76 



THE LUNGS, BY FRANZ EILHARD SCHULZE. 



as well as on the inner surface of the tubular continuations of 

 the bronchia, and in the posterior segment of the lungs of 

 Amphisbsena and Ophidia, it becomes continuous with a wide- 

 meshed system of capillaries that are probably subservient to 

 nutrition. 



Fig. 135. 



Fig. 135. Part of a pulmonary alveolus from the Frog (Rana tem- 

 poraria). The left side is represented destitute of epithelium. Mag- 

 nified 200 diameters. a a, Ends of capillaries ; 6, a heap of small 

 columnar epithelial cells. 



All respiratory capillaries are attached to the alveolar wall 

 by one side only of their circumference, and thus lie with the 

 greater part of their surface freely exposed in the air cavity of 

 the alveolus, except where they are invested by a continuous 

 layer of tesselated epithelial cells. 



Fig. 136. 



Fig. 136. Optical section of an alveolar wall from the edible Frog 

 (Rana esculenta), after being hardened in perosmic acid. Magnified 

 320 diameters. a a a, Lumina of capillary vessels ; 6, nucleated 

 columnar epithelial cells ; c, muscular fibres of the alveolar wall. 



The large polygonal cells of this alveolar epithelium are in 



