With the heart still lying flat in the pericardial sac, the aortic opening is explored by the index-finger of the right hand and the size of the ring estimated. The knife is then introduced on the flat, into the left ventricle, along the left side of the septum, through the aortic opening and as far as possible into the aorta. It is then turned, with the cutting edge upward, and the point pushed through the anterior wall of the aorta. The heart is then drawn downward and slightly raised by the left hand, holding it at the apex by the two flaps of the right ventricle. The knife is then drawn from above downward toward the apex, cutting in succession the anterior wall of the aorta, across the pulmonary artery, through the aortic ring, and the anterior wall of the left ventricle, just to the left side of the septum. By dissecting away the pulmonary artery from the aorta the incision through the former may be avoided. (See Fig. 41.) When desired this cut may be brought down through the septum instead, but if the bundle of His is to be studied in serial sections the cutting of the septum should be avoided. The enterotome or long straight shears may be used for all the incisions except the first ones made into the ventricles. For these the knife is necessary. The incision through the aortic ring usually cuts the anterior segment, but by making the cut more to the right the incision will pass between the anterior and the right posterior flaps. Before the valvular orifices are cut it is often expedient to test the adequacy of the valves by means of water or air. The hydrostatic test is employed to the best advantage in the case of the pulmonary and aortic valves, either by pouring water into the vessels, or by immersing the heart in water and then lifting it up quickly. In the case of the auriculoventricular valves the air-test is carried out by inserting the nozzle of a bellows through an opening made in the ventricular wall and noting the effect of blowing and suction. Graduated cones or balls may be used for more accurate measurement of the orifices, or they may be measured after they have been cut. If sufficient care is exercised in cutting the valvular rings the incision can be carried between the flaps without injury to the latter. This is often desirable in cases of valvular lesion, endocarditis, etc. In such cases the valvular rings may be left uncut, the line of incision being broken by the auriculoventricular ring, when the mitral and tricuspid valves are concerned. The pulmonary and aortic rings may also be left uncut; the incisions are stopped at the rings, and then begun again in the vessel-walls beyond the valves.