312 DISSECTION OF THE EABBIT 



the pleurae, covering the lungs, being closely applied 

 to the parietal layer, lining the thorax, so that the 

 pleural cavities are practically obliterated. 



Each pleura being an airtight sac, the lungs 

 follow the movements of the ribs and diaphragm : 

 so that enlargement of the thoracic cavity causes 

 expansion of the lungs, and hence inspiration. 

 7. The oesophagus is a narrow muscular tube, running 

 through the thorax immediately ventral to the ver- 

 tebral column, and readily seen on raising the left 

 lung. 



VI. DISSECTION ■ OF THE CIECULATOEY SYSTEM. 



A, The Heart and the Boots of the Great Vessels. 



Remove the thymus and ike parietal layer of the peri- 

 cardium, and carefully clean the base of the heart and the 

 roots of the great vessels. 



1. The heart. The ventricular portion of the heart, which 

 forms almost the whole of the exposed ventral sur- 

 face, is marked by a groove which runs from the 

 base obhquely backwards and to the right, indicating 

 the division into right and left ventricles. 



The auricles lie at the base of the heart, on its 

 dorsal aspect, the auricular appendices being the only 

 parts of them visible on the ventral surface. 



a. The right ventricle forms the right side of the 



ventricular portion, but does not reach to the 

 apex. It is soft to the touch. 



b. The left ventricle forms the left side and apex of 



the heart ; and is firmer than the right ventricle, 

 owing to the greater thickness of its walls. 



0. The left auricle is anterior to the left ventricle, 

 and is largely concealed by the great vessels. It 

 is produced ventrally into the ear-like auricular 

 appendix. 



d. The right auricle is placed dorsal to the base of the 



