CHAP, ii.] RESPIRATION. 367 



blood through the thoracic portion of, and thus indirectly on that 

 through the whole of, the vascular system. 



The heart and great blood-vessels are, like the lungs, placed in 

 the air-tight thoracic cavity, and are subject like the lungs to the 

 pumping action of the respiratory movements. Were the lungs 

 entirely absent from the chest, the whole force of the expansion of 

 the thorax in inspiration would be directed to drawing blood from 

 the extra-thoracic vessels towards the heart, and conversely the 

 effect of the contraction of the thorax in expiration would be to 

 drive the blood back again from the heart towards the extra-thoracic 

 vessels. In the presence of the lungs however the free entrance of 

 air into the interior of the chest tends to maintain the pressure 

 around the heart and great vessels within the thorax equal to the 

 ordinary atmospheric pressure on the vessels of the rest of the body 

 outside the thorax ; but it is unable completely to equalise the two 

 pressures. Did the air enter as freely into the lungs as it does into 

 the pleural cavities when wide openings are made in the thoracic 

 walls, the respiratory movements would have very little effect 

 indeed on the flow of blood to and from the heart, just as when 

 such free openings exist they are ineffectual in promoting the 

 entrance and exit of air to and from the lungs. But the air does 

 not pass into the pulmonary alveoli as freely as it would do into a 

 pleural cavity through an opening in the thoracic wall. Before the 

 inspired air can fill a pulmonary alveolus, the walls of the alveolus 

 have to be distended at the expense of the pressure which causes the 

 inspired air to enter. Part of the atmospheric pressure in fact 

 which causes the entrance of the air into the lung is spent in over- 

 coming the elasticity of the pulmonary passages and cells. Conse- 

 quently, any structure lying within the thorax but outside the 

 lungs, is never, even at the conclusion of an inspiration when the 

 lungs are filled with air, subject to a pressure as great as that of the 

 atmosphere. The pressure on such a structure always falls short of 

 the pressure of the atmosphere by the amount of pressure necessary 

 to counterbalance the elasticity of the pulmonary passages and cells. 

 And, since the fraction of the atmospheric pressure w^hich is thus 

 spent in distending the lungs increases as the lungs become more 

 and more stretched, it follows that the fuller the inspiration the 

 greater is the difference between the pressure on structures within 

 the thorax but outside the lungs and the ordinary pressure of the 

 atmosphere. Now we .have seen that the pressure necessary 

 to counterbalance the elasticity of the lungs, when they are com- 

 pletely at rest (in the pause between expiration and inspiration), is 

 in man about 5 to 7 mm. of mercury, and that when the lungs are 

 fully distended, as at the end of a forcible inspiration, the pressure 

 rises to as much as 30 mm. of mercury. Hence at the height of a 

 forcible inspiration the pressure exerted on the heart and great 

 vessels within the thorax is 30 mm. less than the ordinary atmo- 

 spheric pressure of 760 mm., and even when the chest is completely 



