The sesamoid bone known as the fabella—which literally means "little bean"—is found inconstantly in man on the anterior gliding surface of the external head of the gastrocnemius muscle. Rarely, a sesamoid is seen on the medial head of the gastrocnemius muscle. The inconstancy of the fabella in man and of its location about the knee joint, as evidenced by roentgenographic examination, may be misleading in the diagnosis of conditions of the knee joint (fig. 1 A). In rats, cats, rabbits and other animals sesamoids are constantly present1a both in the lateral and in the medial heads of the gastrocnemius muscles (figs. 2 A, and 3). In these animals one or two additional ossicles are present on the popliteus muscle facing the posterior surface of the tibia.
Sesamoids—for example, those on the lateral or medial heads of the gastrocnemius muscles—are each derived from a specific anlage. The fabellae are not