The Vermiform Process or Appendix (processus vermiformis) is a long, narrow, worm-shaped tube, which starts from what was originally the apex of the cecum, and may pass in one of several directions: upward behind the cecum; to the left behind the ileum and mesentery; or downward into the lesser pelvis.
It varies from 2 to 20 cm. in length, its average being about 8.3 cm.
It is retained in position by a fold of peritoneum (mesenteriole), derived from the left leaf of the mesentery. This fold, in the majority of cases, is more or less triangular in shape, and as a rule extends along the entire length of the tube. Between its two layers and close to its free margin lies the appendicular artery.
The canal of the vermiform process is small, extends throughout the whole length of the tube, and communicates with the cecum by an orifice (Orifice of vermiform appendix) which is placed below and behind the ileocecal opening. It is sometimes guarded by a semilunar valve formed by a fold of mucous membrane, but this is by no means constant.
Structure.—The coats of the vermiform process are the same as those of the intestine: serous, muscular, submucous, and mucous. The serous coat forms a complete investment for the tube, except along the narrow line of attachment of its mesenteriole in its proximal two-thirds. The longitudinal muscular fibers do not form three bands as in the greater part of the large intestine, but invest the whole organ, except at one or two points where both the longitudinal and circular fibers are deficient so that the peritoneal and submucous coats are contiguous over small areas.
The circular muscle fibers form a much thicker layer than the longitudinal fibers, and are separated from them by a small amount of connective tissue. The submucous coat is well marked, and contains a large number of masses of lymphoid tissue which cause the mucous membrane to bulge into the lumen and so render the latter of small size and irregular shape. The mucous membrane is lined by columnar epithelium and resembles that of the rest of the large intestine, but the intestinal glands are fewer in number.