PULMONARY CIRCULATION. 100 



It is evident that, under certain circumstances, a larger quantity of blood than usual 

 may pass through these parts without 'necessarily penetrating the true capillaries and 

 thus exerting a modifying influence upon nutrition. The changes which are liable to oc- 

 cur in the quantity of blood, in the force of the heart's action, etc., may thus take place 

 without disturbing the circulation in the capillaries, a provision which the functions of 

 the parts would seem to demand. 



Pulmonary Circulation. The vascular system of the lungs merits the name, which 

 is frequently applied to it, of the lesser circulation. The right side of the heart acts 

 simultaneously with the left, but is entirely distinct from it, and its muscular walls are 

 very much less powerful. The pulmonary artery has thinner and more distensible coats 

 than the aorta and distributes its blood to a single system of capillaries, situated very 

 near the heart. We have seen that the orifice of the pulmonary artery is provided with 

 valves which prevent regurgitation into the ventricle. In the substance of the lungs, 

 the pulmonary artery is broken up into capillaries, most of them just large enough to 

 allow the passage of the blood-corpuscles in a single row. These vessels are provided 

 with a single coat and form a very close net-work surrounding the air-cells. From the 

 capillaries the blood is collected by the pulmonary veins and conveyed to the left auri- 

 cle. There is no great disparity between the arteries 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 circulation. This fact we have often noticed in injecting 

 defibrinated 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 fa- 

 cility. 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 at the same time that 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 pulmonary 

 artery. The only experiments which have not involved opening the thoracic cavity, an 

 operation which must interfere materially with the pressure of blood in the pulmonary 

 artery, as it does with the general arterial pressure, are those of Chauveau and Faivre. 

 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 muscular 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 grave results. It has already been noted that on the right side the lungs are pro- 

 tected by an insufficiency of the auriculo-ventricular valves, which does not exist on the 

 left side, allowing a certain degree of regurgitation when the heart is acting with un- 

 usual force, and thus relieving, to a certain extent, the pulmonary system. This was 

 pointed out by Mr. King, of London, and is called the safety-valve function of the right 

 ventricle. We have noticed, in the heart of the ox, a similar difference between the 

 aortic and the pulmonic semilunar valves. If these be exposed on both sides by cutting 

 away portions of the ventricles, and if a current of liquid be forced against them through 

 the vessels, the aortic valves will be found to entirely prevent the passage of the liquid, 

 even under very great pressure, while the pulmonic valves permit regurgitation under 

 a comparatively inconsiderable force. A little reflection will make it evident that, 



