202 ELEMENTARY BIOLOGY. [CHAP. 



veins, which enter it through the dorsal and 

 posterior walls of the pericardium. 



& The further examination of the structure of the 

 Frog's heart requires a good deal of care and 

 the use of a lens of low magnifying power. In 

 a chloroformed Frog the heart is distended 

 with blood when it ceases to beat. When all 

 signs of contractility have disappeared, the dis- 

 tended heart should be removed from the body 

 by cutting through the adjacent parts in such a 

 manner as to leave the terminations of the 

 veins and the origins of the aortic trunks 

 intact. The organ should next be transferred 

 to a shallow dissecting dish and covered with 

 weak spirit. The right and left walls of the 

 atrium being now carefully slit and the blood 

 which they contain washed away, the delicate 

 septum of the auricles will become visible. By 

 cautiously removing the ventral face of the 

 ventricle, its cavity will be laid open and the 

 auriculo-ventricular opening will be displayed. 

 If the ventral wall of the truncus arteriosus is 

 laid open longitudinally with fine scissors, the 

 valves in its interior will become visible. The 

 pulmonary vein runs along the dorsal aspect of 

 the sinus venosus, between the right anol left 

 superior venae cavas, to the left auricle, into 

 which it opens close to the dorsal attachment 

 of the septum. 



The natural relations of the different divi- 

 sions of the heart should be carefully studied in 

 such a dissection as is described in A. 6. 



