342 CIRCULATION. 



grave results. It has already been noted that on the right 

 side the lungs are protected by an insufficiency of the auri- 

 culo- ventricular valves, which does not exist on the left side, 

 allowing of a certain degree of regurgitation when the heart 

 is acting with unusual force, and thus relieving, to a certain 

 extent, the pulmonary system. This was pointed out by Mr. 

 King of London, and called the safety-valve function of the 

 right ventricle. 1 We have noticed, in the heart of the ox, a like 

 disparity between the aortic and pulmonic semilunar valves. 

 If these be exposed on both sides by cutting away portions of 

 the ventricles, and a current of liquid be forced against them 

 through the vessels, the aortic valves will be found to entire- 

 ly prevent the passage of the liquid, even under very great 

 pressure, while the pulmonic valves permit regurgitation un- 

 der a very inconsiderable pressure. A little reflection will 

 make it evident that when the heart is acting with undue 

 force it is quite as important to relieve the lungs by a certain 

 amount of regurgitation from the pulmonary artery, as by 

 insufficiency of the tricuspid valves. This insufficiency is 

 important, both at the auriculo-ventricular and pulmonic ori- 

 fices, in protecting the delicate structure of the lungs from 

 the variations in force to which the action of both ventricles 

 is constantly liable. 



On microscopic examination of the circulation in the 

 lower animals, as the frog, the movement of blood in the ca- 

 pillaries of the lungs does not present any differences from the 

 capillary circulation in other parts ; except that the vessels 

 seem more crowded with corpuscles, and there is no " still 

 layer " next their walls. 



There are no forces of any moment which are superadded 

 to the action of the right ventricle, in the production of the 

 arterial, capillary, or venous circulation in the lungs; but 

 there are certain conditions which may obstruct the flow of 

 blood through these parts. We have already noted the effect 

 of introduction of air into the veins, in blocking up the capil- 



4 Guy's Hospital Reports, 1837. 



