ON THE ANIMAL FRAME. 299 



withdrawing tlie external pressure of the former 

 and substituting that of the latter. 



When the palm of the hand is placed over the 

 top of the receiver of an air-pump, and the air 

 is exhausted, the pressure of the air on the 

 outside is scarcely felt, but the inside is swollen 

 and feels as if it was drawn or sucked into the 

 receiver. Thus the sensation is on the inside and 

 not without ; the reason is, there is no change of 

 pressure on the outside ; but there is within, and 

 the consequence is a tendency of the air in the 

 hand to escape into the receiver, which occasions 

 the pain and swelling. It is thus also that the 

 issuing of blood in the surgical operation of 

 cupping is effected. 



Though it does not seem of much consequence 

 what the pressure of the air may be on the ani- 

 mal frame, within certain limits, yet sudden 

 changes must always be accompanied with 

 uneasy sensation. Chmbing mountains, or as- 

 cending in a balloon, removes a part of the 

 atmospheric pressure from the body ; this causes 

 the air in the body to tend outwards, and some- 

 times occasions bleedings. To supply oxygen to 

 the lungs a greater volume of air must be breathed, 

 and this seems to produce an acceleration of the 

 pulse. On the other hand, by descending 30 

 2p2 



