THE VASCULAR SYSTEM 23 



dorsal wall of the sinus and wash oat the blood with the 

 blowpipe. Find 



103. the sinu-auricular aperture, leading from the sinus 

 venosus (99) into the right auricle. 



Place the heart ventral side up and remove the ventral wall 

 of the auricles, taking care not to injure the truncus. Find 



104. the inter-auricular septum, separating the two auri- 

 cles (atria). Note the sinu-auricular aperture in the 

 right auricle near the septum and 



105. the aperture of the pulmonary vein, near the sep- 

 tum of the left auricle. 



Again place the heart dorsal side up and remove the dorsal 

 surface of the ventricle, to find 



106. the auriculo-ventricular aperture, divided by the 

 inter-auricular septum, at the cephalic end towards 

 the ventral side, and 



107. the ventriculo-bulbar aperture, leading into the 

 bulbus, also at the cephalic end, but towards the 

 dorsal side and to the right. Note 



108. the semilunar valves by which this aperture is 

 guarded. How many are there? 



Slit open the bulbus and truncus and note 



109. in the bulbus the spiral valve, dividing it longi- 

 tudinally. 



110. In the limbs of the truncus, observe the septa which 

 divide them into three compartments, leading to the 

 three aortic arches on each side. 



2. The Veins 



Returning to the injected frog, identify the cut end of the 

 postcaval vein (101) and follow this caudad, noting its 

 tributaries as follows: 



