The Functional Significance of Semicircular Canal Size

Abstract

A striking feature of the semicircular canal is its small size. In man the radius of curvature of the endolymphatic canal is about 3.3 mm and the internal radius of the lumen 0.15 mm. Furthermore, as will emerge below, it is of more than passing interest that on the whole these dimensions show relatively little change with the size of the animal. Thus, it can be calculated from the data of Jones and Spells (1963) that on average for all animals examined, a 1000-fold increase in body weight (and hence presumably about a 10-fold increase in linear dimensions of the body) is associated with increases of canal radius of curvature and internal radius by factors of only 1.7 and 1.4 respectively. It seems that natural selection in evolution has met with some constraint tending to keep the physical dimensions of these small end-organs within rather closely prescribed size limits, and it is therefore pertinent to enquire into the kind of physiological consequence which might derive from such constraint.