I.] 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 (aortfa 

 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. 



8. Carefully raise the ventricle : lying 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 the 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. 



ft. It will be seen that the two auricles contract 

 together ; immediately after them, the ventricle ; 

 and then, instantly, the truncus arteriosus. 



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

 nosus } it will be found to contract immediately 

 before the auricles. 

 M. 6 



