3^4 



RESPIRATION. 



[CH. XXIV. 



way above described, varies from 5 or 7 mm. of mercury during 

 the pause, to 30 mm. of mercury when the lungs are expanded at 

 the end of a deep inspiration, so that it will be understood that 

 the pressure to which the heart and great vessels are subjected 

 diminishes as inspiration progresses, and at its minimum is less 

 by 30 rnm. than the normal pressure, 760 tnm. of mercury. 

 It will be understood from the accompanying diagram that 

 if there were no lungs in the chest, and its capacity were 

 increased, the effect of the increase would be expended in pumping 



Pig. 326. 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 ; n', when contracted (it must be re- 

 membered 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 ; R, Ll. 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 reflex through A. The position of the mercury in M shows also the suction 

 which is taking place. (Landois.) 



blood into the heart from the veins. With the lungs placed as 

 they are, during inspiration the pressure outside the heart and 

 great vessels is diminished, and they 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 

 is independent of the suction power of the diastole of the auricle 

 about which we have previously spoken. The effect of sucking 

 more blood into the right auricle will, cceteris paribus, increase 



