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“But while the fore legs advance the body by drawing the free lever to the fixed leg-segment, the hind legs do this in exactly the opposite way. The hind leg, namely, seeks to stretch out the tibia, and thus to increase the angle of the knee (R3), thereby giving a push on the ground, by means of which the body is shoved forward a bit.

“Though it might be supposed that the feet would remain stationary during the extension or retraction of the limbs, this never occurs in actual walking. Not merely the upper, but also the lower, thigh is either drawn in or stretched out, as the case may be. The latter then describes a straight line with its point during this scraping or scratching motion (Fig. 115, no), which is obviously the chord to that quadrant which would be drawn by the tibia or foot in a yielding medium, as water, for instance. But even this motion results extremely rarely, and never in actual walking. If we fix our eye anew upon the fore leg at the very moment when it is again retracted, after the resultant ‘fixing,’ we shall then observe that the hip also is simultaneously turned backward in a definite angle. The tibia would describe the arc nq (Fig. 115) by means of the latter alone.