The primary cause leading to the rotation of the intestinal canal and inaugurating the successive stages which produce the adult disposition of the tube is to be found in the rapid increase in length of the small intestine. Numerous convolutions of this tube succeed to the few primary coils noted in the first stages. This condition is illustrated in Fig. 498, taken from an embryo of 4.4 cm. cervico-coccygeal measure, and the arrangement of the intestine is indicated in schema, [Fig. 491]. The cæcum is found nearly in the median line imbedded among the surrounding coils of the small intestine, which by their rapid increase have pushed the pouch cephalad nearly into contact with the caudal surface of the liver.
Three main divisions of the convolutions of the small intestine can be made out, slightly separated from each other in the figure to exhibit the cæcum between them. The proximal (jejunal) set of these convolutions occupy the upper and left part of the abdominal cavity. They are the product of the single larger coil which in the earlier stage (Fig. 497, schema [Fig. 490]) appeared dorsad of the cæcal diverticulum. The distal (ileal) division of small intestinal convolutions has become greatly augmented and lies to the right of the cæcum. The concavity of the pouch is still, as in the earlier stages, directed to the right and the entrance of ileum into colon takes place from right to left. The caudal part of the abdominal cavity is occupied by an intermediate set of transition convolutions which join the proximal and distal divisions. In the two stages just described (Figs. 497 and 498, Schema [Figs. 490] and [491]), the initial step in the intestinal rotation has been taken, i. e., the beginning of the colon has been displaced cephalad from its original position in the caudal and left part of the abdominal cavity by the pressure of the rapidly growing coils of the small intestine and now lies transversely ventrad of the duodenum, having crossed the duodeno-colic neck or isthmus of the primitive umbilical loop (cf. [Fig. 487], C).
At first the distal coils of the small intestine occupy a position behind as well as to the right of the cæcum, forming a dorsal retro-cæcal division connected by intermediate convolutions with the ventral division occupying the lower and left portion of the abdominal cavity. The apex of the cæcum is frequently imbedded among these terminal coils of the ileum. With the continued growth of the small intestines a further displacement of the cæcum cephalad and to the right takes place, while at the same time the terminal ileal coils pass downwards and to the left, from a retro-cæcal into a subcæcal position, thus permitting a direct apposition of the cæcum to the dorsal parietal (prerenal) peritoneum. The last steps in this process of withdrawal of the original voluminous dorsal (retro-cæcal) division of ileal convolutions are well seen in the preparation shown in Fig. 499, taken from an embryo of 6.7 cm. vertex-coccygeal measure, and corresponding to the schematic stages represented in [Figs. 490] and [491]. The cæcum in this preparation has not yet completed its rotation and still turns its concavity upwards and to the right, with the apex imbedded among the terminal convolutions of the ileum.
The ileo-cæcal junction takes place from right to left in a downward direction. Nearly the entire mass of the small intestine is situated below and to the left of cæcum and colon, but a terminal ileal coil still occupies, although evidently in the process of withdrawal, the retro-cæcal position, separating the cæcum from direct contact with the dorsal parietal peritoneum. The withdrawal of this terminal coil of the small intestine is accompanied, or immediately followed, by a further turn of the colon cephalad and to the right, which brings it into contact with the caudal surface of the liver and completes the rotation, producing a change in the relative positions of the terminal ileal coils and the cæcum. In the stages illustrated in Figs. 498 and 499 and shown schematically in [Figs. 490] and [491], the terminal coils of the ileum pass from right to left behind the cæcum to enter the colon, and the concavity of the cæcal pouch is directed upwards and to the right. After the final rotation has occurred (schema, [Fig. 492]) the ileum enters the large intestine from the left and from below, and the concave border of the cæcum is directed caudad and to the left. This change in relative position has been accomplished by a revolution of the colon and cæcum through an arc of 180° around its own long axis carrying the cæcum above and behind the small intestine and bringing it into contact with the dorsal prerenal parietal peritoneum. At the same time the terminal coils of the ileum turn downwards and to the left. If this final step in the rotation of the large intestine fails to occur, with otherwise normal development of the parts, the ileum will persist in entering the large intestine from right to left after the cæcum has obtained its final lodgment in the right iliac fossa. We have had occasion to refer previously to the significance of these instances of partially arrested development (cf. [p. 61], [Figs. 123], [127] and [128]).
In Figs. 500 and 501, taken from an embryo of 4.9 cm. vertex-coccygeal measure, the final rotation of the cæcum from the position occupied in [Fig. 498] has occurred and the concavity of the pouch is directed caudad and towards the left. At the same time the escape of the terminal ileal coils from behind the cæcum and beginning of the colon has not yet taken place and hence the colon is still kept by these coils from direct opposition to the dorsal prerenal parietal peritoneum. The condition presented by this preparation can be schematically indicated by [Figs. 492] and [493]. The rotation has carried the beginning of the colon (Fig. 500), with the cæcal bud and appendix curved on itself and turning its concavity to the left, into the subhepatic position. The greater part of the small intestinal coils lie now below and to the left of the cæcum, but the terminal ileal convolutions (Fig. 500) still occupy a retro-cæcal position, separating the pouch and the colon from the dorsal parietal peritoneum. In Fig. 501 the right lateral view of the same embryo is shown with the cæcum and colon depressed and turned to the left. The termination of the ileum reaches the ileo-colic junction by passing behind the cæcum, and the immediately adjacent ileal coils are still retro-cæcal, intervening between the pouch and the dorsal parietal peritoneum.
In the next succeeding stage (schema, [Fig. 494]) these coils of the ileum turn downward and to the left so as to lie below and mesad to the cæcum and colon, thus permitting the direct apposition of the large intestine to the parietal prerenal peritoneum. The terminal ileum now passes from below and to the left upwards and to the right to its junction with the colon. This freeing of the dorsal surface of cæcum and colon from contact with the coils of the small intestines, and the consequent direct apposition of the same to the dorsal parietal peritoneum influences to a great extent the subsequent arrangement of the parts, because it affords the conditions necessary to the fixation of the colon and mesocolon by adhesion to the parietal peritoneum (cf. [p. 81]).
[Fig. 499], taken from an embryo of 6.7 cm. vertex-coccygeal measure, illustrates this stage, which is encountered in the majority of instances and during which the retro-cæcal coils of the terminal ileum are withdrawn (schema, [Fig. 493]). The convolutions of the small intestine have greatly increased in size and number. The retro-cæcal ileal coils, compared with Fig. 500, have shifted their position caudad and to the left, so as to lie below and ventrad of the beginning of the colon. Only a single coil remains behind the cæcum and appendix, intervening between these structures and the ventral surface of the right kidney, and this coil is in the process of withdrawal from the dorsal position as indicated by the superficial and short course of the coil which connects it with the remaining ventral convolutions. As soon as the withdrawal of this single remaining dorsal coil is completed the entire mass of the small intestines will occupy a position ventrad, caudad and to the left of the cæcum and colon ([Fig. 494]), which will then rest directly against the dorsal parietal peritoneum investing the ventral surface of the right kidney.