Geotropic Orientation of Young Rats. by W. J. Crozier A~d G. Pincus

In describing the geotropic conduct of young rats (RaUus norvegicus) t we have stressed the fact that to obtain data suitable for quantitative treatment of the gravitational orientation it is necessary to employ individuals of the closest possible similarity. This has meant, in practice, employing litter mates from lines long inbred, under external conditions as nearly uniform as possible. The result has demonstrated that, with these precautions taken, it is possible to formulate precisely the connection between the intensity of excitation and the extent of the geotropic orientation. On a creeping plane inclined at angle ~ to the horizontal, and in which the effective gravitational component is therefore proportional to sin a, the orientation path is one defined by an angle # on the plane, such that # = K log sin a C. The precision of the orientation increases according to the same equation. In addition, cos # = K sin ~. The speed of progression bears similar relations to the angle a. I t was proposed to interpret these results as signifying that on an inclined plane the rat orients upward until the difference between the work done by the legs of the two sides is reduced to a certain (constant) fraction of the total. I t is possible to entertain this view because the differential postures of the legs encourage it, and because they are extended in the plane of creeping. I t is supported by the effects of increasing the mass lifted during creeping, as by attaching weights to the base of the animal's tail. This conception of the geotropic excitation controlling the amount of orientation as a proprioceptive matter is strengthened by further findings in this laboratory regarding the