202 CIRCULATION. 



lungs, where, in order to facilitate the respiratory processes, 

 the walls of the capillaries are very thin. The lungs them- 

 selves are exceedingly delicate, and an effusion of blood, or 

 considerable congestion, would be liable to be followed by 

 serious consequences. To prevent this, the right ventricle is 

 not permitted to exert all its force, under all circumstances, 

 upon the blood going into the pulmonary artery ; but when 

 the action of the heart is exaggerated from any cause, the 

 lungs are relieved by a slight regurgitation, which takes 

 place through the tricuspid valve. The lungs are still further 

 protected by the sufficiency of the mitral valve, which pro- 

 vides that no regurgitation shall take place into their substance 

 from the left heart. In the systemic circulation the capilla- 

 ries are less delicate ; extravasation of blood would not be 

 followed by any serious results, and the circulating fluid is 

 made to pass through a considerable extent of the elastic 

 vessels, before it begins to be distributed in the tissues. It is 

 evident that on the left side there is no necessity for such a 

 provision, and it does not exist. 



Aortic and Pulmonic Valves. The action of the semi- 

 } unar valves is nearly the same upon both sides. In the in- 

 tervals of the ventricular contractions, they are closed, and 

 prevent regurgitation of blood into the ventricles. The sys- 

 tole, however, overcomes the resistance of these valves, and 

 forces the contents of the ventricles into the arteries. During 

 this time the valves are applied, or nearly applied, to the 

 walls of the vessel ; but as soon as the ventricles cease their 

 contraction, the constant pressure of the blood, which, as we 

 shall see hereafter, is very great, instantaneously closes the 

 openings. 



The action of the semilunar valves can be seen by cutting 

 away a portion of the ventricles in the heart of a large ani- 

 mal, securing the nozzles of a double syringe in the aorta and 

 pulmonary artery, and forcing water into the vessels. In 

 performing this experiment, it will be noticed that while the 



