HORSE-BACK RIDING. 41 



sensitive to the influence of heat and cold. From cold 

 the muscular fibre contracts and lessens the calibre of 

 the vessels, the blood circulates less abundantly in 

 their cavities ; and paleness of the tissues which they 

 traverse and a lower temperature are the conse- 

 quences. 



Heat, on the contrary, seems to paralyze these 

 nerves, and the muscular fibre no longer reacts against 

 the pressure from within, which, encountering less 

 resistance, dilates the vessel just in proportion. The 

 parts become red, swollen, gorged with blood, and 

 the temperature is increased. It is specially on the 

 surface and the extremities that we may observe 

 these phenomena, because the medium in which we 

 live is daily subjected to numerous influences which 

 lower the temperature ; it borrows, so to speak, from 

 the heat of the body with which it comes in contact. 



The slight conductibility of the human body pre- 

 serves the internal organs from these daily losses, and 

 the blood which renews itself continually maintains at 

 the surface a relative warmth. But still the sensation 

 of cold causes the contraction of the capillaries over 

 the whole surface of the body, and the circulation is 

 enfeebled just in proportion to the energy, and, per 

 contra, it is frequently accelerated in the substance of 

 the organs contained in the visceral cavities. If the 

 cooling is slight, if the contraction in the capillaries is 

 infinitesimal, there is only a slight increase of tension 

 in the great arteries ; but if the capillary net-work is 



