INFLUENCE OF RESPIRATORY PRESSURE ON THE HEART. Ill 



in diameter and six to eight inches iu length; the tube is bent at a right angle, 

 and communicates with a small metal capsule about the size of a saucer (T), over 

 which a membrane composed of collodion and castor oil is loosely stretched. To 

 this membrane is attached a glass-rod (H) used as a writing-style, which records 

 its movements on a glass-plate (S) moved by clock-work. A small valve (K) is 

 placed on the side of the tube (D), which enables the experimenter to breathe 

 when necessary. The tube (D) is held in an air-tight manner between the lips, 

 the nostrils being closed, the glottis open, and respiration stopped. Fig. 36, 

 A, B, C, are curves obtained in this way. In them we observe 



(a) At the moment of the first sound (1.), the respiratory gases undergo a sharp 

 expiratory movement, because at the moment of the first part of the ventricular 

 systole the blood of the ventricle has not left the thorax, while venous blood is 

 streaming into the right auricle through the venaa cav*, and because the dilating 

 branches of the pulmonary artery compress the 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 by (a) 

 the muscular mass of the ventricle occupying slightly less bulk during the contrac- 

 tion, and (/3) owing to the thoracic cavity being slightly increased by the fifth 

 intercostal space being pushed forward by the cardiac impulse. 



(b) Immediately after (1. ), there follows a strong inspiratory current of the respira- 

 tory 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 vente cavae. 



(c) After the second sound (at 2.), indicated sometimes by a slight depression in 

 the apex of the curve, the arterial blood accumulates, and hence there is another 

 expiratory movement in the curve. 



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

 inspiratory movement of the gases. 



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

 occurs. 



60. Influence of the Respiratory Pressure on the 

 Dilatation and Contraction of the Heart. 



The variation ia 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 move- 

 ments of the heart, as was proved by Carson in 1820, and by Donders 

 in 185 4. 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 (excluding 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 

 cavities of the heart ; the heart in diastole is small and contains a small 



