Chap. XLIII.] SITTING. 511 



walking. If, however, the feet be separated from 

 one another, the base of support is enlarged, and stand- 

 ing becomes more easy. 



As regards different parts of the body, the vertical 

 from the centre of gravity of the head passes in front 

 of the atlas articulation ; hence the head tends to 

 fall forward. The centre of gravity of head and 

 trunk, including the arms, is in front of the tenth 

 dorsal vertebra, at the level of the xiphoid process of 

 the breast bone. It is nearer the front the shorter the 

 individual happens to be. The vertical passes behind 

 a line joining the hip joints, and the tendency of head 

 and trunk is to fall backwards. This is overcome by 

 muscular effort aided by the ligaments, ileo-femoral, 

 fascia lata, etc. The perpendicular through the centre 

 of gravity of head, trunk, and thighs falls slightly 

 behind the knee joints, so that the tendency is still to 

 fall backwards. But the vertical from the centre of 

 gravity of the body as a whole passes in front of the 

 ankle joint ; hence the tendency to fall forward. 



In sitting* the body is supported on the tuberosities 

 of the ischia, and the legs are thrown out of action. 

 The vertical through the centre of gravity may pass 

 between the tubera, in front of them, or behind them. 

 In the two last cases, muscular effort or some support 

 prevents the body falling forwards or backwards. 

 The arm leaning on a table, for instance, gives sup- 

 port to the forward inclination, and the back of a chair 

 prevents the backward displacement. In the first 

 case, slight muscular effort maintains the balance. 



WaSking. The feature of walking is that the 

 body never entirely leaves the ground, but its weight 

 passes alternately from one foot to the other. The 

 dynamics of walking are shown in Fig. 218, where 

 the body is represented with one leg j perpendi- 

 cularly under the centre of gravity G, while the 

 other is behind, resting on the ground by the ball 



