294 VERTEBRATE ZOOLOGY 



Slit open the dorsal wall of the right auricle and its appendage, 

 being careful not to injure the caval veins, and wash out the blood 

 inside. Note the network of muscles on the inner surface ; also the 

 openings of the caval veins into the auricle. Observe the auricular 

 septum, which separates the two auricles ; also the large auriculo- 

 ventricular aperture into the right ventricle. 



Open the right ventricle by slitting the pulmonary artery longi- 

 tudinally and continuing the incision to the hinder end of the 

 ventricle. Note the thinness of the walls of the ventricle, and the 

 muscular ridges on the inner surface. Guarding the auriculo- 

 ventricular aperture is the large tricuspid valve, which consists of 

 a long membranous flap attached at its free end to the wall of the 

 ventricle by long tendinous and muscular cords, the chordae 

 tendineae. The opening of the ventricle into the pulmonary artery 

 is guarded by the three pouched semilunar valves, which permit 

 the flow of blood in one direction only. Look for them with the 

 aid of the blowpipe. 



Slit open the dorsal wall of the left auricle, but without cutting 

 the pulmonary veins. Note the openings of the pulmonary veins, 

 and the large auriculo-ventricular aperture. 



Open the left ventricle by slitting the aorta and continuing the 

 incision through the ventricle to the apex of the heart, and study 

 the interior. Note the thickness of the walls. The auriculo- 

 ventricular aperture is guarded by the bicuspid, or mitral, valve, 

 which consists of two distinct flaps joined with the ventricular 

 wall by chordae tendineae. The opening into the aorta is guarded 

 by three semilunar valves, one of which is dorsal and the other 

 two lateral. Note the two prominent muscular ridges in the wall 

 of the ventricle. 



Exercise 17. Draw a diagrammatic view showing the four cavities 

 of the heart and their apertures and valves. 



Cut off the apex of the heart by a transverse incision a short 

 distance from its end, in order to show the two ventricles and the 

 relative thickness of their walls. 



Exercise 18. Draw the cross section. 



Exercise 19. Draw a diagram of the entire vascular system. 



