The spiral fold of the caecum in the ostrich (Struthio camelus).

The caecum of the ostrich was observed to contain a fold that spiralled approximately 30 times, transforming the lumen into a long, spiral-shaped cavity. The fold consisted of mucosa, muscularis mucosa and submucosa. The proximal part of the fold also contained a central core of muscle originating from the inner circular muscle layer of the caecal wall. The distal part was expanded to form a 'frill', thereby enlarging the surface area of the fold. Blood vessels entered the fold at the mesenteric border from where they were distributed to the fold and caecal wall. From its origin near the caecorectal junction, the fold rapidly increased in height to attain 2.75 +/- 0.95 cm at the 9th turn of the spiral. Thereafter it gradually decreased and disappeared approximately 14 cm from the apex of the caecum. The surface area of the spiral fold averaged 955.75 cm2 and constituted 54% of the total caecal mucosal surface. It is suggested that the fold plays an important role in the absorption of volatile fatty acids and other metabolites produced by microbial fermentation of cellulose and hemicellulose.