338 DISSECTION OF THE RABBIT. 



fiA. 



plane behind the heart, and closely applied to the 

 oesophagus. 

 4. The diaphragm is a thin partition; separating the thoracic 

 cavity from the much larger abdominal cavity. Its 

 marginal portion is muscular, the fibres arising from 

 the inner surface of the hinder ribs and from the 

 vertebral column, and converging to be inserted into 

 the thinner tendinous central portion. The muscular 

 margin is specially developed in the dorsal region, 

 forming two stout pillars arising from the lumbar 

 vertebrEB. 



The diaphragm is arched so as to present a very 

 convex surface towards the thorax : by contraction 

 of its muscular portion, in the act of inspiration, this 

 convexity is diminished and the thoracic cavity 

 thereby enlarged. The small size of the thoracic 

 relatively to the abdominal cavity should be noticed. 



6. The phrenic nerves are a pair of slender white cords 

 lying between the heart and the lungs, and easily 

 sten on pressing these apart. Posteriorly they 

 divide into branches supplying the muscular portion 

 of the diaphragm (see fig. 63, p. 364). 



Their origin from the fourth cervical nerves will 

 be seen in the dissection of the neck (p. 368). 



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 peri- 

 cardium, the posterior part remaining vacant. 



During life the lungs are distended so as to 

 almost fill the thoracic cavity, the visceral layer of 

 the pleurse, covering the lungs, being closely applied 

 to the parietal layer lining the thorax, so that the 

 pleural cavities are practically obliterated. 



