In STOOPING (fig. 28) or SQUATTING (fig. 29), as in picking up any thing from the ground, the lower limbs and the trunk are bent in a zigzag manner; the heels are raised; and the plane of gravity falls, in front of the ankles, over the balls of the toes. Now we recognise one of the advantages which accrues to man from the great length of his thigh. For the head and upper part of the trunk are advanced so far in _front_ of the feet, that it would be impossible to maintain a balance at all, even upon the balls of the toes, and we should necessarily fall forwards, were it not that, owing to the length of the thigh, the lower part of the trunk is carried backwards to a plane _behind_ the heels, and so serves to maintain the equilibrium. _Walking._ [Illustration: Figs. 30. 31. 32. Walking.] [Illustration: Figs. 33. 34. 35. Walking.] Let us next consider the part which the foot performs in WALKING. To understand this it is necessary to consider its positions and movements in the several stages of a step. When first placed upon the ground the foot (R, fig. 30) is a little in advance of the body; and the heel comes first (fig. 33) into contact with the ground. The toes quickly follow; and the body, then, passes, vertically, over, the ankle and the key-bone of the instep. The foot (R, fig. 31 and fig. 34) now rests steadily upon the heel and the balls of the toes; the other foot (L) leaves the ground, so that the whole weight is borne by one foot; and the plantar arch of that foot expands a little, so as to cause slight lengthening of the foot, under the weight that is laid upon it. Much yielding of the arch is, however, prevented by the ligaments that brace the arch (fig. 7), and by the muscles that are disposed beneath it. Next, the heel (fig. 35) is raised by the action of the calf muscle, and the weight of the body is thrown forwards, over the balls of the toes, while the other foot (L, fig. 32) is carried onwards, and is placed upon the ground ready to receive the weight and commence its carrying work. When this has been done the foot is withdrawn from the ground; and, in the withdrawal, a final impulse onward is given, so as to throw the weight of the body fairly over to the other foot. The fore part of the foot is then raised, and the knee is bent a little. By these means the toes are kept clear of the ground, while the foot is swung forward, beside the other, so as to be ready again to rest upon the ground and bear the weight of the body. In each complete step, therefore, there is a period during which the foot rests upon the ground, and a period in which it is swinging in the air. In walking the former period is considerably longer than the latter; and at the commencement, and at the end, of that period (figs. 30 and 32) the other foot is also upon the ground, so that it is only during the middle of the time (fig. 31) in which the foot rests upon the ground that it has to bear the whole weight of the body.