MOTION AND LOCOMOTION. 149 



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

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

 hanging 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 short- 

 ening, the very time at which, we have just seen, it works 

 at most mechanical 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 working of the skeletal muscles, strength 

 being sacrificed for variety, ease, rapidity, extent, and ele- 

 gance 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 leap- 

 ing, running, or falling. In all postures the condition of 

 stability is that the vertical line drawn through the centre 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 proportionate to the extent of this 

 base, for the wider it is the less is the risk of the perpendicu- 

 lar through the centre of gravity falling outside of it on slight 

 displacement. 



The Erect Posture. This is pre-eminently 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 co-operation of many muscles, co-ordinated 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 centre of gravity of the whole adult Body lies 



