90 ELEMENTARY BIOLOGY. [CHAP. 



then be placed in tolerably strong spirit until tho- 

 roughly set. 



i. When sufficiently hardened, remove the lungs and 

 adjacent parts, so as to isolate the heart together 

 with the cut ends of the great vessels. Examine as 

 under. 



a. Ventral aspect. Work over the external charac- 

 ters described in § i. 

 /;. Dorsal aspect. Note especially — 



a. The sinus venosus ; spacious and elongated, 

 receiving the three caval veins. 



ft. The atrium ; if the heart be well distended a 

 slight constriction will be seen subdividing 

 this into a larger right auricle and a smaller 

 left auricle. 



y. The pulmonary veins (cf. § 7) ; seen to con- 

 verge immediately in front of the sinus ve- 

 nosus, to enter the inner border of the left 

 auricle. 



S. The sinu-auricular aperture ; seen on opening 

 up the sinus venosus and removing its con- 

 tents. It communicates with the cavity of 

 the right auricle and is guarded by two mem- 

 branous flaps {sinu-auricular valves). 



ii. General dissection from the front. 



Pin down ventral surface uppermost, and dissect 

 with great care, under water. Remove the front 

 wall of the ventricle, atrium and truncus arteriosus, 

 and lay open the bases of the aortic arches ; care- 

 fully remove the coagulated blood which fills them 

 and wash clean. 



