1102 WALKING. [Book ni. 



the muscle has contracted, assist its return to its proper position 

 of rest. When a point has to be fixed the two sets of muscles 

 which act as antagonists on the point are both thrown into con- 

 traction, in proportion to their relative effect. If the action of 

 one muscle (or set of muscles) is to be dominant its antagonist 

 may take no part in the action, being neither contracted nor 

 relapsed ; but there are reasons for thinking that, in many cases 

 at all events, the action of one muscle though remaining domi- 

 nant is tempered and guarded so to speak by a concomitant 

 feebler action of its antagonist; on the other hand we have evi- 

 dence as in the case of the ocular muscles (§ 595) that it may 

 be assisted by an inhibition, a relaxation of the antagonist. 

 These several phases are governed by the nervous system, and 

 the behaviour of antagonistic muscles and groups of muscles 

 affords many instances of what we have so often insisted upon, 

 namely, that nearly all the various movements of our body are 

 coordinate movements, and that in many cases the coordination 

 is extremely complex. 



§ 684. The erect posture, in which the weight of the body 

 is borne by the plantar arches, is the result of a series of con- 

 tractions of the muscles of the trunk and legs, having for their 

 object the keeping the body in such a position that the line of 

 gravity falls within the area of the feet. That this does require 

 muscular exertion 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 can- 

 not 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 thoracic 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. 



§ 685. In walking advantage is taken of this forward posi- 

 tion of the centre of gravity, and the tendency to fall forwards 

 is utilized to swing each leg in turn forwards after the fashion 

 of a pendulum. In each step there is a moment at which the 



