Anatomy

The vermiform appendix is present only in humans, certain anthropoid apes and the wombat (a nocturnal, burrowing Australian marsupial). It is a blind muscular tube with mucosal, submucosal, muscular and serosal layers. Morphologically, it is the undeveloped distal end of the large caecum found in many lower animals. At birth, the appendix is short and broad at its junction with the caecum, but differential growth of the caecum produces the typical tubular structure by about the age of 2 years (Condon). During childhood, continued growth of the caecum commonly rotates the appendix into a retrocaecal but intraperitoneal position (Fig. 59.1). In approximately a quarter of cases, rotation of the appendix does not occur resulting in a pelvic, suhcaecal or paracaecal position. Occasionally, the tip of the appendix becomes extraperitoneal lying behind the caecum or ascending colon. Rarely, the caecum does not migrate during development to its normal position in the right lower quadrant of the abdomen. In these circumstances the appendix can be found near the gall bladder or, in the case of situs inversus viscerum, in the left iliac fossa causing diagnostic difficulty if appendicitis develops (Fig. 59.2).

The position of the base of the appendix is constant, being found at the confluence of the three taeniae coli of the caecum which fuse to form the outer longitudinal muscle coat of the appendix. At operation, use can he made of this to find an elusive appendix, as gentle traction on the taeniae coli, particularly the anterior taenia, will lead the operator to the base of the appendix.

The mesentcry of the appendix or mesoappendix arises from the lower surface of the mesentery of the terminal ileum, and itself is subject to great variation. Sometimes as much as the distal third of the appendix is bereft of mesoappendix. Especially in childhood, the mesoappendix is so transparent that the contained blood vessels can he seen (Fig. 59.3). In many adults it becomes laden with fat, which obscures these vessels. The appendicular artery, a branch of the lower division of the ileocolic artery, passes behind the terminal ileum to enter the mesoappendix a short distance from the base of the appendix. It then comes to lie in the free border of the mesoappendix. An accessory appendicular artery may be present but, in most people, the appendicular artery is an ‘end-artery’, thrombosis of which results in necrosis of the appendix (syn. gangrenous appendicitis). Four, six or more lymphatic channels traverse the mesoappendix to empty into the ileocaecal lymph nodes.

Microscopic anatomy

The appendix varies considerably in length and circumference. The average length is between 7.5 and 10 cm. The lumen is irregular, being encroached upon by multiple longitudinal folds of mucous membrane lined by columnar cell intestinal mucosa of colonic type (Fig. 59.4). Crypts are present but are not numerous. In the base of the crypts lie argentaffln cells (Kultschitzsky cells) which may give rise to carcmnoid tumours (vide in Ira). The appendix is the most frequent site for carcinoid tumours which may present with appendicitis due to occlusion of the appendiceal lumen.

The submucosa contains numerous lymphatic aggregations or follicles. This profusion of lymph tissue has promoted the concept that the appendix is the human equivalent of the avian bursa of Fabricius as a site of maturation of thymus-independent lymphocytes. While no discernible change in immune function results from appendicectomy, the prominence of lymphatic tissue in the appendix of young adults seems important in the aetiology of appendicitis (vide infra).