CHAP, viz.] SOME SPECIAL MECHANISMS. 1481 



over, after 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 contraction, 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 relaxed ; but there are reasons for thinking that, in many 

 cases at all events, the action of one muscle though remaining 

 dominant is tempered and guarded so to speak by a concomitant 

 feebler action of its antagonist ; there is no satisfactory evidence of 

 the occurrence of a relaxation in 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. 



921. The erect posture, in which the weight of the body is 

 borne by the plantar arches, is the result of a series of contractions 

 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 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 

 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. 



922. In walking advantage is taken of this forward position 

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

 utilised to swing each leg in turn forwards after the fashion of a 

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

 resting vertically on one leg, say the right, while the other is 

 inclined obliquely behind. The two legs and the plane of the 

 ground form a right-angled triangle, of which the left leg is the 



