PRESSURE AND COMPRESSIBILITY. 



virtue of which they transmit freely, and luidiminished, the 

 pressure in all directions. The fluids which fill the entire vascular 

 system are exposed, as well as the surface of the body, to the 

 pressure of the atmosphere, which enters the lungs and all the 

 cavities and open parts of the organs. These fluids transmit that 

 pressure to all the inner parts of the body, so that the skin and 

 integuments are pressed by them outwards by a force exactly 

 equal to that with which the air presses the external surface of the 

 skin inwards. These outward and inward pressures are neces- 

 sarily always equal, because, in fact, they are one and the same 

 pressure, i.e., that of the air, the pressure on the external surface 

 acting inwards, being the immediate action of the air, and the 

 pressure of the internal fluids acting outwards being the same 

 pressure of the air transmitted by those fluids to the inside of the 

 skin and integuments. 



10. That this outward pressure, transmitted by the fluids which 

 fill the organs under the skin, is really at all times in operation, 

 and that it is only counteracted by the immediate pressure of the 

 external air upon the skin, is rendered conspicuously manifest 

 in the well-known surgical operation of cupping. In that pro- 

 cess the open mouth of the cupping-glass being pressed upon the 

 skin so as to exclude all communication with the external air, 

 the air within the cup is withdrawn, or partially withdrawn, by 

 means of a syringe attached to the glass. The moment the skin 

 within the glass is relieved from even a small part of the pressure 

 of the external air by this means, the outward pressure of the 

 fluids under the skin begins to take effect, being no longer resisted ; 

 it swells up the skin within the glass, and when the skin thus 

 dilated is punctured with the lancet, the blood is propelled from 

 it by the force of the pressure of the fluids under the skin acting 

 outwards. 



11. The free transmission of pressure in all directions is a pro- 

 perty which air has in common with water and other liquids. It 

 has, however, another quality eminently characteristic, which is 

 not found in liquids, or any other form of matter. The property 

 we refer to is unlimited compressibility. 



Let a tube A B (fig. 1) be provided, open at one end A, and closed 

 at the other B, and let a solid plug p be made to fit it air-tight. 

 Let an opening, governed by a stop-cock, be provided at c. When 

 the plug is inserted at A, the air inclosed by it in the tube will be 

 in its natural state, provided the stop-cock be open, and the 

 plug will be pressed upon it by the amount of the atmospheric 

 pressure. If we suppose the plug to have the magnitude of a 

 square inch, this pressure will be fifteen pounds. 



The stop-cock c being closed, let the plug P be pressed down 



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