The different forms of disease and deformity affecting the valves, which have been adverted to in the preceding remarks, necessarily cause obstruction to the circulation, attended with regurgitation of the blood and increase of pressure in some of the cavities of the heart. Any obstruction arising out of disease of the valves is likely to occasion enlargement of the heart in one or another part of the organ according to the particular valves affected.

Obstructive disease of the aortic valves gives rise to thickening of the walls of the left ventricle, while the same condition of the pulmonary valves would similarly affect the right.

Enlargement of the heart may arise from increase in the development of the muscular structure (hypertrophy of the muscular walls), or from dilatation of the cavities from extra pressure from within. But it is pretty generally recognized that the same obstruction which gives rise to hypertrophy will also occasion dilatation of the cavities, hence it is found that both conditions usually exist to a greater or less extent at the same time.

The heart of the horse, which in a normal condition weighs about seven or eight pounds, may be increased in this disease to twelve or more pounds.

Hypertrophy and dilatation are much more common in the left ventricle than in the right, for the reason that the valves of the former are more liable to sprain and disease than those of the latter. It will be remembered that the left ventricle sends the blood over the entire body, while the function of the right is confined to distributing it over the lungs. The work entailed in the former act being so much greater than that of the latter, the chances of obstruction to the blood flow are correspondingly increased. Thus the aorta, the vessels of the kidneys or the liver, or those of one or more of the various organs of the body, may by disease become narrowed, and tend to obstruct the circulation, which in time the heart would attempt to overcome, and its increased effort would have the effect which follows all muscular work, of sooner or later causing thickening of its walls. If the obstruction continued, or for any reason became worse, dilatation of the heart would follow hypertrophy. In these cases the valves which guard the orifice are required to stretch in order to fill up the still larger opening, or they sprain and become diseased.

It does not necessarily follow that because the heart is enlarged its walls should be thickened. This will frequently depend upon the amount of dilatation which it has undergone. In some cases the walls are much thinner than normal owing to the hypertnyphy not having kept pace with the dilatation.