LUNGS. 



[ 403 ] 



LUNGS. 



^er. The former is composed of a basis 

 homogeneous areolar tissue, with nume- 

 us elastic fibres, vessels, and nerves. 

 The elastic fibres surround the air-cells in 

 e form of elegant wavy bundles and sepa- 

 te fibres, which anastomose and constitute 

 iense network, most obvious at those parts 

 lere several cells are in contact with each 

 her ; whilst in other parts the areolar ele- 

 3nt supporting the numerous capillaries 

 edominates, and the elastic elements are 

 3re sparing and slender. The epithelium 

 of the pavement kind, not ciliated, con- 

 ting of rounded or polygonal nucleated 

 [Is, about 1-2000" in diameter. 

 The terminal bronchi do not end in sepa- 

 :e air-cells, but in a group of them, and 

 ve no direct communication with each 

 ler, but open into a common cavity, with 

 lich the bronchus also communicates (fig. 

 4). These groups of air-cells form the 



Fig. 424. 



Pwo pulmonary lobules, n, a, with the air-cells, b, b, 

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

 vly born. 



Magnified 25 diameters. 



bules of the lungs, and are separated from 

 ch other by areolar tissue mixed with 

 iclear fibres, containing in adult ani- 

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

 stinct or isolated granules, sometimes also 

 y^stals. The lobules are best seen in the 

 ngs of young animals. 

 These smaller or primary lobules are ag- 

 egated to form larger secondary lobules — 

 e lobules of descriptive anatomists, and 

 e outlines of which in adults are principally 

 apped out by lines of pigment. 



Fig. 425. 



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

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



<»!si6^^6i&i 





Capillaries of the human lung. 

 Magnified 6o 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 (PI. 31 . fig, 34) each lung- 

 may be compared to a single lobule of a lung 



2d2 



