In studying endocarditis we have seen that the valves are frequently altered in their structure; we have now to consider these alterations more specifically, and their effects on the heart and circulation. It is not usual to designate anything as valvular disease unless it interferes with the function of a valve or orifice. The function of a valve is to close an orifice under certain circumstances, and we speak of valvular disease when the alterations are such as either to obstruct the orifice or to interfere with its closure by the valve. Hence valvular lesions may be divided into two kinds, namely, narrowing or stenosis of the orifice, and insufficiency of the valve. In referring to these same lesions, as they affect the current of blood in the heart, we speak of obstruction of an orifice and of regurgitation through the orifice.

Valvular disease occurs much more frequently in the valves of the left side than in those of the right, and hence 'we have chiefly to do with the mitral and aortic valves. We have already seen that in the foetus it is the valves of the right side which are most frequently affected, and we have connected this with the fact that these valves are more liable to variations of pressure in the foetus than in the adult.