42 HORSE-BACK RIDING. 



considerably contracted, the quantity of blood re- 

 maining the same, the arterial tension can only check 

 the circulation to a certain extent, and for the double 

 reason that the circulation is less active and the 

 quantity of blood sent to the surface, where it would 

 be cooled, smaller, its temperature rises, and this 

 heat paralyzes the vaso-motors which govern the 

 capillaries of the viscera ; the calibre of these vessels 

 perceptibly augments, and the organs which they 

 permeate are gorged with blood. This is the ordi- 

 nary cause of visceral congestions. Such at any rate 

 is their fatal mechanism — and it is explained by the 

 inertia of the vaso-motors. 



We have already seen how muscular exercise in- 

 creases and re-establishes the surface circulation by 

 augmenting the internal combustion, and thus giving 

 an impulse to the blood, accelerating its motion ; and 

 by raising the temperature of the surface of the body, 

 all these influences combined triumph over the con- 

 tracted vessels, they gradually relax, the blood re- 

 enters, and with it heat. This process continues until 

 the tension is equal or nearly so in all the capillaries 

 of the body. The blood rushes into them and no 

 longer gorges the viscera; these lose their congestion 

 in consequence, the vaso-motors come out of their 

 torpid condition in proportion as the heat circulates, 

 instead of concentrating in the centres. 



What we have just said concerning muscular exercise 

 in general applies especially to horse-back riding, and 



