. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. MOTION. 461 allowed to accompany the movements of the legs without muscular effort. When the body is put in motion, the momentum generated re- quires an equal force in the opposite direction to stop it, for which purpose the trunk is thrown back; and this, with the resistance of the feet on the ground, will commonly suffice. When, however, this is not the case, the motion must be arrested gradually, or, as it often happens when the plane of position is composed of ice, the leg goes on without the trunk, arid the cen- tre
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. MOTION. 461 allowed to accompany the movements of the legs without muscular effort. When the body is put in motion, the momentum generated re- quires an equal force in the opposite direction to stop it, for which purpose the trunk is thrown back; and this, with the resistance of the feet on the ground, will commonly suffice. When, however, this is not the case, the motion must be arrested gradually, or, as it often happens when the plane of position is composed of ice, the leg goes on without the trunk, arid the cen- tre of gravity comes to the ground. In walking, the trunk is also elevated and depressed at each step vertically, as well as oscillated in other directions. By the assist- ance of a rod graduated into millimetres, which was carried at the head of the trochanter major by the ambulator, and viewed through a tele- scope, Weber was enabled to ascertain the amount of the elevation and depression of the trunk. He found that when the length of the steps treading on the whole sole of the foot measured feet, the mean elevation and de- pression were inches. The plane in which the rod vibrated, and the magnitude of the oscillations, did not appear to vary materially, whether the speed was accelerated or retarded. In walking on the ball of the great toe the mean elevation and depression of the trunk was inch. Estimate of the forces employed in walking. The forces which we have to estimate in walk- ing are, first, that of the extension of the leg; secondly, the gravity of the body ; thirdly, the resistance which the body encounters in pro- gression. Of these, the first is that force which the leg exerts by its extension to bring it into a straight line, as if both of its extremities (viz. the head of the femur, and that part of the foot which is in contact with the ground) endeavoured to push each other away. The direction of this force depends on the position of the extremi- tie
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