348 DISSECTION OF THE RABBIT. 



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 

 v the lumbar vertebrae. 



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. 



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



lying between the heart and the lungs, and easily 

 seen on pressing these apart. Posteriorly they 

 divide into branches supplying the muscular portion 

 of the diaphragm (see fig. 65, p. 374). 



Their origin from the fourth cervical nerves will 

 be seen in the dissection of the neck (p. 377). 



6. The pleurse. 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 dther 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 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 air-tight sac, the lungs 

 follow the movements of the ribs and diaphragm ; 



