222 THE BODY AT WORK 



needed to throw into the aorta the blood which has been 

 received by the right auricle. 



The expansion of the chest influences the flow of blood in 

 yet another way. The heart and the great vessels which join 

 and leave it are themselves enclosed within the chest, subject 

 to the negative pressure produced within that cavity by the 

 elasticity of the lungs. The lungs pull upon the pericardium, 

 the membranous covering of the heart. When this pull is 

 increased owing to the forcible expansion of the chest, blood is 

 sucked into the great veins, just as air is sucked into the wind- 

 pipe. The thick- walled aorta, containing blood at high pressure, 

 does not feel the effect of slight variations in the pressure round 

 it. The soft-walled veins are expanded during inspiration to a 

 not inconsiderable degree. What relief a deep yawn gives by 

 hastening a languid circulation ! Leaning over an account-book 

 late in the afternoon, every condition is unfavourable to the 

 flow of blood. It accumulates in the legs and in the abdomen. 

 The head is thrown back and the mouth opened wide, while the 

 chest expands in a long deep inspiration. Down on the liver, 

 stomach, and intestines presses the flattened diaphragm, 

 squeezing their blood towards the heart. The negative pressure 

 within the chest sucks this up, and draws down the blood con- 

 tained in the great veins of the neck. The capillaries of the 

 lungs are widened, allowing blood to pass more quickly from 

 the right side to the left side of the heart. The heart responds 

 to the call upon it, throwing all that it receives into the aorta. 

 Only a great effort of the will had kept the pale brain at work ; 

 in the attic it suffers more than organs on the lower storeys 

 from insufficient pressure. For a short time after the yawn 

 it finds itself nourished with an adequate supply of blood. 



The negative pressure in the thorax is considerable at all 

 times. If a manometer a U -shaped tube with mercury in its 

 loop be connected with a cannula passed through the wall of 

 the chest, the difference of level of the mercury in the two limbs 

 of the U is a measure of the force with which the lungs are 

 endeavouring to shrink away from the chest-wall. Even at the 

 end of expiration the mercury in the limb next the chest stands 

 about 6 millimetres higher than the mercury in the outer 

 limb. During a deep inspiration the pressure in the chest 

 falls 30 millimetres below the atmospheric pressure. Hence a 



