HORSE- BACK RIDING. 31 



head and thorax, which, endowed, so to speak, with 

 movement independent of those of the trunk, seem 

 to be subject to some foreign influence, though they 

 have in fact received the same impulse, but trans- 

 formed and exaggerated by the force of gravity act- 

 ing most strongly upon the parts furthest removed 

 from the trunk, and the more easily in proportion to 

 their mobility. 



This force when strongly applied may cause the 

 fall of the rider, but when utilized and applied judi- 

 ciously renders his seat firm and secure. 



The sacro-lumbar and long dorsal muscles, by 

 drawing the chest and head backwards, cause the 

 centre of gravity to fall behind the perpendicular line, 

 and oppose a certain resistance to its displacement 

 forward. The strength of the. muscular contraction, 

 in order to effect this object, must be in proportion 

 to the impulse imparted to the trunk. 



In galloping, the rider is conscious only of an oscil- 

 lation backwards and forwards alternately, and the 

 flexors of the thigh, the psoas and sacro-lumbar mus- 

 cles are especially called on to restore any consider- 

 able displacement, to recover the centre of gravity, 

 whether it be thrown forwards or back, according to 

 the need, while the adductors fix the knees. 



We may in this way explain the theory of horse-back 

 riding in reference to its mechanical action, and if 

 from it we may infer that gravity contributes towards 

 restoring the equilibrium which it has helped to de- 



