RESPIRATORY WAVE IN BLOOD-PRESSURE CURVE. 299 



change of several millimetres which we see in the respiratory 

 undulation in the arterial pressure. We must, therefore, seek 

 the explanation in the changes it causes in the great veins. 



It has been suggested that by facilitating the flow from the 

 great veins into the thorax, by a kind of sucking action, the 

 amount of blood entering the right auricle during inspiration may 

 be increased, and thus the left ventricles may be better filled and 

 made to beat more actively, so as to cause an elevation in the 

 arterial pressure. 



But this view appears to leave the pulmonary circulation out 

 of the question in a way hardly justifiable, since it must be tra- 



Fio. 138. 



Blood-pressure and Respiratory Tracings recorded synchronously record- 

 ing surface moving from right to left showing that the variations in press- 

 ure in the arteries (continuous line) and in the thoracic cavity (dotted line) 

 do not exactly correspond, the latter continuing to fall after the blood 

 pressure has commenced to rise. 



versed by the blood before the increased inspiratory inflow to the 

 right auricle can affect the left ventricle or the systemic arteries. 

 The sequence of events may be read as follows. During inspi- 

 ration the negative pressure on the right heart is increased ; the 

 atmospheric pressure acting on the tributaries of the superior 

 vena cava is unchanged, while the pressure in the abdominal 

 cavity is increased, and the inferior vena cava compressed by the 

 muscular action. The blood then flows more readily during in- 

 spiration into the right heart, and consequently the lungs receive 



