THE LUNGS. 145 



Outside of the cartilaginous layer the trachea is surrounded 

 by fibrous and areolar tissue continuous with that of the 

 adjacent structures. 



The Lungs. 



The lungs are made up of air- vesicles or alveoli, the ulti- 

 mate functional divisions of the organ ; bronchial tubes, the 

 air-passages leading to the air-vesicles ; connective sustentacu- 

 lar tissue ; an ample system of bloodvessels, besides lymph- 

 atics and nerves; and a serous covering derived from the 

 pleura. 



The lung-substance is more or less distinctly divided into 

 pyramidal lobules, consisting of the groups of alveoli cor- 

 responding to terminal divisions of the bronchi and blood- 

 vessels ; the lobules are to a greater or less extent marked off 

 from one another by septa, although closely packed together. 



The connective tissue of the lung, its sustentacular frame- 

 work, may be divided into the superficial investment of the 

 organ and the interalveolar, peribronchial, and interlobular 

 tissues. The surface of the lung is surrounded by elastic and 

 areolar tissue, lying beneath the pleura and continuous with 

 its subserous layer. 



The free surface is covered by the visceral portion of the 

 pleura. 



The interalveolar connective tissueis a delicate tissue, mostly 

 elastic, between, surrounding, and forming a supporting frame- 

 work for the air-vesicles ; it comprises the larger part of the 

 pulmonary connective tissue, and to it are due the well-known 

 elasticity and contractile tendency of the lungs. 



The peribronckial connective tissue is a firmer tissue of the 

 white-fibrous variety which forms definite sustentacular tracts 

 accompanying, supporting, and surrounding in a single sheath 

 the bronchi and their concomitant bloodvessels in all their 

 ramifications and subdivisions throughout the lungs. 



The interlobular connective tissue consists of more or less 

 definite thin fibrous septa separating the lobules of the organ. 



Pigment, usually black, is often abundantly deposited in 

 patches in the interalveolar and interlobular tissue ; it con- 

 sists of particles inspired into the air- vesicles and removed 

 into the interior by the lymphatics. 



10 Hist. 



