Analysis of the Geotropic Orientation of Young Rats. Vii

Extension of analysis of the functional basis of geotropic excitation and response in young rats has made it desirable to obtain, for additional genetically stabilized lines, further tests of the quantitative reproducibility of orientation data as secured from successive generations in these lines over a period of several years; and of the measures of variation of performance as these are related to the exciting vector and to the extent of orientation. Procedures are illustrated whereby the significance of measurements can be checked automatically. It is shown that differences apparent in the geotropic behavior of three inbred lines of R. norvegicus are quantitatively recoverable over a period of nine generations. The constant, characteristic features for each inbred line concern: the extent of upward orientation, absolutely and as a function of the inclination of the substratum; the threshold slope for orientation; the dispersion of mean orientation-angles as governed by the slope of substratum; the dependence of the dispersion of the relative variation of observed orientation-angles upon the intensity of excitation; and the proportion of the total variation of response which is modifiable as a function of the slope of surface. It is also shown that when for two lines of rats the curves connecting orientation-angle with inclination of substratum differ in position and in details of form, the curves none-the-less undergo distortions of homologous type when rats of these lines creep geotropically with the same additional load in the form of a mass attached at a corresponding position on the back; and that shifting this mass to another position induces a quite different modification of the curve. These effects are discussed in terms of the view that orientation during geotropic creeping is controlled by the adjustment of sensorially equivalent tension-excitation in the legs of the two sides of the body, and that the frequency distributions of thresholds for excitation within the several groups of receptor units concerned differ quantitatively among the inbred stocks, but are statistically constant within each line.