46 HORSE-BACK RIDING. 



it cannot take a large part in the act of respiration. 

 By its contraction, however, the thoracic cavity is 

 enlarged and gives entrance to a greater quantity of 

 air. The diaphragm takes part also to some extent, 

 though passively, in the act of expiration. The 

 viscera pass on to it the shock received by them at 

 each measure of the trot, which it resists, thereby 

 causing a slight relaxation of its fibres ; from this fol- 

 lows a contraction in the thoracic cavity, and the ex- 

 pulsion of a certain quantity of air. It may also, 

 when the motion of the horse is very gentle and the 

 rider makes no muscular resistance, take a more 

 active part in the respiration. 



It is to the inspiratory muscles and to the fact that 

 the respiration becomes costal, that the increased 

 capacity of the lungs is due. The diaphragm being 

 contracted and the ribs powerfully raised, the chest 

 finds itself considerably dilated, and the air fully fills 

 the lungs. By the relaxation of the muscles, the ribs 

 descend, the capacity of the lungs is diminished, and 

 the vitiated air which they contained escapes. 



But it is possible to produce a contrary effect. The 

 violent reaction of the trot of certain animals causes 

 the whole mass of the abdominal viscera to be thrust 

 forcibly against the diaphragm, thereby brusquely 

 expelling the air from the lungs. Respiration then 

 becomes painful and synchronous with the gait of 

 the horse, each expiration being short and sono- 

 rous. The inspiratory muscles are in a state of per- 



