CHAP. VIL] SPECIAL MUSCULAR MECHANISMS. 663 



is shewn by the facts, that a person when standing perfectly at 

 rest in a completely balanced position falls when he becomes 

 unconscious, and that a dead body cannot be set on its feet. 

 The line of gravity of the head falls in front of the occipital 

 articulation, as is shewn by the nodding of the head in sleep. 

 The centre of gravity of the combined head and trunk lies at 

 about the level of the ensiform cartilage, in front of the tenth 

 dorsal vertebra, and the line of gravity drawn from it passes 

 behind a line joining the centres of the two hip-joints, so that the 

 erect body would fall backward were it not for the action of the 

 muscles passing from the thighs to the pelvis assisted by the 

 anterior ligaments of the hip-joints. The line of gravity of the 

 combined head, trunk and thighs falls moreover a little behind 

 the knee-joints, so that some, though little, muscular exertion is 

 required to prevent the knees from being bent. Lastly, the line of 

 gravity of the whole body passes in front of the line drawn 

 between the two ankle-joints, the centre of gravity of the whole 

 body being placed at the end of the sacrum ; hence some exertion 

 of the muscles of the calves is required to prevent the body falling 

 forwards. 



In walking, there is in each step a moment at which the body 

 rests vertically on the foot of one, say the right leg, while the other, 

 the left leg, is inclined obliquely behind with the heel raised and 

 the toe resting on the ground. The left leg, slightly flexed to avoid 

 contact with the ground, is then swung forward like a pendulum 

 the length of the swing or step being determined by the length of 

 the leg ; and the left toe * is brought to the ground. On this left 

 toe as a fulcrum, the body is moved forward, the centre of gravity 

 of the body describing a curve the convexity of which is upward 

 and the left leg necessarily becoming straight and rigid. As the 

 body moves forward, a point will be reached similar to that with 

 which we supposed the step to be started, the body resting 

 vertically on the left foot, and the right leg being directed behind 

 in an oblique position. The movement on the left foot however 

 carries the body beyond this point, and in doing so swings the 

 right leg forward until it is the length of a step in advance of its 

 previous position, and its toe in turn forms a fulcrum on which the 

 body, and with it the left leg, is again swung forward. Hence 

 in successive steps the centre of gravity, and with it the top of the 

 head, describes a series of consecutive curves, with their convexities 

 upwards, very similar to the line of flight of many birds. 



Since in standing on both feet the line of gravity falls between 

 the two feet, a lateral displacement of the centre of gravity is 

 necessary in order to balance the body on one foot. Hence in 

 walking the centre of gravity describes not only a series of vertical, 



1 This indicates perhaps what should be done rather than the actual practice; 

 most people put the heel to the ground first, the contact with the toe coming later. 



