EESPIEATION. 



229 



Fig. 242 A, portion of the 

 lung of a hog. The terminal 

 vesicles are filled with mer- 

 cury, and of the natural size. 

 B, the same part seen under 

 a simple lens. 



Delicp.te arcuate fibres, of the nature of elastic tissue, sur- 

 round these terminal vesicles, and 

 hold them distended, whilst the 

 vessels spread freely over their sur- 

 face (fig. 242). 



[ 390. The development of the 

 lungs is extremely interesting. In the 

 embryo of the bird and mammal 

 they first appear in the shape of a 

 simple, and then of a double projec- 

 tion from the esophagus (fig. 244, ), 

 which soon divides more distinctly 

 into two, becomes separated from 

 this part, and is finally supported 

 upon a pedicle the future trachea 

 (fig. 244, 5). In birds these little 

 sacs are then drawn out into hollow tubes, which pass over 

 into the paral- 

 lel pipes above 

 described ( 

 387). In the 

 mammalia 

 they divide, 

 after the man- 

 ner of branch- 

 es, into twigs 

 and minute 

 vesicles (figs. 

 241 and 242), 

 which advance 

 in develop- 

 ment, and be- 

 come the future 

 terminal cells 

 (fig. 242, B). 



[ 391. The 



capillary vas- Fig. 243. Small portion of lung from the body of 



cular net- work a man examined shortly after death, under a magnify- 



of the lunffs * n ^ P ower ^ ^0 times. The vessels, b, b, &c., still 



P. ' turgid with blood, include very minute islets of paren- 



ac Y chyma between them ; the semicircular fibres, c, a, a, 



Stated, exhi- surround the smallest terminal cells of the lungs. 



