HEAllT AND GEEAT VESSELS. 339 



Each pleura being an air-tight sac, the lungs 

 foUo^ 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. 



8. The pneumogastric nerves. At the anterior end of the 



thorax the pneumogastric nerves lie parallel to the 

 phrenic nerves, and a little nearer the median plane. 

 Further back the pneumogastrics lie dorsal to the 

 heart and to the roots of the lungs, and close to the 

 vertebral column ; the left nerve lying on the oeso- 

 phagus, the right nerve a little to its right side. 

 The nerves are readily seen on turning aside the 

 lungs and heart. 



V. DISSECTION OF THE CIECULATOEY SYSTEM. 



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



Bemove the thymus and the parietal layer of the peri- 

 cardium, and carefully 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. 



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. 



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