134 Dr. E. Klein on the Anatomy of [Jan. 29, 



condition, of polyhedral cells (not flattened as commonly described) 

 arranged in a single layer. This is well seen in guineapigs, less di- 

 stinctly in rabbits, rats, dogs, and cats. If the lung is not distended, the 

 endothelium of the surface very much resembles an epithelium, the cells 

 being polyhedral, or in the form of short columns ; they are markedly 

 granular, and have distinct nuclei. Even in the moderately distended 

 lung, the endothelium of the pleura pulmonum is by no means of the same 

 morphological character as that on the costal pleura. Between the endo- 

 thelium of the one and that of the other organ there exists the same dif- 

 ference as between that of the ovary and that of the peritoneum the one 

 consisting of polyhedral, or shortly columnar, granular cells with very marked 

 nuclei, the other of very flattened, almost hyaline, endothelial plates. 



(6) The pleura pulmonum is a very thin connective-tissue membrane, 

 provided, like other serous membranes, with a rich network of elastic 

 fibres. In the lungs of the rat, rabbit, cat, and dog the pleura pulmonum 

 seems to consist, for the most part, of elastic networks. In the matrix, 

 there is generally one layer of flattened connective-tissue corpuscles to be 

 found. 



Beneath the proper pleural membrane, there exists, in the guineapig, a 

 membrane which consists of non-striated muscular fibres, arranged in 

 bundles which form a meshwork. In the normal condition the bundles 

 are relatively thin, and the meshwork which they form has elongated large 

 meshes. In the distended lung the meshes are of a much greater diameter 

 than in the collapsed lung ; in the latter they form a more continuous mem- 

 brane. The muscular bundles have, in general, a radiating direction from 

 the apex towards the basis of the lung ; and it is further to be noted that 

 they are most abundant on the external surface, viz. that directed towards 

 the anterior wall of the chest, and the internal surface, viz. that directed 

 towards the mediastinum ; whereas on the posterior surface the bundles 

 are scanty, and become more and more so the nearer the vertebral column 

 is approached. This distribution of the muscular tissue is therefore in 

 perfect agreement with the proportion in which the different parts of the 

 lung participate in the respiratory movement, the fibres being most richly 

 distributed over those parts of the pulmonary surface which are subject to 

 the greatest extent of excursions, and vice versa. In rats and rabbits, as 

 well as in cats and dogs, bundles of un striped muscular fibres occur spa- 

 ringly ; at any rate there are none on the posterior surface of the lung of 

 these animals. As soon as the superficial parts of the lung become the 

 seat of a chronic inflammatory process (e. g. tuberculosis, chronic pneu- 

 monia), the muscular bundles increase in breadth and number to such 

 a degree, that they form a continuous membrane, chiefly in those parts of 

 the surface which correspond to the diseased portions of the lung. 



1. Subpleural lymphatics. The meshes of the muscular membrane of 

 the lung of guineapigs are lined by a single layer of flattened endothelial 

 cells, constituting, in fact, a communicating system of lymphatic sinuses. 



