288 



HANDBOOK OF PHYSIOLOGY. 



it never does so entirely, since part of the pressure of the air which is 

 drawn into the lungs through the trachea is expended in overcoming 

 their elasticity. The amount thus used up increases as the lungs 

 become more and more expanded, so that the pressure inside the thorax 

 during inspiration, as far as the heart and great vessels are concerned, 

 never quite equals that outside, 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 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 



Fig. 216. Diagram of an apparatus illustrating the effect of inspiration upon the heart and 

 great vessels within the thorax. I, the thorax at res ; 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 thora~ is enla.ged ; H. the heart; v, the veins entering it, 

 and A, the aorta : nl, U, the right and left lung : T, th trachea: M, mercurial manometer in con- 

 nection 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.) 



its minimum is less by 30 mm., than the normal pressure, 760 mm. of 

 mercury. It will be understood from the accompanying diagram how, 

 that if there were no lungs in the chest, if its capacity were increased, 

 the effect of the increase would be expended in pumping blood into the 

 heart from the veins. With the lungs placed as they are, during in- 

 spiration 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 



