MOTION AND LOCOMOTION 107 



favorable to the muscle. Those who have practised the gym- 

 nastic feat of raising one's self by bending the elbows when hang- 

 ing by the hands from a horizontal bar know practically that if 

 the elbow-joints are quite straight it is very hard to start; and 

 that, on the other hand, if they are kept a little flexed at the 

 beginning the effort needed is much less; the reason being of 

 course the more advantageous direction of traction by the biceps 

 in the latter case. 



Experiment proves that the power with which a muscle can 

 contract is greatest at the commencement of its shortening, the 

 very time at which, we have just seen, it works at most me- 

 chanical disadvantage; in proportion as its force becomes less 

 the conditions become more favorable to it. There is, however, 

 it is clear, nearly always a considerable loss of power in the work- 

 ing of the skeletal muscles, strength being sacrificed for variety, 

 ease, rapidity, extent, and elegance of movement. 



Postures. The term posture is applied to those positions of 

 equilibrium of the Body which can be maintained for some time, 

 such as standing, sitting, or lying, compared with leaping, run- 

 ning, or. falling. In all postures the condition of stability is that 

 the vertical line drawn through the center of gravity of the Body 

 shall fall within the basis of support afforded by objects with 

 which it is in contact; and the security of the posture is propor- 

 tionate to the extent of this base, for the wider it is the less is the 

 risk of the perpendicular through the center of gravity falling 

 outside of it on slight displacement. 



The Erect Posture. This is preeminently characteristic of 

 man, his whole skeleton being modified with reference to it. 

 Nevertheless the power of maintaining it is only slowly learnt 

 in the first years after birth, and for a long while it is unsafe. 

 And though finally we learn to stand erect without conscious 

 attention, the maintenance of that posture always requires the 

 cooperation of many muscles, coordinated by the nervous system. 

 The influence of the latter is shown by the fall which follows a 

 severe blow on the head, which may nevertheless have fractured 

 no bone nor injured any muscle: the concussion of the brain, as 

 we say, "stuns" the man, and until its effects have passed off 

 he cannot stand upright. In standing with the arms straight 

 by the sides and the feet together the center of gravity of the 



