ASPIRATION OF THE THORAX. 367 



fibrous tissue being, for example, not unfrequently found 

 developed on the liver, from the pressure of the lower ribs 

 forced against it by a tight corset. 



The Aspiration of the Thorax. As already pointed out, 

 the external air cannot press directly upon the contents of 

 the thoracic cavity, on account of the rigid framework which 

 supports its walls; it still, however, presses on them indi- 

 rectly through the lungs. Pushing on the interior of these 

 with a pressure equal to that exerted on the same area 

 by a column of mercury 760 mm. (30 inches) high, it dis- 

 tends them and forces them against the inside of the 

 chest-walls, the heart, the great thoracic blood-vessels, the 

 thoracic-duct, and the other contents of the chest-cavity. 

 This pressure is not equal to that of the external air, 

 since some of the total air-pressure on the inside of the 

 lungs is used up in overcoming their elasticity, and it is 

 only the residue which pushes them against the things out- 

 side them. In expiration this residue is eqnal to that ex- 

 erted by a column of mercury 754 mm. (29.8 inches) high. 

 On most parts of the Body the atmospheric pressure acts, 

 however, with, full force. Pressing on a limb it pushes the 

 skin against the soft parts beneath, and these compress the 

 blood and lymph vessels among them; and the yielding 

 abdominal walls do not, like the rigid thoracic walls, carry 

 the atmospheric pressure themselves but transmit it to the 

 contents of the cavity. It thus comes to pass that the- 

 blood and lymph in most parts of the Body are under a, 

 higher atmospheric pressure than they are exposed to in 

 the chest, and consequently these liquids tend to flow into 

 the thorax, until the extra distension of the vessels in which 

 they there accumulate compensates for the less external 

 pressure to which those vessels are exposed. An equili- 

 brium would thus very soon be brought about were it not 

 for the respiratory movements, in consequence of which the 

 intra-thoracic pressure is alternately increased and dimin- 

 ished, and the thorax comes to act as a sort of suction- 

 pump on the contents of the vessels of the Body outside it; 

 thus the respiratory movements influence the circulation 

 of the blood and the flow of the lymph. 



