500 NEGATIVE PRESSURE IN THORAX. [BOOK n. 



ure 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 pres- 

 sure of the atmosphere. Now we have seen that the pressure 

 necessary to counterbalance the elasticity of the lungs, when 

 they are completely 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 atmospheric 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 

 5 mm. mercury) below that of the atmosphere. We may add 

 that any obstacle to the free ingress of the inspired air, any diffi- 

 culty 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 tho- 

 racic expansion is directed to increasing 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 

 atmosphere on the vessels outside the thorax. During expira- 

 tion 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 inspi- 

 ration the pressure bearing on the right auricle and the venae 

 cavee 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 cavse 

 and right auricle is no longer assisted, and in consequence less 



