Planar relationships of the semicircular canals in rhesus and squirrel monkeys

Abstract

The technique of principal-component analysis was used to define anatomically the semicircular canal planes of the rhesus and squirrel monkeys with respect to the stereotaxic coordinate system. The analyses were performed on a series of points obtained from the dissected osseous labyrinths. A planar equation was defined for each canal plane in the stereotaxic coordinate systema and angles were calculated between the 3 ipsilateral canal planes, between synergistic canal pairs and between each canal plane and the stereotaxic planes. The data frrm both species are similar: the ipsilateral canal planes are nearly orthogonal; synergistic pairs of canal planes are approximately parallel with angles of 2°–12° between pairs in the rhesus monkey and 13°–16° between pairs in the squirrel monkey. The horizontal canal planes from angles of 22° and 18° with the horizontal stereotaxic plane in the rhesus and squirrel monkeys, respectively. A head position of 15° (pitch nose-down) was calculated to produce an optimal head position in both species for maximally stimulating the horizontal canals and minimally stimulating the vertical canals during horizontal angular acceleration. The radii of curvature (R) of the horizontal, anterior and posterior canals were also measured for both species using a calibrated reticle. These measurements indicate that the anterior canal of both species has the largest raduis of curvature. This anatomical information is discussed in relation to the available physiological data.