These _orifices_ differ considerably in size. Those between the auricles and ventricles are considerably larger than those at the commencements of the great arteries. Those on the right side of the heart too are greater than those on the left. They vary with the form of the heart. Thus in dilatation of an auricle and ventricle on one side of the heart, the auriculo-ventricular opening becomes equally widened and the valves remaining disproportionately small the blood is allowed to rush back into the auricle during ventricular contraction. The left auriculo-ventricular opening has been known to become contracted in some very flat and shallow chests; the blood failing to circulate freely through the lungs and to reach the left side of the heart in a full supply this orifice accommodates its size to the amount, and may become so narrowed that it forms a serious obstacle to the blood flow and a series of morbid changes result following the backward course of the circulation. The auricle first becomes overdistended and its muscular walls increase in thickness and consistency; the lungs tend next to suffer from a passive congestion, and lastly the right side of the heart becomes engorged and enlarged. Any obstruction in the aorta which conveys the blood from the left side of the heart equally leads to dilatation of its internal cavity and abnormal thickness of its walls. The imperfection of the valves is one of the most serious results of such changes in heart structure. The sounds by which such imperfection may be recognized will be presently noticed, meanwhile the mode of testing this in the heart of the dead animal will be referred to. If due to structural changes in the valves themselves, the new deposits, the cicatrices, the lacerations, etc., will be visible to the eye. Though no such disease-changes are seen the valves may still manifest imperfection by failing to fulfill their normal function when put to the test. Water is poured into one or other of the great arteries which arise from the ventricles, the vessel being held vertically, and if it fails to descend into the heart the valvular action is perfect. The auriculo-ventricular valves may be equally tested by filling the ventricle and observing whether there is a reflux into the auricle.