_a_, Colon; _b_, cæcum; _c_, ileum; _d_, ileo-cæcal opening; _e_,
circular depression around end of ileum. (After P. Schumann.)
]
The mucous membrane of the cæcum contains numerous solitary glands; they
are found also in the first part of the colon.
The =rectum= is almost completely covered with peritoneum, the line of
peritoneal reflection being under the second or third coccygeal
vertebra. At the junction of the rectum and anus the mucous membrane has
a stratified, squamous epithelium, and contains the =anal glands=. A
small opening on either side leads into two lateral =anal sacs= (Sinus
paranales); these are usually about the size of a hazelnut, and contain
a dirty gray, fatty substance, which has a peculiar and very unpleasant
odor. The skin which lines these pouches contains coil glands. Further
back the skin contains large sebaceous glands and peculiar =circumanal
glands=.
The retractor ani muscle is extensive. It arises from the shaft of the
ilium, the pubis, and the symphysis pelvis, and passes upward and
backward to end on the first coccygeal vertebræ and the sphincter ani
externus. The retractors, together with the coccygei, form a sort of
pelvic diaphragm, analogous to that of man.