The lower end of the ileum ends by opening (Ileal orifice; Orifice of ileal papilla) into the medial and back part of the large intestine, at the point of junction of the cecum with the colon.
The opening is guarded by a valve, the ileal papilla (Colic valve) consisting of two segments or lips, which project into the lumen of the large intestine.
If the intestine has been inflated and dried, the lips are of a semilunar shape. The upper one (Ileocolic lip; Superior lip), nearly horizontal in direction, is attached by its convex border to the line of junction of the ileum with the colon; the lower lip (Ileocaecal lip; Inferior lip), which is longer and more concave, is attached to the line of junction of the ileum with the cecum. At the ends of the aperture the two segments of the valve coalesce, and are continued as narrow membranous ridges around the canal for a short distance, forming the frenulum of ileal orifice (frenula of the valve). The left or anterior end of the aperture is rounded; the right or posterior is narrow and pointed. In the fresh condition, or in specimens which have been hardened in situ, the lips project as thick cushion-like folds into the lumen of the large gut, while the opening between them may present the appearance of a slit or may be somewhat oval in shape.
Each lip of the valve is formed by a reduplication of the mucous membrane and of the circular muscular fibers of the intestine, the longitudinal fibers and peritoneum being continued uninterruptedly from the small to the large intestine.
The surfaces of the valve directed toward the ileum are covered with villi, and present the characteristic structure of the mucous membrane of the small intestine; while those turned toward the large intestine are destitute of villi, and marked with the orifices of the numerous tubular glands peculiar to the mucous membrane of the large intestine. These differences in structure continue as far as the free margins of the valve. It is generally maintained that this valve prevents reflux from the cecum into the ileum, but in all probability it acts as a sphincter around the end of the ileum and prevents the contents of the ileum from passing too quickly into the cecum.