THE RESPIRATORY MECHANISM. 393 



men, is often called " abdominal respiration," as distinguished 

 from breathing by the ribs, called " costal " or " chest breath- 

 ing." In both sexes the diaphragmatic breathing is the 

 most important, but, as a rule, men and children use the ribs 

 less than adult women. Since both abdomen and chest alter- 

 nately expand and contract in healthy breathing, anything 

 which impedes their free movement is to be avoided; and the 

 tight lacing which used to be thought elegant a few years 

 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 ex- 

 treme cases of tight lacing some organs are often directly 

 injured, weals of fibrous tissue being, for example, not unfre- 

 quently 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 distends 

 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 overcom- 

 ing 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 at- 

 mospheric 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 tho- 

 racic 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 distention of the vessels in 



