Anatomy
The vermiform appendix is present only in
humans, certain anthropoid apes and the wombat (a nocturnal, burrowing
Australian marsupial). It is
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).