THE FROG. 8 1 



the xiphisternum with forceps, and remove the entire 

 ventral portion of the shoulder-girdle, being very 

 careful not to injure the underlying blood-vessels. 

 Raise the pericardium in like manner and remove 

 sufficient of it to expose the heart. 



a. External anatomy of the heart. 



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

 tricle) with the apex directed backwards. 



^. The truncus arteriosus ; a sub-cylindrical part, 

 arising from the right side of the base of the 

 ventricle and dividing anteriorly into two {aortio 

 arches). 



y. The atrium; thin-walled, rounded, lies on the 

 dorsal aspect of the truncus, in front of the 

 ventricle. Its subdivision into the two auricles 

 is not visible from this aspect. 



h. Carefully raise the ventricle : lyitig beneath it 

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

 venosus ; an elongated thin-walled sac, seen to 

 be in immediate relationship to the atrium and 

 in connexion with the great veins, which enter 

 it through th-e dorsal wall of the pericardium. 



b. The pulsation of the heart, 



a. Watch the movement carefully ; it is a regularly 

 alternating series of contractions and dilatations. 



/?. It will be seen that the two auricles contract 

 together ; immediately after them, the ventricle ; 

 and then, instantly, the truncus arteriosus. 



7. Raise the ventricle so as to see the sinus ve- 

 nosus ; it will be found to contract immediately 

 before the auricles. 



M. 6 



