RESPIRATION: THE MECHANISM OF BREATHING 361 



back, and is still indulged in by some who think a distorted form 

 beautiful, seriously impedes one of the most important functions 

 of the Body, leading, if nothing worse, to shortness of breath and 

 an incapacity for muscular exertion. In extreme cases of tight 

 lacing some organs are often directly injured, weals of 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 indirectly through the 

 lungs. Pushing on the interior of these with a pressure equal to 

 that exerted on the same area by a column of mecury 760 mm. 

 (30 inches) high, it distends them and forces them against the in- 

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

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

 The pressure against these organs is not equal to that of the ex- 

 ternal 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 outside them. In 

 expiration this residue is equal to that exerted 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 disten- 

 tion of the vessels in which they there accumulate compensates 

 for the less external pressure to which those vessels are exposed. 

 An equilibrium would thus very soon be brought about were it 

 not for the respiratory movements, in consequence of which the 

 intrathoracic pressure is alternately increased and diminished, 

 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 respira- 



