A Theoretical and Comparative Study of the Functional Dependence of the Semicircular Canal upon Its Physical Dimensions

It has long been known that, in contrast to the range of variation of body dimensions, the semicircular canals of different animals are approximately similar in size. In the present study, measurements of the internal radius (r) and radius of curvature (R) of the endolymphatic canal were made in 87 species, comprising 46 mammals, 17 birds, 17 fishes and 7 reptiles, using specimens, and photographic records of specimens, already available. Using the principles of dynamical similarity, theoretical arguments are adduced which predict that if Steinhausen’s (1933) interpretation of semicircular canal function is correct there should be very slow increases, systematic but discreet, of r and R with body mass (m). The results of the measurements largely confirmed the theoretical prediction, the actual relations obtained for all species together being: log10100r2 = 0·0945 (±0·0549) log10 m+0·2519 and log10100R = 0·0761 (±0·0402) log10 m + 2·3797 (where r and R are measured in mm and m in kg), similar relations being obtained for each class examined. It is shown that the observed changes in r and R can to a large extent account respectively for the changes in canal sensitivity and the time constant of cupular return which the theory suggests should be called for by changes in animal shape and body mass.