MINUTE STRUCTURE OF THE AIR-CELLS. 



901 



presence of nuclei, however, situated in the capillary meshes, and of larger 

 size than those which belong to the capillary walls, is allowed on all hands ; 

 and the majority of recent observers declare the existence of exceedingly 



Fig. 629. 



f 



Fig. 629. DIAGRAMS ILLUSTRATING THE PROGRESSIVE ADVANCE IN THE CELLULAR 

 STRUCTURE OF THE LUNGS OP KEPTILES. 



A, the upper portion of the lung of a serpent : the summit has cellular walls, the 

 lower part forms merely a membranous sac. B, lung of the frog, in which the cellular 

 structure extends over the whole internal surface of the lung, but is more marked at the 

 upper part. C, lung of the turtle : the cells here have extended so as to occupy nearly 

 the whole thickness of the lung. 



delicate squamoua epithelial cells. These, according to Eberth, lie in the 

 capillary meshes, from one to three in each, but leave the surfaces of the 

 capillary vessels uncovered. According to others, they join each other over 

 the capillary blood-vessels. 



Fig. 630. 



Fig. 630. FRAGMENT OF THE INJECTED LUNG OF A YOUNG PIG, SHOWING THE MINUTE 

 STRUCTURE OF THE VESICLES (from Hirschmann and Chrzonszczewsky). 



a, the areolar and elastic tissue supporting the vesicles ; 6, the cavities of two of the 

 vesicles partially cut through ; c, the ineshes of the pulmonary capillaries, the latter 

 being filled with dark colouring matter, and the meshes being occupied by regular 

 hexagonal epithelial cells, which in various places are seen to meet each other over the 

 capillary vessels ; d, the intervesicular pulmonary vessels. In this instance, the cells 

 observed in each mesh have been more numerous than they are said by Eberth to be in 

 the human subject. 



