STRUCTURE OF THE LUNGS OF REPTILES AND AMPHIBIA. 77 



immediate contact with each other, laterally cover that surface 

 of the capillaries which is turned towards the cavity of the 

 air passage with a thin transparent laminar expansion, and 

 send conical processes, usually consisting of the cell nucleus, 

 with a little surrounding granular protoplasm, into the capillary 

 meshes, to so great a depth, indeed, that they reach to the 

 connective-tissue stroma of the alveolar wall, and thus com- 

 pletely fill the spaces of the capillary plexus. 



These tap-shaped prolongations proceeding from the nucleus 

 and granular protoplasm of each epithelial cell, are usually found 

 at the angles of the cells, so that several cones may lie in juxta- 

 position, and occupy one capillary mesh. Still many cells occur, 

 whose nucleated process is attached nearer to their centre, and 

 with it completely fill a single capillary mesh * Whilst now the 

 respiratory surfaces of the lungs in Reptiles and Amphibia are 

 lined by similar pavement epithelial cells,f the free borders of 

 all the more prominent septa and ridges, as well as the internal 

 surface of the bronchial continuation, are covered in general by 

 a somewhat flattened columnar ciliated epithelium, in some 

 parts of which numerous cup-shaped cells are distributed. 

 The entire surface of the posterior non-respiratory segment of 

 the lungs of the Snake and Amphisbsena is lined by a single 

 but continuous layer of small polygonal, slightly granular 

 pavement epithelial cells. 



* Whilst the results of my investigations in regard to the pulmonary 

 epithelium of the Amphibia agree in all essential points with those of 

 Elenz and C. Schmidt, I differ from these observers in one point relating 

 to the alveolar epithelium of the reptilian lungs, as I found all the epithe- 

 lial cells, however flat they might be, contain nuclei, whilst I was unable 

 to discover any structureless non-nucleated plates. On filling the lungs 

 with Mliller's solution, and immersing them in it at the same time, the 

 epithelium investing the respiratory cavities of Amphibia is not only ren- 

 dered perfectly distinct, with all the cell contour lines, but this layer is 

 wholly or partially raised and broken up into its constituent cells. 



t We meet also here and there, especially in the lungs of the Frog, 

 amongst the epithelium of the alveoli, with rounded groups varying from 

 ten to thirty in number, of more cylindrical cells, which collectively fill a 

 large capillary mesh, and appear to be to some extent subservient to the 

 performance of a similar secretory function to that of the cup cells 

 (fig. 135 6). 



