XIII.] THE FKOG. 193 



and partly concealed by it, will be seen a delicate 

 sac in which pulsations are going on ; this sac is the 

 pericardium : if it be opened and removed very care- 

 fully, the heart and some of the great blood-vessels 

 will be laid bare ; clean carefully the two great trunks 

 (aortic arches) which diverge from the anterior end 

 of the heart, following each to the point of its division 

 into three vessels. 



The heart, 

 i. Note the general form of the organ, 



a. Its posterior conical thick-walled portion (ven- 

 tricle} with the apex turned backwards. 



6. The truncus arteriosus: a sub-cylindrical part, 

 arising from the right side of the base of the 

 ventricle and dividing anteriorly into the two aortic 

 arches. 



c. The atrium: thiri-walled, rounded, lies on the 

 dorsal aspect of the truncus and ventricle. The 

 separation between the two auricles is not visible 

 externally. 



d. Carefully raise the ventricle: lying beneath it 

 (that is, on its dorsal side) will be seen another 

 division of the heart, the sinus venosus; it lies 

 between the atrium and the great systemic veins, 

 which enter it through the dorsal and posterior 

 walls of the pericardium. 



e. 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 



M. 13 



