209

_Arvicola pennsylvanicus_, ileo-colic junction and caecum of, 211

Asymmetrical type of ileo-colic junction, 223

_Ateles_, ileo-caecal folds of, 261
_ater_, ileo-colic junction and caecum of, 214

Atresia ani, 24, 28

Axial mesoderm, connection with splanchnic and somatic mesoderm, 22

_Bassaris astuta_, ileo-colic junction of, 212

Batrachians, stomach of, 44, 46

_Belone_, alimentary canal of, 40

Biliary ducts in _Accipenser_, 145
in Amphibians, 145
arrangement of, 145
in birds, 145
in _Buceros_, 145
in calf, 145
in dog, 145
in _Galeopithecus_, 145
in _Lophius_, 145
in _Lutra_, 145
in Monotremes, 145
in _Phoca_, 145
in Reptilia, 145
in sheep, 145
in _Tarsius_, 145
in _Trigla_, 145
in _Xiphias_, 145

Birds, folds of intestine in, 196
glandular stomach of, 50
biliary ducts in, 145
ileo-colic junction of, 203
muscular stomach of, 50
venous system of, 161

Blastoderm, 20
layers of, 21
primitive, 20

Blastodermic vesicle, 20

Blastomeres, 20

Blastula, 20

Blastosphere, 20

Body-cavity, development of, 21
primitive condition of, 29

Body-wall, 22

_Boselaphus tragocamelus_, ileo-colic junction and caecum of, 210

_Bos indicus_, ileo-colic junction and caecum of, 210
spiral colon of, 233

_Bradypus marmoratus_, ileo-colic junction of, 208
stomach of, 51

Brunner's glands, 194

_Buceros_, biliary ducts in, 145

Bursa epiploica in lower forms, 187

Caeca of the anthropoidea, compared with the human, 247

Caecal gastric appendices of _Dicotyles_, 48

Caecum and appendix, changes in position during development, 239
development of, 237
morphology of, 237
variations of, 244
descent of, 76, 243
of embryo, shape of, 245
first appearance in human embryo of, 53
function of, 219
non-descent in adult, 75
persistent subhepatic position in adult, 75
in the Rodentia, 229
shape of, and origin of appendix, 245
types of, 245
in the Ungulata, 229

Calf, biliary ducts in, 145

Camel, gastric water-cells, 49

Canal, medullary, 21, 28
neuro-enteric, 23

_Canis familiaris_, ileo-colic junction and caecum of, 212

_Capra aegagrus_, ileo-colic junction and caecum of, 209

Cardiac vein in Anure Amphibian, 158

Cardinal veins, anterior, 147
posterior, 147

Carnivora, gastric diverticula of, 48
ileo-colic junction of, 212
stomach of, 46, 47

Carnivorous birds, stomach of, 50

_Casuarius_, duodenum, biliary and pancreatic ducts of, 115
intestinal villi of, 195

_Castor fiber_, ileo-colic junction and caecum of, 211
stomach of, 46

Cat, development of pancreas, 115
dorsal mesogastrium, spleen and pancreas, 126
lesser peritoneal sac, 128
spleen, pancreas and great omentum, 127

Caudal artery, 64
vein in Selachian, 154
in Urodele Amphibian, 156

Caudate lobe, 170

Cebidae, ileo-colic junction of, 214

_Cebus leucophaeus_, ileo-colic junction and caecum of, 216
_monachus_, ileo-colic junction and caecum of, 216

Cell-body, 19

Cellulae coli, 199

_Ceratodus_, spiral intestinal valve in, 119

_Cercoleptes caudivolvulus_, ileo-colic junction of, 212

_Cercopithecus campbellii_, ileo-colic junction and caecum of, 214
_pogonias_, ileo-colic junction and caecum of, 214
_sabaeus_, ileo-caecal folds of, 264
ileo-colic junction and caecum of, 214

_Cervicapra_, intestinal folds of, 196

_Cervus sika_, ileo-colic junction and caecum of, 210
spiral colon of, 233

Cetacea, ileo-colic junction of, 209

Cetaceans, stomach of, 49

Changes in position during development of caecum and appendix, 239

Cheek pouches, 48
of _Macacus nemestrinus_, 48

Cheiroptera, ileo-colic junction of, 212

Chelonians, liver of, 144
stomach of, 45, 46

_Chelydra_, alimentary canal of, 58
pancreas of, 117
_serpentaria_, ileo-colic junction of, 201

Chick, development of liver in, 143
development of pancreas in, 115

_Chlamydophorus_, ileo-colic junction of, 207

_Choloepus didactylus_, ileo-colic junction of, 207

_Chrysothrix sciureus_, ileo-colic junction and caecum of, 214

Cleft, uro-genital, 27

Cloaca, development of, 24
division of, in higher vertebrates, 27
in human embryos, 26
in _Platypus anatinus_, 26
structure of, in lower vertebrates, 25
in _Iguana tuberculata_, 25

Cloacal membrane, 24
anal segment, 28
uro-genital segment, 28

Coeliac axis, 65

Coelom, composition and derivation of walls, 29
development of, 21
primitive condition of, 29

Colic bend of the Manidae, 234
loop in _Phascolarctos_, 234

Colico-phrenic ligament, 109

Colon, ascending, adhesions of, 81
position of, in foetus, 84
and caecum of _Lagomys pusillus_, 232
descending, adhesion of, 81
relation of, to left kidney, 83
position as influenced by foetal liver, 77
spiral coil of, 233
structural modifications of, 230

_Coluber natrix_, stomach of, 44

Common bile duct, 145

Comparative anatomy of hepatic venous circulation, 154
of liver, 144

Comparison of human and anthropoid caeca, 247

Connective tissue and muscular fibers, derivation of, 30

Coprodaeum, 25

Coronary artery, 181
ligaments of liver, 173

_Corvus_, caeca of, 203

Costo-colic ligament, 109

Crocodiles, stomach of, 46, 51

Crop, 48

_Cryptobranchus alleghaniensis_, ileo-colic junction of, 201

Cyclostomata, divisions of alimentary canal of, 40, 42
intestinal canal of, 191
spiral intestinal valve of, 119

_Cyclothurus didactylus_, ileo-colic junction and caeca of, 207

_Cyclura teres_, ileo-colic junction and caecum of, 202

_Cynocephalus anubis_, ileo-colic junction and caecum of, 214
_babuin_, ileo-colic junction and caecum of, 214
_porcarius_, ileo-colic junction and caecum of, 214
_sphinx_, ileo-colic junction and caecum of, 214

Cynoidea, ileo-colic junction of, 212

Cynomorpha, ileo-colic junction of, 213

_Cyprini_, stomach of, 44

Cystic duct, 146
development of, 142

Cysto-enteric duct, 145

_Dasyprocta agouti_, ileo-colic junction and caecum of, 211
spiral colon of, 234

_Dasypus sexcinctus_, ileo-colic junction and caeca of, 207

_Dasyurus viverinus_, ileo-colic junction of, 206

Descent of caecum, 243

Derivatives of entodermal intestinal tube, 34

Deuteroplasm, 19

Development of caecum and appendix, 237
of cystic duct, 142
of gall-bladder, 142
of liver, 141
in amniota, 143
in chick, 143
in Elasmobranchs, 143
in Teleosts, 143
of portal circulation, 147
of spiral colon, 233
of transverse colon, 244
of vascular system of liver, 145

_Dicotyles_, caecal gastric appendices of, 48
_torquatus_, ileo-colic junction and caecum of, 209

_Didelphis_, ileo-caecal folds of, 263
_virginiana_, ileo-colic junction and caecum of, 205

Digitiform gland of Selachians, 201

Dipnoeans, intestinal canal of, 191
spiral intestinal valve in, 119

Diverticulum caecum vitelli in birds, 35
in _Urinator imber_, 35
_lumme_, 35
vateri, 114

Dog, biliary ducts in, 145

Dorsal mesentery, early condition and derivation, 32
in lower vertebrates, 32
smooth muscular fiber of, 33
mesogastrium, area of adhesion to parietal peritoneum, 106
developmental changes in direction and extent, 103
definition of, 100, 101
gastro-splenic segment, 108
redundant omental growth, 105
spleen and pancreas in cat, 126
vertebro-splenic segment, 108
vascular ileo-caecal fold, 262
fold, influence on shape of appendix, 271

Double caecal pouches of birds, 203

Ducts of Cuvier, 147
in Selachian, 155
in Urodele amphibian, 156
omphalo-mesenteric, 22
of Santorini, 111
vitello-intestinal, 22
of Wirsung, 111
development of, 112

Ductus venosus, 149
changes after birth in, 152

Duodenal antrum, 194
fold of cat, 92
of _Hapale vulgaris_, 93
inferior, 95
of _Nasua rufa_, 92
superior, 95
fossae, 92
superior, 94
vascular relations, 95, 96
loop, 54

Duodeno-colic neck, 57

Duodeno-jejunal fossa in the cat, 93

Duodenum, adhesion of, 67
development of, 53
peritoneal relations of infra-colic segment, 81
of supra-colic segment, 81
suspensory muscle of, 33
with biliary and pancreatic ducts, of _Casuarius_, 115

_Echidna hystrix_, ileo-colic junction and caecum of, 204

_Echelus conger_, alimentary canal of, 54
ileo-colic junction of, 200
intestinal mucosa of, 197
endgut of, 199
pyloric appendix of, 120

Ectoderm, 21

Edentata, ileo-colic junction of, 206
types of ileo-colic junction and caecum in, 218

Egg, development of, 20
structure of, 19

Elasmobranchs, development of liver in, 143

_Elephas indicus_, ileo-colic junction and caecum of, 210

Embryonal intestinal hernia, 52

Embryonic shield, 20

Embryo, separation of, 20

Endgut of _Echelus_, 199
extent and contained segments, 38
function of, 198
in lower vertebrates, 199

Enteric canal, primitive condition of, 29

Entoderm, 21
derivatives of, 28

Entodermal intestinal tube, derivatives of, 34

Epiblast, 21

Epiploic bursa, 107
early stages of, 104

Epithelium of alimentary canal, derivation of, 30

_Erethizon dorsatus_, ileo-colic junction and caecum of, 211

_Esox_, alimentary canal of, 40

_Eunectes marinus_, ileo-colic junction and caecum of, 203

External iliac artery, 63
perineal folds, 28

Foetus at term, venous system of, 162

Falciform ligament, as part of ventral mesogastrium, 165

_Felis_, ileo-colic junction and caecum of, 212
_leo_, ileo-colic junction and caecum of, 212

Fish, development of pancreas, 115
folds of intestine, 196
ileo-colic junction of, 200

Fissipedia, ileo-colic junction of, 212

Fissure, transverse anal, 27

Folds, ileo-caecal, 260

Follicles, solitary, 196

Foramen of Winslow, 174
boundaries in adult human subject, 184
caudal boundary, 178
in lower mammals, 183
relation to duodenal adhesion, 184
in _Tamandua bivittata_, 183

Foregut, comparative anatomy of, 42
divisions of, 191
extent and contained segments, 38

Formative yolk, 19

Fossa duodeno-jejunalis, 96
ileo-caecal, 260
intersigmoidea, 97
of Treitz, 92, 96

Function of caecum, 219
of pyloric appendices, 221
of pyloric caeca, 221
of spiral fold of intestinal mucous membrane, 220

Furrow, primitive intestinal, 22

_Gadus callarias_, ileo-colic junction of, 201
pyloric appendices in, 120

_Galeopithecus_, biliary ducts in, 145
ileo-colic junction and caecum of, 213

Gall-bladder, development of, 142
occurrence of, 144

Gastric diverticula of Carnivora, 48
of Herbivora, 48
of Omnivora, 48

Gastro-hepatic omentum, as part of ventral mesogastrium, 165

Gastro-splenic omentum, 109

Genito-urinary sinus, 27
tract, male, in _Platypus anatinus_, 26

Germinal area, 20
membrane, 20
spot, 19
vesicle, 19

Glands of Lieberkuehn, 194

Glandular stomach of birds, 50

_Gobius_, stomach of, 45

_Gorilla savagei_, ileo-colic junction and caecum of, 216

Graafian follicle, 19

Greater curvature, first appearance of, 41

Groove, medullary, 21
primitive intestinal, 22

_Halicore_, ileo-colic junction of, 208

_Halmaturus derbyanus_, ileo-caecal folds of, 263
ileo-colic junction and caecum of, 205
stomach of, 47

_Hapale jacchus_, ileo-colic junction and caecum of, 214
_vulgaris_, duodenal fold, 93

_Heloderma suspectum_, ileo-colic junction of, 211

Hepatic antrum of lesser sac, 170
artery, 65
development of, 179
in relation to foramen of Winslow, 180
relation to duodenal adhesion, 182
relation to primitive dorsal mesentery, 182
cylinders, 143
ducts, 145
flexure, formation of, 76
recess of lesser sac, 177
ridge, 142
veins, 148
venous circulation, comparative anatomy of, 154
direction of current, 152
summary of development, 153

Hepatic-portal system in Selachian, 155
in Urodele Amphibian, 157
vein in _Iguana_, 160

Hepato-cystic duct, 145

Hepato-enteric duct, 145

Herbivora, gastric diverticula of, 48
stomach of, 46, 47

Herbivorous birds, stomach of, 50

Herons, caecum of, 204
stomach of, 50

_Hippopotamus_, ileo-colic junction of, 209

Human caeca compared with those of the Anthropoidea, 247

_Herpestes griseus_, ileo-colic junction and caecum of, 212
_ichneumon_, ileo-colic junction and caecum of, 212

_Hyaena striata_, ileo-colic junction and caecum of, 212

_Hylobates hoolock_, ileo-colic junction and caecum of, 216

Hypoblast, 21

Hyracoidea, ileo-colic junction of, 210

_Hyrax capensis_, ileo-colon, ileo-caecum and colic caeca of, 210
large intestine and caeca of, 234

Iguana, abdominal vein of, 160
caecal pouch and valves of, 202
hepatic-portal vein of, 160
post-cava of, 159
renal-portal system of, 159
sciatic vein of, 160
segmental veins of, 161
_tuberculata_, ileo-colic junction and caecum of, 201
cloaca in, 25
ventral mesogastrium of, 166

Ileo-caecal folds, aberrant types of, 276
of the anthropoid apes, 274
of _Ateles_, 261
of _Cercopithecus sabaeus_, 264
of _Didelphis_, 263
and fossae, 260
of _Halmaturus derbyanus_, 263
of _Mycetes fuscus_, 264
smooth muscular fibers of, 33
fossa, anterior, 267
posterior, 271
fossae, aberrant types of, 276

Ileo-colic artery, 262
junction of _Accipenser sturio_, 201
of _Alligator mississippiensis_, 201
of Amphibia, 201
of the Arctoidea, 212
of the Ailuroidea, 212
of _Arvicola pennsylvanicus_, 211
of the Anthropoidea, 213
of the Anthropomorpha, 216
of the Artiodactyla, 209
of the Arctopithecini, 214
of _Arctopithecus marmoratus_, 208
of _Ateles ater_, 214
of _Bassaris astuta_, 212
in birds, 203
of _Boselaphus tragocamelus_, 210
of _Bos indicus_, 210
in the Carnivora, 212
of _Canis familiaris_, 212
of _Capra aegagrus_, 209
in cases of arrested intestinal rotation, 241
of _Castor fiber_, 211
of the Cebidae, 214
of _Cebus_, 215
_leucophaeus_, 216
_monachus_, 216
of _Cercoleptes caudivolvulus_, 212
of _Cercopithecus campbellii_, 214
_pogonias_, 214
_sabaeus_, 214
of _Cervus sika_, 210
of the Cetacea, 209
of _Chlamydophorus_, 207
of Cheiroptera, 212
of _Chelydra serpentaria_, 201
of _Choloepus didactylus_, 207
of _Chrysothrix sciureus_, 214
of _Corvus_, 203
of _Cryptobranchus alleghaniensis_, 201
of _Cyclothurus didactylus_, 207
of _Cyclura teres_, 202
of _Cynocephalus_, 213
_anubis_, 214
_babuin_, 214
_porcarius_, 214
_sphinx_, 214
of the Cynoidea, 212
of the Cynomorpha, 213
of _Dasyprocta agouti_, 211
of _Dasypus sexcinctus_, 207
of _Dasyurus viverinus_, 206
of _Dicotyles torquatus_, 209
of _Didelphis virginiana_, 205
of _Echelus conger_, 200
of _Echidna hystrix_, 204
of the Edentata, 206
effect of rotation on position of, 59
of _Elephas indicus_, 210
of _Erethizon dorsatus_, 211
of _Eunectes marinus_, 203
of _Felis_, 212
leo, 212
in fish, 200
of the Fissipedia, 212
of _Gadus callarias_, 201
of _Galeopithecus_, 213
of _Gorilla savagei_, 216
of _Halicore_, 208
of _Halmaturus derbyanus_, 205
of _Hapale jacchus_, 214
of _Heloderma suspectum_, 204
of the herons, 204
of _Herpestes ichneumon_, 212
of _Hippopotamus_, 209
of _Hyaena striata_, 212
of _Hylobates hoolock_, 216
of _Hyracoidea_, 210
of _Hyrax capensis_, 210
of _Iguana tuberculata_, 202
of the Insectivora, 213
of _Lagothrix humboldtii_, 215
of Lamellirostra, 203
of _Lemur macaco_, 213
_mongoz_, 213
of the Lemuroidea, 213
of _Lepus cuniculus_, 211
of _Macacus_, 214
_cynomolgus_, 214
_ochreatus_, 214
_pileatus_, 214
_rhesus_, 214
of mammalia, 204
of _Manatus americanus_, 208
of _Manis longicauda_, 208
of Marsupialia, 204
of Monotremata, 204
of _Midas geoffrei_, 214
_ursulus_, 214
of _Monodon_, 209
of _Mustela_, 212
of _Mycetes cabaya_, 214
_fuscus_, 215
of _Myoxus_, 211, 212
of _Myrmecophaga jubata_, 207
of _Nasua rufa_, 212
of _Necturus maculatus_, 201
non-vascular serous folds, 262
of _Nycticebus tardigradus_, 213
of _Nyctipithecus commersonii_, 214
of _Ornithorhynchus anatinus_, 204
of _Orycteropus_, 208
of _Oryx leucoryx_, 210
of _Otolicnus crassicaudatus_, 273
of _Paradoxurus typus_, 212
of _Perameles nasuta_, 206
of the Perissodactyla, 210
of _Phascolarctos cinereus_, 205
of _Phascolomys wombat_, 206
of _Phocaena_, 209
of _Phoca vitulina_, 212
of _Physeter_, 209
of the Pinnipedia, 212
of the piscivorous divers, 203
of _Pithecia satanas_, 215
of _Pleuronectes maculatus_, 201
of the Primates, 213
of the Proboscidea, 210
of _Proteles lalandii_, 212
of _Pseudemys elegans_, 201
of _Pteropus medius_, 212
of _Rana catesbiana_, 201
of the Ratitae, 203
of Reptilia, 201
of the Rodentia, 211
serial review in Vertebrata, 200
of the Sirenia, 208
of _Simia satyrus_, 216
of _Strix_, 203
of _Struthio africanus_, 204
of _Sus scrofa_, 209
asymmetrical type, 223
symmetrical type, 221
of _Tamandua bivittata_, 208
of _Tapirus americanus_, 210
of _Tarsius spectrum_, 213
of _Tatusia novemcincta_, 207
of _Taxidea americana_, 212
of _Tolypeutes_, 207
of _Trichosurus vulpinus_, 205
of _Troglodytes niger_, 217
types of, and caecum, 217
in Edentata, 218
in Marsupialia, 218
of _Ursus_, 212
of the Ungulata, 209
of _Vulpes fulvus_, 212
vascular mesenteric folds of, 262
of _Xenurus_, 207
of _Zalophus gillespiei_, 212

Iliac vein in Urodele Amphibian, 157

Inferior mesenteric artery, 67

Infra-colic compartment, secondary parietal peritoneum of, 85, 86

Insectivora, ileo-colic junction of, 213

Intermediate duodenal fold, 96
non-vascular ileo-caecal fold, 262

Internal iliac artery, 63
perineal folds, 27

Intestinal blood vessels, effect of intestinal rotation on, 59
canal of _Amphioxus_, 191
of Amphibians, 191
of Cyclostomata, 191
diverticula, 193
of Dipnoeans, 191
of Teleosts, 191
folds of mucosa, 193
non-differentiated, of lower vertebrates, 191
folds in Amphibia, 196
in birds, 195
in fish, 196
furrow, primitive, 22
glandular apparatus in lower vertebrates, 195
groove, primitive, 22
juice, function of, 194
mucous membrane of _Cervicapra_, 196
of _Echelus conger_, 197
_Lophius_, 197
lymphoid tissue, 196
of _Phocaena_, 196
of _Thalassochelys_, 197
rotation, 58
arrest of development, 60
demonstration in cat, 67
spiral fold, function of, 193
vascular supply, 63
in cases of non-rotation, 67
villi of Carnivora, 195
of _Casuarius_, 195
of Ophidia, 195
of _Ursus maritimus_, 195

Intestine in early human embryo, 52
general consideration of, 51
large, and caeca of _Hyrax_, 234
functions of, 198
length of, 199
of monkeys, 199
of rodents, 199
width of, 199
small, 192
absorbing apparatus, 195
divisions of, 194
length of, 192
secretory apparatus, 194
structure of, 194
villi, 195

Isthmus, duodeno-colic, 57

Jejuno-ileum, development of, 54

_Labrus_, stomach of, 44

_Lagomys pusillus_, colon and caecum of, 232

_Lagothrix humboldtii_, ileo-colic junction and caecum of, 215

Lamellirostra, ileo-colic junction and caeca of, 203

Lateral vein in Selachian, 155

_Lemur macaco_, ileo-colic junction and caecum of, 213
_mongoz_, ileo-colic junction and caecum of, 213

Lemuroidea, ileo-colic junction of, 213

_Lepus cuniculus_, ileo-colic junction and caecum of, 211
saccus lymphaticus of, 211

Lesser curvature, first appearance of, 41

Ligament, colico-lienale, 109
of ductus venosus, 152
gastro-lienale, 110
lieno-renale, 109
phrenico-lienale, 109

Ligamenta coli, 199