THE NECTURUS 49 



much smaller than the opening into the right atrium. Cut 

 an opening into the atria so as to expose the inside. Note 

 the inter-atrial septum and the fenestrations that make it 

 incomplete. 



Atrioventricular Opening 



The atria open into the ventricle by a single passage 

 protected by valves. The blood from the two atria is 

 forced through into the ventricle at the same time. This 

 opening is valved and supplied with muscles to regulate it. 

 Which atrium is the larger? 



Ventricle 



The walls of the ventricle are to be slit so as to show the 

 atrioventricular opening and also the opening into the 

 truncus. The walls of the ventricle are heavy in compari- 

 son with the thin walls of the atria. 



Truncus Arteriosus 



Slit the truncus from the ventricle to the efferent vessels. 

 Is there a septum dividing it into two parts as in the 

 frog? Note the series of valves that prevent a return. 

 What is the type of valve found here? The enlargement 

 of the truncus is a pressure equalizer so that there may 

 be a steady stream of blood going to the gills. The part 

 of the truncus with valves is called the conus arteriosus, 

 while the enlarged part is the bulbus arteriosus. 



