The cecum and the vermiform appendix
The cecum is the first part of the large intestine. It begins caudally from the ileocecal valve and ends blindly in the right iliac fossa. Typically the cecum is located intraperitoneally in the right lower abdomen and has a length of 5 to 7 cm. Due to an incomplete rotation of the umbilical loop during embryogenesis however it may lie quite variably. Therefore clinically one differentiates between three important variations: mobile cecum (completely covered by peritoneum), free cecum (with its own mesocecum) and fixed cecum (secondary retroperitoneal). As in the colon taeniae, haustra and semilunar folds are found in the cecum but no appendices epiploicae.
The vermiform appendix is attached dorsomedially to the end of the cecum where all three taeniae converge. It is 2 to 15 cm long and lies often intraperitoneally retrocecal (65%) or in the lesser pelvis (30%). The appendix is attached to the posterior abdominal wall by the mesoappendix. Here taeniae, haustra, semilunar folds and appendices epiploicae are all absent.
The cecum is supplied by the anterior and posterior cecal arteries and the appendix by the appendicular artery (all branches of the ileocolic artery from the superior mesenteric artery). The venous blood drains through the correspondent veins into the superior mesenteric vein. As the colon both the cecum and appendix are innervated by the superior mesenteric plexus whereas the parasympathetic fibers come from the vagus nerve (cranial nerve X).
The microscopic structure of the cecum is equal to that of the colon: mucosa (columnar epithelium with crypts, contains goblet and endocrine cells), submucosa (with blood vessels and lymph nodes), muscularis (strongly pronounced inner circular musculature, outer longitudinal musculature almost restricted to the taeniae) and serosa/adventitia.
Histologically the appendix looks quite similar to the colon and cecum. A distinctive feature is however the numerous lymph follicles and the parafollicular tissue in the connective tissue layer of the mucosa (lamina propriae mucosae) and the submucosa. The crypts are particularly deep so that the follicles are in close contact to the intestinal lumen. M-cells (microfold cells) are found in the epithelium which access antigens from the intestinal lumen. As the appendix lacks taeniae it has a regular outer longitudinal musculature.
The main tasks of the cecum are the absorption of water and salts and the lubrication of the feces with mucus. Especially components from plant-rich food (e.g. cellulose) are bacterially decomposed here. This explains why herbivores have considerably larger ceca in comparison to carnivores.
The appendix is part of the GALT (gut-associated lymphatic tissue) and fulfills immunological functions. Furthermore it is assumed that it serves as a “safe house” for enterobacteria (e.g. in case of diarrhea). On the picture you can see an inflamed vermiform appendix which was removed operatively.