LUNGS. 



[ 403 ] 



LUNGS. 



layer. The former is composed of a basis 

 of homogeneous areolar tissue, with nume- 

 rous elastic fibres, vessels, and nerves. 



The elastic fibres surround the air-cells in 

 the form of elegant wavy bundles and sepa- 

 rate fibres, which anastomose and constitute 

 a dense network, most obvious at those parts 

 where several cells are in contact with each 

 other ; whilst in other parts the areolar ele- 

 ment supporting the numerous capillaries 

 predominates, and the elastic elements are 

 more sparing and slender. The epithelium 

 is of the pavement kind, not ciliated, con- 

 sisting of rounded or polygonal nucleated 

 cells, about 1-2000" in diameter. 



The terminal bronchi do not end in sepa- 

 rate air-cells, but in a group of them, and 

 have no direct communication with each 

 other, but open into a common cavity, with 

 which the bronchus also communicates (fig. 

 424). These groups of air-cells form the 



Fig. 424. 



Fig. 425. 



Two pulmonary lobules, a, a, with the air-cells, b, b, 

 and the terminations of the bronchi, c, c ; from an infant 

 newly born, 



Magnified 25 diameters. 



lobules of the lungs, and are separated from 

 each other by areolar tissue mixed with 

 nuclear fibres, containing in adult ani- 

 mals (fig. 425) black pigment in the form of 

 distinct or isolated granules, sometimes also 

 crystals. The lobules are best seen in the 

 lungs of young animals. 



These smaller or primary lobules are ag- 

 gregated to form larger secondary lobules 

 the lobules of descriptive anatomists, and 

 the outlines of which in adults are principally 

 mapped out by lines of pigment. 



Outer surface of the lung of a cow, the air-cells of 

 which were injected with wax. a, a, a, air-cells ; b, b, 

 boundaries of the (primary) lobules. 



Magnified 30 diameters. 



The lobular structure of the lungs is best 

 shown in the lungs of foetal animals injected 

 from the trachea or bronchi. 



The capillaries of the lungs are extremely 

 minute and very difficult to inject fully ; and 



Fig. 426. 



Capillaries of the human lung. 

 Magnified 60 diameters. 



the finest injection is required for the pur- 

 pose. 



In the lower vertebrate animals, the 

 structure of the lungs is much simpler than 

 in the higher. Thus in the Triton each 

 forms a simple tubular sac, whilst in the 

 frog and toad (PL 31. fig. 34) each lung 

 may be compared to a single lobule of a lung 



2o2 



