xiii.] THE FROG. 201 



3. Note the smooth moist membrane (pleuroperitoneum) 

 lining the inside of the body-cavity and covering the 

 outside of the contained viscera. 



4. The liver will be readily recognized as a great 

 brownish mass covering a great part of the other 

 abdominal viscera: lying in a cleft in its anterior 

 border 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 carefully, 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. 



5. The heart. 



a. Note the general form of the organ. 



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

 tricle) with the apex turned backwards. 



b. 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: thin-walled, rounded, lies on the 

 dorsal aspect of the truncus and ventricle. The 

 separation between the two auricles is not visi- 

 ble 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 



