284 



PULMONARY MUCOUS TISSUE. 



few metamorphoses and scarcely any epithelial remains: 

 they even show a tendency to waste away with age ; and the 

 walls which supported them also falling away, what is called 



preparation which he thought necessary for the study of the pul- 

 monary lobules (desiccation, bichloride of mercury, water of am- 

 monia, and, finally, iodine. Now the pulmonary epithelium is the 

 most delicate of all the tissues, and requires the same process of 

 preparation as the most delicate epitheliums of the serous tissue. 

 Elenz (in 1864), by means of nitrate of silver, ascertained the 

 existence of a pulmonary epithelium in all the vertebrated animals, 

 and his observations have been since confirmed by others. Schmidt 

 (op. ciV.), by employing the same method, arrived at the following 



Fig. 75. Pulmonary epithelium.* 



conclusions: in the mammalia the pulmonary vesicles of the em- 

 bryo are lined with regular cells, all of uniform size; in the new- 

 born animals some of these cells become larger and cover the 

 capillaries, while the rest remain unchanged, united together in 

 groups in the meshes of the capillaries (Fig. 75). Finally, in 

 adults the groups consist of a smaller number of cells, and many 

 of them are quite isolated. The large cells which divide them 

 appear to be partly united, resembling membranous layers, ex- 

 tremely simple and almost amorphous. 



The arguments against the existence of the pulmonary epithe- 

 lium which have been drawn from the study of comparative 

 anatomy have all proved false in the light of fuller investigation. 

 The pond-loach (coUtis fossilis) is a singular fish, which swallows 



* lj Capillary vessels. 2, Interstice in the capillaries (the white in the dia- 

 gram is a portion of the capillary network; the dotted lines represent the 

 meshes or interstices of this network). 3, Outline of the epithelial cells. 

 4, Nuclei of the cells, usually found in a mesh. 



