THE LUNGS. 347 



sixteen to twenty incomplete cartilaginous rings connected 

 by fibrous tissue and muscular fibres ; the deficiency of 

 the rings is posterior. In the interval between their ends 

 extends the trachealis muscle, a band of involuntary mus- 

 cular fibres disposed longitudinally. The mucous mem- 

 brane is of the ciliated variety. 



THE PLEURAE. 



The pleurae are two flattened serous sacs interposed 

 between the lungs and the walls of the chest. Each 

 lung is entirely invested except at the hilum ; the pleurae 

 are then reflected over the pericardium, inner surface of 

 the chest, and thoracic surface of the diaphragm ; a fold 

 of pleura extending from the root of the lung to the 

 diaphragm is called the broad ligament of the lung. 

 The parietal layer extends upward about one and a half 

 inches above the first rib; it is impressed by the subclavian 

 artery, which rests upon it. 



THE LUNGS. 



The lungs are a double organ, right and left, sepa- 

 rated by the structures in the middle mediastinum ; they 

 consist of five lobes, these of myriads of lobules, and 

 each lobule of clusters of air-cells. 



Each lung is conoidal in form, and presents an apex, 

 a base, an anterior and posterior border, an inner and 

 outer surface. The apex projects above the level of the 

 first rib ; the base is broad, concave, and rests on the 

 upper surface of the diaphragm ; the anterior border is 

 thin, sharp, and meets the anterior border of the other 

 lung, except at the lower third, where that of the left 

 lung recedes, exposing the pericardium to the extent of 

 the area of a silver dollar ; the posterior border is 

 bluntly rounded and broad, and longer than the ante- 

 rior ; the anterior surface is convex and smooth, and is 



