The alternate rotation of the trunk upon the limb and the limb upon the trunk is well seen in fig. 27, p. 59.
At _A_ of fig. 27 the trunk (_g_) is observed rotating on the left foot (_f_). At _D_ of fig. the left leg (_h_) is seen rotating on the trunk (_a_, _i_): these, as explained, are complementary movements. At _A_ of fig. the right foot (_c_) is firmly placed on the ground, the left foot (_f_) being in the act of leaving it. The right side of the trunk is on a lower level than the left, which is being elevated, and in the act of rolling over the foot. At _B_ of fig. the right foot (_m_) is still upon the ground, but the left foot having left it is in the act of swinging forward. At _C_ of fig. the heel of the right foot (_n_) is raised from the ground, and the left foot is in the act of passing the right. The right side of the trunk is now being elevated. At _D_ of fig. the heel of the right foot (_o_) is elevated as far as it can be, the toes of the left foot being depressed and ready to touch the ground. The right side of the trunk has now reached its highest level, and is in the act of rolling over the right foot. The left side of the trunk, on the contrary, is subsiding, and the left leg is swinging before the right one, preparatory to being deposited on the ground.
From the foregoing it will be evident that the trunk and limbs have pendulum movements which are natural and peculiar to them, the extent of which depends upon the length of the parts. A tall man and a short man can consequently never walk in step if both walk naturally and according to inclination.[39]
[39] “The number of steps which a person can take in a given time in walking depends, first, on the length of the leg, which, governed by the laws of the pendulum, swings from behind forwards; secondly, on the earlier or later interruption which the leg experiences in its arc of oscillation by being placed on the ground. The weight of the swinging leg and the velocity of the trunk serve to give the impulse by which the foot attains a position vertical to the head of the thigh-bone; but as the latter, according to the laws of the pendulum, requires in the quickest walking a given time to attain that position, or _half_ its entire curve of oscillation, it follows that every person has a certain measure for his steps, and a certain number of steps in a given time, which, in his natural gait in walking, he cannot exceed.”