Much light is thrown on the adult anatomy of the parts and on the origin of the variations observed by the study of their embryonic history. In considering the factors which determine the variations in the position, size, and shape of the appendix it must be remembered that the rudimentary character of this structure is responsible for many of the aberrant conditions encountered.
As a part of the general cæcal pouch which persists in an early developmental stage and which we can regard as destined for further reduction and ultimate elimination in the course of evolution, the appendix shares with other vestigial structures a wide range of variation. Consequently the study of the development of this portion of the alimentary tract enables us to gain a clearer view of the primary arrangement of the structures and to trace the causes which are active in determining the adult conditions most frequently encountered.
At the time when the umbilical loop of the intestine has formed and has begun to protrude into the cavity of the umbilical cord (fifth to sixth week), the first indication of the future cæcum appears as a circumscribed thickening of the returning or ascending limb of the intestinal loop a short distance from the apex (Figs. 486-488). This rudiment indicates the derivation of the future definite intestinal segments from the elements of the loop. The descending limb, apex (site of embryonic vitelline duct, Meckel’s diverticulum of adult) and a short succeeding portion of the ascending limb furnish the ileum and jejunum. The rest of the ascending limb develops into cæcum and appendix, ascending and transverse colon. The increase in the length of the intestine is not uniform. The formation of convolutions begins in the seventh week in the apex and subsequently in the descending limb. By the eighth week a considerable number of jejuno-ileal coils have resulted from the growth in length of these parts of the original umbilical loop, while the growth of the segment which furnishes the colon is at this time still inconsiderable ([Fig. 489]). In the meanwhile the thickening of the tube which forms the first rudiment of the cæcum has developed into a small sac-like enlargement of the gut, budding from the left and dorsal aspect of the ascending limb, crescentic in shape, turning its concavity toward the parent tube. In the majority of instances examined the small outgrowth is packed closely between the incipient ileal convolutions, lying under cover of the more prominent bulging coils of the umbilical protrusion, between them and a single coil of larger arc situated dorsally and belonging to the jejunal or proximal portion of the small intestine (Fig. 490). [Fig. 497], taken from an embryo of 11 mm. cervico-coccygeal length, represents this stage in the development of the umbilical loop. The arrangement of the cæcum which we can assume as the typical condition at this stage and which determines in part the subsequent final arrangement of the structures, is illustrated by this relation of the cæcal bud to the surrounding incipient convolutions of the small intestine, with the larger part of these coils situated ventrad of the cæcum and only a single coil of larger curve placed dorsally; the cæcal pouch, derived from the ascending limb of the umbilical loop, is situated between these two divisions, turning its concave border to the right and embracing the parent tube. At the time when the human cæcum first appears as a distinct structure it forms a small conical pouch with blunt extremity whose shape is well illustrated by the cæcum of some of the new-world monkeys, as Mycetes fuscus, the brown howler monkey ([Figs. 449] and [450]). The outgrowth develops rapidly in length and very soon assumes a distinct crescentic shape, gradually tapering toward the extremity, a type which is found reproduced in the cæcum of Ateles ater, the black-handed spider monkey ([Fig. 443]). There is as yet no constriction or demarcation separating the distal segment (future appendix) from the proximal part (cæcum proper) but the entire pouch gradually narrows funnel-like to its termination.
Figs. 488-492.—Series of schematic figures illustrating stages in the rotation of the intestinal canal.
Figs. 493-496.—Series of schematic figures illustrating stages in the rotation of the intestinal canal.
II. CHANGES IN THE POSITION OF THE CÆCUM AND APPENDIX DURING NORMAL DEVELOPMENT, DEPENDING UPON THE ROTATION OF THE INTESTINE AND THE SUBSEQUENT DESCENT OF THE CÆCUM.