124 



INFLUENCE OF RESPIRATORY PRESSURE ON THE HEART. 



overcoming the outward traction, the blood-vessels of the lungs will 

 be distended with blood. Much less blood will be driven out of the 

 left heart, so that the pulse may even be temporarily arrested. The 

 result is an overdistended, enlarged heart and ' the presence of an 

 increased amount of blood in the lesser circulation, as compared with 

 the greater. 



As, when the breathing is normal, the tension of the pulmonary air 

 is diminished during inspiration and increased during expiration, this 

 normal alternation of pressure tends to assist the circulation: inspira- 

 tion hastens the venous and lymphatic flow through the venae cavae 

 (if the axillary or the jugular vein is opened during an operation, air 

 may be sucked in and cause death) and thus favors complete diastole ; 



FIG. 36. Apparatus for the Demonstration of the Influence of Respiratory Expansion (II) and Contraction (I) 

 of the Thorax on the Heart and the Circulation. 



expiration hastens the movement of blood into the arterial system and 

 favors systolic emptying of the heart. At the same time the val- 

 vular arrangement of the heart secures a constant direction to the 

 accelerated blood-current. 



The elastic traction of the lungs also exerts a favorable influence on 

 the lesser circulation, which is contained entirely within the thorax; 

 for the blood within the pulmonary capillaries is under the same pres- 

 sure as the pulmonary air, while that of the pulmonary veins is under 

 lower pressure, as the elastic traction of the lungs by distending the 

 left auricle necessarily hastens the flow of blood from the pulmonary 

 veins into the left auricle. On the other hand, the elastic traction of 



