244 THE BIOLOGY OF THE ANIMAL 



tricle and through some of the blood-vessels. 

 "With sharp, fine scissors carefully cut away 

 enough of the ventral surface of the ventricle, 

 truncus arteriosus, auricles, and aortic arches to 

 expose their cavities. With a stream of water 

 from a pipette wash the clotted blood out of the 

 cavities. The study of certain points will be 

 facilitated if the preparation be placed in a deep 

 watch-glass containing fifty per cent, alcohol, and 

 held near a window where the light may shine 

 through the membranes. Examine the ventri- 

 cle. What is the character of its wall? How 

 thick is it ? What is its color ? How does it dif- 

 fer from the walls of the auricles ? From that 

 of the truncus arteriosus? Notice that the 

 cavity of the ventricle is undivided. On the right 

 of the preparation look for the auriculo - ven- 

 tricular aperture. What is its shape? Into 

 what does it open ? Look for the two auriculo- 

 ventricular valves. How are they connected 

 with the walls of the ventricle? Between the 

 two auricles find the inter-auricular septum. 

 What is its structure? In which direction does 

 it extend? What is its position with regard to 

 the truncus arteriosus? To the auriculo- ventric- 

 ular aperture ? In the right auricle, near the sep- 

 tum, look for the opening of the sinus venosus, 

 the sinu-auricular aperture. Near the top 

 of the left auricle find close to the septum the 

 aperture of the pulmonary veins. Examine 

 the truncus arteriosus and note its connection 

 with the ventricle. From what part of the latter 

 does the truncus start ? Note the longitudinal 

 valve which divides the truncus into two parts. 



