THE HEART 405 



from each other. Near its centre is a 

 thin oval patch, the fossa ovalis, marking 

 the position of the fenestra ovalis of the 

 embryo, 

 iv. The right auriculo-ventricular aperture is a 

 large crescentic opening in the posterior wall 

 of the auricle. 



b. The left auricle. 



i. The openings of the pulmonary veins are in a 

 small recess of the dorsal wall of the auricle. 



ii. The left auriculo-ventricular aperture is circular. 



2. The ventricles. 



Cut across the ventricles transversely, about the middle of 

 their length. Note the shape of the cavities, and the thickness 

 of their walls. 



a. The right ventricle wraps round the right side of 



the left ventricle. Its outer wall is thin, and its 

 cavity crescentic in transverse section. 



i. The right auriculo-ventricular valve is a mus- 

 cular flap, hanging into the ventricular cavity, 

 on the right side of the auriculo-ventricular 

 aperture. It is connected by chordae tendinese 

 with the wall of the ventricle. 



ii. The aperture of the pulmonary artery is at the 

 extreme anterior end and left side of the 

 ventricle. 



Pass a seeker from the ventricle along the pulmonary 

 artery, and lay it open with scissors. 



a. The semilunar valves are three crescentic 

 pocket-lilie flaps, guarding the entrance to 

 the artery. 



b. The left ventricle has very thick spongy walls, ex. 



cept at the apex, where it is thin. Its cavity is 

 somewhat irregular in shape, but is more or less 

 circular in transverse section. 



