306 



RESPIRATION 



elasticity. The amount thus used up increases as the lungs become more 

 and more stretched, so that the intrathoracic pressure during inspiration, as 

 far as the heart and great vessels are concerned, never quite equals the intra- 

 pulmonary pressure, and at the conclusion of inspiration is considerably 

 less than the atmospheric pressure. It has been ascertained that the amount 

 of the pressure used up in the way above described varies from 5 to 7 mm. of 

 mercury in ordinary inspiration, to 30 mm. of mercury at the end of a deep 

 inspiration. So it will be understood that the pressure to which the heart 

 and great vessels are subjected diminishes as inspiration progresses, and at 



FIG. 242. Diagram of an Apparatus Illustrating the Effect of Inspiration upon the 

 Heart and Great Vessels within the Thorax. I, The thorax at rest; II, during inspiration; 

 D represents the diaphragm when relaxed; D', when contracted (it must be remembered 

 that this position is a mere diagram), i. e., when the capacity of the thorax is enlarged; H, 

 the heart; V, the veins entering it, and A, the aorta; Rl, LI, the right and left lung; T, the 

 trachea; M, mercurial manometer in connection with pleura. The increase in the capacity 

 of the box representing the thorax is seen to dilate the heart as well as the lungs, and so to 

 pump in blood through V, whereas the valve prevents reflux through A. The position of 

 the mercury in M shows also the suction which is taking place. (Landois.) 



its summit is less by from 7 to 30 mm. than the normal atmospheric pres- 

 sure, 760 mm. of mercury. It will be understood from the accompanying 

 diagram how an increase in the volume of the thorax will have the effect of 

 pumping blood into the heart from the veins. During inspiration the pres- 

 sure outside the heart and great vessels is diminished, and they, by virtue of 

 their elasticity, have therefore a tendency to expand and to diminish the intra- 

 vascular pressure. The diminution of pressure within the veins passing 

 to the right auricle and within the right auricle itself, will draw the blood 

 into the thorax, and so assist the circulation. This suction action of the 

 thorax is the cause of the slight negative pressure of the ventricles previously 

 described. The effect of more blood in the right auricle will, cateris paribus, 

 increase the amount passing through the right ventricle, and through the 



