RESPIRATORY ORGAN'S THE LUNGS. 575 



ciliated as in the larynx, and differs in no respect from the- latter. 

 The glands in the mucous membrane of the trachea are numerous; 

 the larger occurring more in the posterior wall, externally to the 

 muscles and the whole mucous membrane, while the smaller are 

 more numerous on the anterior wall, and just exterior to the elastic 

 layer of the membrane. The larger have a scaly epithelium in 

 their casca; while the^ smaller, being only simple or bifurcated 

 follicles in the thickness of the membrane itself, have a conoidal 

 epithelium. 



The bloodvessels have their larger branches running longitudi- 

 nally, while the superficial capillary plexus is close beneath the 

 basement-membrane. The lymphatics are abundant; commencing 

 (in one case) in wide meshed plexuses, 75^^ to 4^0^ f an i nc ^ 

 broad, of thin-walled vessels giving off cascal processes. (Kb'lliker.) 



4. The lungs are, structurally, to be regarded as two compound 

 racemose glands; and an accurate knowledge of the structure of 

 one of the lobules, therefore, implies that of a whole lung. The 

 lungs are invested, however, externally, by a serous membrane, the 

 pleura ; which, like the peritoneum, forms a closed cavity, and con- 

 sists of two portions the pleura costalis lining the thoracic cavity, 

 and the pleura pulmonalis, directly adherent to the lung. In struc- 

 ture, also, the pleura entirely corresponds with the peritoneum (see 

 p. 523); the parietal layer being the thicker and most adherent, 

 and its epithelium being the simple scaly variety. The pulmonary 

 layer is, however, the more vascular. Nerves, with fine and coarser 

 fibres, are sent to the parietal layer, from the phrenic and the sym- 

 pathetic (Luschka)', and Kolliker has seen medium and thick nerve- 

 fibres accompanying the branches of the bronchial arteries in the 

 pleura costalis, and occasionally, large scattered ganglion-cells also. 

 .The lung proper consists 1st, of the continuations of the trachea 

 (the bronchi and their subdivisions) into the air-cells; %dly, the ves- 

 sels and nerves ; and, 3c%, the connective tissue binding all these 

 elements together in the lobules. 



1. The bronchi and their subdivisions in the lung have the same 

 structural elements as the trachea, on a diminished scale ; except 

 that the cartilaginous rings entirely disappear in the finer subdi- 

 visions (under ^ of an inch), and in the finest the fibrous tunic- 

 coalesces with the mucous. The smooth muscular fibres constitute 

 a completely continuous layer in the smaller subdivisions, and ter- 

 minate J- of an inch short of the last air-cells to which they lead. 



