The jejunum occupies the left upper portion of the abdomen, and the ileum is positioned in the right abdomen and upper part of the pelvis. No distinct anatomic demarcation exists between jeju-num and ileum. Visual examination of the luminal surface of the small intestine reveals mucosal folds, the plicae circulares. More numerous in the proximal jejunum, the plicae circulares decrease in number in the distal small bowel and are absent in the terminal ileum. Aggregates of lymphoid follicles are scattered throughout the small intestine but are found in highest concentration within the ileum, where they are designated Peyer's patches.
Peyer's patches normally are more prominent during infancy and childhood than they are in adulthood. The small bowel is in continuity with the colon at the ileocecal valve, which comprises two semilunar lips that protrude into the cecum. The ileocecal valve provides a barrier to the retrograde flow of colonic contents into the small intestine. This barrier appears to be a function of the angulation between the ileum and cecum that is maintained by the superior and inferior ileoceal ligaments, and a true tonic, sphincter-type pressure does not appear to be present in this region.
Colon and Rectum The colon is a tubular structure approximately 30 to 40 cm in length at birth in the full-term infant. In the adult, the colon measures 1. 5 m, about one quarter of the length of the small bowel. The diameter of the colon is greatest in the cecum (7. 5 cm) and narrowest in the sigmoid (2. 5 cm). The colon is continuous with the small intestine proximally at the ileocecal valve and ends distally at the anal verge ( Fig. 93-1 ).
The external appearance of the colon differs from that of the small bowel, because the longitudinal muscle fibers of the colon coalesce into three discrete bands called teniae, located at 120-degree intervals about the colonic circumference: tenia liberis, tenia omentalis, and tenia mesocolica. The teniae start at the base of the appendix and extend continuously to the proximal rectum. Outpouchings of the colon, the haustra, are found between the teniae. Semilunar folds characterize the mucosa between the haustra. Sacs of peritoneum filled with adipose tissue, the appendices epiploicae, are found on the surface of the colon.