Fig. 502.—Human embryo, 6.6 cm. vertex-coccygeal measure. Liver removed. (Columbia University, Study Collection.)

This stage is illustrated in Fig. 502, taken from an embryo of 6.6 cm. vertex-coccygeal measure. The cæcum and appendix here occupy the subhepatic position, well to the right of the median line and in the background of the abdominal cavity. The terminal retro-cæcal ileal coils of the embryo shown in Figs. 500 and 501 have descended caudad and to the left, thus freeing the dorsal surface of cæcum and colon and permitting direct contact with the prerenal parietal peritoneum.

In the succeeding stages the cæcum gradually descends along the background of the right lumbar region from the subhepatic position to the right iliac fossa, producing by this descent the ascending colon as a distinct segment of the large intestine.

It will be observed that in the stage shown in Fig. 502 (schema, [Fig. 494]) the large intestine passes from the cæcum to the splenic flexure transversely from right to left across the upper part of the abdominal cavity, caudad and ventrad of the stomach and cephalad of the coils of the small intestine.

In the following stages the disproportionately large size of the embryonic liver compels the colon, as the cæcum descends, to assume an oblique position. When the cæcal descent is completed the colon traverses the abdominal cavity in contact with the caudal surface of the liver passing from the right iliac fossa obliquely cephalad and to the left to the splenic flexure where it becomes continuous with the descending colon, which segment has early assumed its definite position in the background of the abdominal cavity on the left side ([Fig. 495]). This oblique position of the colon is seen in Figs. 503 and 504. During this stage the increase in the length of the colon may lead to the arrangement seen in [Fig. 505], where the future transverse segment of the large intestine is bent caudad in form of an arch whose summit extends nearly to the pelvis. This condition at times persists in the adult, in cases of unusually long large intestine, and recalls the normal arrangement found in many of the cynomorphous monkeys in whom the transverse colon forms an extensive V-or U-shaped loop, with the apex directed caudad toward the pubic symphysis ([Fig. 506]). In other instances in the human fœtus this part of the large intestine is thrown into a number of shorter irregular coils ([Fig. 507]).

Fig. 503.—Human embryo, 7.6 cm. vertex-coccygeal measure. Liver andsmall intestine from the duodeno-jejunal to the ileo-colic junction removed.(Columbia University, Study Collection.)

Fig. 504.—Human fœtus, 10.6 cm. vertex-coccygeal measure. Liver andgreater part of small intestine removed. (Columbia University, Study Collection.)

Fig. 505.—Human fœtus, 20.4 cm. vertex-coccygeal measure. (ColumbiaUniversity, Study Collection.)

Fig. 506.—Abdominal viscera of Macacus rhesus, rhesus monkey, hardenedin situ. (Columbia University Museum, No 1817.)

Fig. 507.—Abdominal viscera of human fœtus at term, hardened insitu; hepatic flexure formed, and ascending and transverse colon differentiated.(Columbia University Museum, No. 1816.)

 

Fig. 508.—Human fœtus of 10.7 cm. vertex-coccygeal measure. Liverand small intestine from the duodeno-jejunal to the ileo-colic junctionremoved. The colon already presents an ascending, transverse anddescending segment. The appendix is retro-cæcal, curved, with the tipturned down, under cover of the ileo-colic junction and mesentery.(Columbia University, Study Collection.)

Normally, however, in the process of further development and with the relative decrease in the size of the liver, the hepatic flexure ([Fig. 505]) becomes defined and passes cephalad and to the right, taking up the slack of the bent segment and establishing the typical ascending and transverse colon as seen in Fig. 508 (schema, [Fig. 496]).

III. VARIATIONS OF ADULT CÆCUM AND APPENDIX.

The study of the variations of the adult cæcum and appendix involves the consideration of the following points: