INFLUENCE OF EESPIBATORY PRESSURE ON THE HEART. 



87 



accompanying bronchi. The blood of the right ventricle has not yet left the thorax, it passes 

 merely into the pulmonary circuit. The expiratory movement is diminished somewhat (o) by 

 the muscular mass of the ventricle occupying slightly less bulk during the contraction, and (j8) 

 owing to the thoracic cavity being slightly increased by the fifth intercostal space being pushed 

 forward by the cardiac impulse. 



(2) Immediately after (1) there follows a strong inspiratory current of the respiratory gases. 

 As soon as the blood from the root of the aorta reaches that part of the aorta lying outside the 

 thorax, more blood leaves the chest than passes into it simultaneously through the venae cava?. 



(3) After the second sound (at 2), indicated sometimes by a slight depression in the apex of 



A 



Fig. 64. 

 Landois' cardio-pneumograph, and the curves obtained therewith. A and B, from man, 1 and 



2 correspond to the periods of the first and second heart-sounds ; C, from dog ; D, method 



of using the apparatus, 

 the curve, the arterial blood accumulates, and hence there is another expiratory movement in 

 the curve. 



(4) The peripheral wave-movements of the blood from the thorax cause another inspiratory 

 movement of the gases. 



(5) More blood flows into the chest through the veins, and the next heart-beat occurs. 



60. INFLUENCE OF THE RESPIRATORY PRESSURE ON THE HEART. 



The variation in pressure to which all the intra-thoracic organs are subjected, 

 owing to the increase and decrease in the size of the chest caused by the respiratory 

 movements, exerts an influence on the movements of the heart. Examine first the 

 relations in different passive conditions of the thorax, when the glottis is open. 



The diastolic dilatation of the cavities of the heart, besides the pressure of the 

 venous blood and the elastic stretching of the relaxed muscle-wall, is fundamentally 

 due to the elastic traction of the lungs. This is stronger the more the lungs are 

 distended (inspiration), and is less active the more the lungs are contracted 

 (expiration). Hence it follows : 



(1) When the greatest possible expiratory effort is made, (of course, with the 

 glottis open), only a small amount of blood flows into the heart ; the heart in 

 diastole is small and contains a small amount of blood. Hence the systole must 

 also be small, thus causing a small pulse-beat. 



(2) On taking the greatest possible inspiration (with the glottis open), and 

 therefore causing the greatest stretching of the elastic tissue of the lungs, the elastic 

 traction of the lungs is, of course, greatest = 30 mm. Hg (Bonders), and may 

 interfere with the contraction of the thin-walled atria and appendices, in con- 

 sequence of which these cavities do not completely empty themselves into the 

 ventricles. The heart is in a state of great diastolic distension, and filled with 



