THE RABBIT. 327 



the upright position of the heart) by a semi-lunar fold, the 

 valve of Thebesius. 



318. The aperture of the coronary vein, by which 

 the blood is brought back from the substance of the heart ; 

 it is a small round opening just within the margin of the 

 tunnel-like aperture of the left precaval. 



319. The apertures of the pulmonary veins into 

 the left auricle. 



XXXVII. Cut away both auricles so as to expose the 

 bases of the ventricles : cut away all but about 

 an eighth of an inch of the aorta and pulmonary 

 artery : pour water into the ventricles through the 

 auriculo-ventricular apertures. Note : 



320. The auriculo-ventricular valves, two sets of 

 membranous flaps, which, when the filled ventricles are 

 squeezed, come together at their edges and close the 

 auriculo-ventricular apertures ; but when the pressure is 

 released fall down into the ventricles. On the left side are 

 two flaps, together constituting the mitral or bicuspid 

 valve ; on the right side are three flaps, together con- 

 stituting the tricuspid valve. 



321. The semilunar valves, of which there are three 

 at the origin of both the pulmonary artery and aorta : 

 all three valves, in each case, are in one plane, and 

 when there is fluid in the artery, come together by 

 their edges, and close the passage to the ventricle; but 

 when the ventricle is squeezed, the pressure forces them 

 aside, and causes them to flap back against the walls of 

 the artery. 



322. The sinuses of Valsalva, slight dilatations of 

 the aorta and pulmonary artery, just anterior to (above) 

 the semilunar valves. 



323. The apertures of the coronary arteries, by which 



