THOEACIC VISCEEA 321 



and conical in form ; the apex being directed back- 

 wards and slightly to the left ; and the base, with the 

 roots of the great vessels, forwards. 



3. The lungs are a pair of pink spongy bodies at the sides 



of the thorax, and filling the greater part of its 

 cavity. They collapse when the thorax is opened. 



Find the trachea in the middle line of the neck, recognising 

 it by its transverse cartilaginous rings. Free it from the 

 sv/rrounding parts about the middle of its course; make a 

 longitudinal incision in it ; insert a blowpipe, and pass a 

 ligature loosely round the trachea behind it ; inflate the lungs 

 through the blowpipe, and when they are fully distended tie 

 the Ugature firmly, so as to retain the air in the lungs. 



The lungs lie quite free in the thoracic cavity, 

 except at their roots, where the blood-vessels and 

 •bronchi enter them. The left lung is divided into 

 two lobes; and the right into four, of which the 

 smallest -and hindmost lies across the median plane 

 behind the heart, closely appUed 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 

 vertebra. 



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 white cords lying 

 between the heart and the lungs, and readily seen 

 on pressing these apart. Posteriorly they divide into 



