386 CHORD AT A. 



anterior mesenteric and posterior mesenteric arteries, which 

 should be identified (see page 388) 



The oesophagus may then be cut through near the dia- 

 phragm and the rectum near the anus, and if the mesentery 

 be carefully cut through the whole alimentary system may 

 be removed and unravelled. The thoracic cavity should 

 now be opened by cutting through the ribs on either side 

 and between the diaphragm and the sternum. The cavity 

 is almost entirely filled by the two lungs and the heart. The 

 trachea can be traced down the neck (see Plate XT 1 1.). 

 Just where it emerges from the buccal cavity there is a 

 cartilaginous larynx which forms the organ of voice. It 

 is formed of thyroid and cricoid cartilages modified from 

 branchial arches in the embryo (see page 417). The trachea 

 throughout its course is distended by cartilaginous rings. It 

 passes into the thoracic cavity anteriorly and divides into 

 two bronchi which lead to the lungs in which they branch. 

 (These are best seen on removal of the blood-vessels.) 



The lungs are of a bright-red colour, spongy, and lying 

 quite free in the cavity around them. The left lung has two 

 lobes, the right has four. Each lung is envel- 

 esp a ory. ^p^^ |^y ^ layer of peritoneum called the pleura, 

 which has the same relationship to the lung as has the 

 pericardium to the heart. The outer layer of the pleura is 

 pushed against the ribs and the inner adheres to the lung. 

 Between the two is the pleural cavity, which is practically 

 squeezed out of existence in the living animal by the ex- 

 pansion of the lungs. Between the two pleura is a space, 

 the mediastinal space, nearly filled by the heart and peri- 

 cardium. 



Hence the perivisceral coelom in the rabbit is divided 



into no less than four separate parts — the pericardial 



cavity, two pleural cavities and the abdominal 



* ^^' cavity. Between the last and the other three 

 is the diaphragm. The diaphragm is innervated by a pair 

 of phrenic nerves arising from the fourth spinal nerve in 

 the neck. They may be easily seen passing down between 

 heart and lungs. The capacity of the thorax is increased by 

 raising of the ribs, caused by contraction of the intercostal 

 muscles and by the lowering of the diaphragm. Air is 

 in this way inspired or drawn into the lungs. Expiration 



