HORSE-BACK RIDING. 29 



interferes to check this disturbance of his equilibrium, 

 or recover it, if it be lost. Two forces contribute to 

 these results : the proper management of the weight 

 of the body and the muscular contraction. The cen- 

 tre of gravity, which is simply the point of union of 

 the forces resulting from weight, contributes greatly to 

 the firm maintenance of the seat, if it falls directly 

 and vertically upon the saddle, but if it is greatly dis- 

 placed, it includes the whole body, and increases the 

 effect of the movement communicated to it. 



In walking, which is a regular gait, this displace- 

 ment is next to nothing ; therefore we will not insist 

 upon the inconsiderable movements caused by it, 

 since all the active interference of the rider is confined 

 to a small pressure of the knees determined by the 

 adductor muscles of the thighs. Nevertheless the 

 rider can scarcely avoid a slight swaying of the upper 

 portions of the body against which the sacro-lumber 

 and long dorsal muscles react, and hold back the 

 spine and thorax and with them the centre of gravity 

 from their constant tendency to fall forward. 



This almost permanent contraction of the muscles 

 brought into action — and which might be termed a 

 state of active immobility, since its effect is to fix 

 the points upon which it acts and maintain them in 

 a quiet state — becomes fatiguing if kept up for any 

 length of time. 



The trot is of all gaits the one requiring the great- 

 est number of movements on the part of the rider. 



