322 DISSECTION OF THE BABBIT 



branches supplying the muscular portion of the 

 diaphragm. 



Their origin from the fourth cervical nerves will 

 be seen in the dissection of the neck (p. 349). 



6. The pleurae. Each side of the thorax is lined by a 



glistening membrane, the pleura, which is reflected 

 over the lung at its root in the same way as the 

 peritoneum is reflected over the abdominal viscera. 

 The right and left pleural sacs are separated from 

 each other by the mediastinal cavity, the greater part 

 of which is occupied by the heart and pericardium, 

 the posterior part remaining vacant. 



During life the lungs are distended so as to 

 almost fill the thoracic cavity, the visceral layer of 

 the pleurffi, 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 air-tight 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. 



TI. DISSECTION OF THE CIBCULATOBY SYSTEM. 



A. The Heart and the Roots of the Great Vessels. 



Remove the thymus and the ^parietal layer of the peri- 

 cardium, and ca/refully clean the base of the heart and the 

 roots of the great vessels, avoiding injury to the nerves. 



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 obliquely backwards and to the right, indicating 

 the division into right and left ventricles. 



