648 NEGATIVE PRESSURE IN THORAX. [BOOK n. 



ticity of the pulmonary passages and alveoli. Consequently, 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. 

 And, since the fraction of the atmospheric pressure which 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 

 at rest, at the end of an expiration, the pressure on the heart and 

 great vessels is slightly (by about 5mm. mercury) below that of the 

 atmosphere. We may add that any obstacle to the free ingress 

 of the inspired air, any difficulty in the full expansion of the 

 pulmonary alveoli, of course increases the negative pressure to 

 which the thoracic structures outside the lungs are subjected by 

 the expansion of the chest. Hence when the trachea is closed 

 a very large part of the thoracic expansion is directed to in- 

 creasing the negative pressure around the heart and great blood 

 vessels. 



During an inspiration then the pressure around the heart and 

 great blood vessels becomes considerably less than that of the atmo- 

 sphere on the vessels outside the thorax. During expiration this 

 pressure returns towards that of the atmosphere, but in ordinary 

 breathing never quite reaches it. It is only in forcible expiration 

 that the pressure on the thoracic vascular organs reaches or exceeds 

 that of the atmosphere. But if during inspiration the pressure 

 bearing on the right auricle and the venae cavse becomes less than 

 the pressure which is bearing on the jugular, subclavian, and other 

 veins outside the thorax, this must result in an increased flow from 

 the latter into the former. Hence during each inspiration a larger 

 quantity of blood enters the right side of the heart. This probably 

 leads to a stronger stroke of the heart, and at all events causes 

 a larger quantity to be ejected by the right ventricle ; this 

 causes a larger quantity to escape from the left ventricle, and thus 

 more blood is thrown into the aorta, and the arterial pressure 

 proportionately increased. During expiration the converse takes 

 place. The pressure on the intra-thoracic blood vessels returns to 

 the normal, the flow of blood from the veins outside the thorax 

 into the venae cavae and right auricle is no longer assisted, and in 



