478 THE MECHANICAL FUNCTIONS. 



tilde of standing, would be thrown down by a very 

 small impulse. It is evident, indeed, that in the 

 living body, if the centre of gravity were at any 

 moment to pass beyond the base, no muscular effort 

 which could then be made would avail to prevent 

 the body from falling. But the actions of the 

 muscles are continually exerted to prevent the 

 yielding of the joints under the weight of the body, 

 which tends to bend them. In quadrupeds less 

 exertion is requisite for that purpose ; and standing 

 is in them, as we have seen, a posture of compara- 

 tive repose : in man it requires nearly as great an 

 expenditure of muscular power as the act of walk- 

 ing. Soldiers on parade experience more fatigue 

 by remaining in the attitude of standing, than they 

 would by marching during an equal time. Strictly 

 speaking, indeed, it is impossible for even the 

 strongest man to remain on his legs, in precisely 

 the same position, for any considerable length of 

 time. The muscles in action soon become fatigued, 

 and require to be relieved by varying the points of 

 support, so as to bring other muscles into play. 

 Hence the weight of the body is transferred alter- 

 nately from one foot to the other. The action of 

 standing consists, in fact, of a series of small and 

 imperceptible motions, by which the centre of 

 gravity is perpetually shifted from one part of the 

 base to another ; the tendency to fall to any one 

 side being quickly counteracted by an insensible 

 movement in a contrary direction. Long habit has 

 rendered us unconscious of these exertions, which 

 we are, nevertheless, continually making ; but a 

 child learning to walk finds it difficult to accom- 

 plish them successfully. It is one among those arts 



