304 MANUAL OF PHYSIOLOGY. 



in the arterial system. Owing to the lungs being very elastic 

 and constantly tending to shrink away from the costal pleura, as 

 may be seen when the thoracic cavity is opened and the lungs 

 collapse, the pressure in the pleural cavity is less than that of 

 the atmosphere which distends the lungs, i. e., the pleural pressure 

 is negative. All the viscera in the thoracic cavity are habitually 

 under influence of the negative pressure. Thus the elastic lungs 

 exert a kind of traction on the pericardium, and tend to cause a 

 negative pressure within the heart and great systemic vessels, 

 both arteries and veins. The influence is, of course, greater in 

 the thin-walled veins, in which the pressure is minimal, than in 

 the thick-walled arteries, where the pressure is so high that they 

 feel but little the intrathoracic decrease. 



The amount of traction exercised on the pericardial contents 

 by the lungs varies with the respiratory movements, being slightly 

 increased during inspiration, and decreased during expiration. 

 The differences which are thus produced, however, during ordi- 

 nary respiration are very slight (probably 1 mm., mercury), when 

 compared with the mean negative pressure, which, while the 

 thorax is in an intermediate state of extension, is probably 

 about 10 mm., mercury. So slight a variation as 1 mm., mer- 

 cury, could not, by direct action on the aortic arch, cause the 

 change of several millimetres which we see in the x 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 eleva- 

 tion 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- 

 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 



