ii MECHANICS OF LOCOMOTOE APPAKATUS 107 



distance becomes gradually less, and in the erect posture is almost 

 negligible. On the other hand, the working line of the quadriceps 

 muscle which extends the knee remains at approximately the same 

 distance from the axis of the knee-joint during the movement. 



III. Leaving the study of the various positions that may be 

 assumed and the different movements that may be performed 

 by each part of the skeleton, we must here confine ourselves to 

 studying the different postures and movements of the body as a 

 whole in progression. 



In both standing and walking the position and the displace- 

 ment of the centre of gravity and of the line of action of gravity 

 of the whole body are of great importance. 



Every part of the body gravitates according to the vertical 

 line that falls from it to the earth. This infinite number of 

 perpendiculars which only meet at the centre of the earth, and 

 may therefore be regarded as parallels, may be replaced by one 

 single perpendicular line representing the sum of the component 

 forces ; this is known as the line of gravity. Whatever position 

 is assumed by the body, so long as it preserves the same form, 

 the lines of gravity corresponding with each posture intersect 

 at the same point which is known as the centre of gravity. 



In all bodies which are not geometrical in form and consist 

 of a heterogeneous mass, the centre of gravity can only be deter- 

 mined by experiment. This is done by suspending the body by a 

 cord successively in two different positions ; the directions of the 

 cord prolonged through the body give two lines of gravity, and 

 the point at which they intersect is the centre of gravity. The 

 exact determination of the centre of gravity of the human body 

 is much more difficult, since it is not a rigid body, and undergoes 

 changes of form. 



Borelli (1679) and the Webers, starting from the assumption that 

 in well-formed individuals the line of gravity must lie in the median 

 sagittal plane, or the plane of symmetry of the body, attempted 

 merely to ascertain the height of the centre of gravity, that is, its 

 distance from the sole of the foot and apex of the head, without 

 defining its position on the transverse vertical plane. For this 

 purpose they laid a man on his back upon a board supported on 

 a metal wedge, and placed the whole in equilibrium like the arms 

 of a balance. The vertical plane perpendicular to the length of 

 the body through the wedge that supports the board must pass 

 through the centre of gravity. They found that this plane was 

 nearer the crown of the head than the sole of the foot. If the 

 total height of a man be taken as 1000, the centre of gravity 

 would be found at 570 from the sole and at 430 below the crown. 



These observations were controlled by Harless and by Meyer, 

 who found values that did not vary more than 3 per cent from 

 those of the earlier observers. Harless found that in woman the 



