The general shape of the cupula conformsto the receptor area from which it arises. In the vertical canals of the frog, the cristae have an overall dumbbell shape; the cupula has corresponding enlarged masses which project from the expanded receptor ends while the central portion, which is about one third as thick, completes the diaphragm. The horizontal canal crista has a single enlarged end (club shaped) and consequently has a single cupular mass extending along one side of the ampulla up to the apex. A thinner portion extends to the opposite ampullar wall. In the vertical canals the enlarged masses end before the summit such that the thinner center reaches the apex.The cupula forms a tight diaphragm across the ampulla which is most easily displaced at the center. The central zone of the crista serves to position hair cells about one third the distance to the center of the ampulla while the ends of the crista are more peripherally located in relation to the diaphragm. A graded response to diaphragmatic displacement results. The apex of the cupula is most labile to detachment but not initial displacement. This region can act as a relief valve such that once displacement of the diaphragm has reached its maximum for the base of the cupula, the apical end is displaced and ultimately detached. This mechanism serves to prevent damage to the receptor structure and its cupular relationship.The cupula is composed of two components: filamentous material (protein) and an amorphous substance (mucopolysaccharide). The protein is related to supporting cells and the distal end of the kinocilium. The mucopolysaccharide is secreted from cuboidal cells which surround the receptor area and may serve as a binding substance.From the saddle-shaped receptor surface ciliary tufts consisting of 3 or 4 rows of short stereocilia project into the lumen and a single row of 5 or 6 of these processes continues across the subcupular space (60–70 µm) accompanied by a single kinocilium. The kinocilium continues into the filamentous and mucopolysaccharide matrix of the cupula to form a linkage between the receptor cells and the cupula. Scattered binding filaments are found between the kinocilium and adjacent stereocilia which are similar to those described in the saccule and utricle.