PULMONARY CIRCULATION. 341 



the left auricle. There is no great disparity between the ar- 

 teries and veins of the pulmonary system as regards capacity. 

 The pulmonary veins in the human subject are not provided 

 with valves. 



The blood in its passage through the lungs does not meet 

 with the resistance which is presented in the systemic circu- 

 lation. This fact we have often noticed in injecting defibrin- 

 ated blood through the lungs of an animal just killed. We 

 have also observed that an injection passes through the lungs 

 as easily when they are collapsed as when they are inflated. 

 The anatomy of the circulatory system in the lungs and of 

 the right side of the heart shows that the blood must pass 

 through these organs with comparative ease. The power of 

 the right ventricle is evidently less than half that of the left, 

 and the pulmonary artery will sustain a much less pressure 

 than the aorta. 



The two sides of the heart act simultaneously ; and while 

 the blood is sent by the left ventricle to the system, it is sent 

 by the right ventricle to the lungs. Some physiologists have 

 endeavored to measure the pressure of blood in the pulmo- 

 nary artery. The only experiments which have not involved 

 opening the thoracic cavity, an operation which must inter- 

 fere materially with the pressure of blood in the pulmonary 

 artery, as it does with the general arterial pressure, are those 

 of Chauveau and Faivre. 1 These observers measured the 

 pressure by connecting a cardiometer with a trocar intro- 

 duced into the pulmonary artery of a living horse, through 

 one of the intercostal spaces, and found it to be about one- 

 third as great as the pressure in the aorta ; an estimate which 

 corresponds pretty nearly with the comparative power of the 

 two ventricles, as deduced from the thickness of their muscu- 

 lar walls. 



Anatomy teaches us that the capillaries of the lungs have 

 exceedingly delicate walls ; and it is evident that rupture of 

 these vessels from excessive action of the heart would lead to 



1 LONGET, Traite de Plysiologie, Paris, 1861, tome i., pp. 886, 887. 



