RESPIRATORY MEMBRANE. 285 



pulmonary emphysema ensues, a change which is so often 

 observed in old people. This is not the case, however, in 

 pathological conditions: when irritated, this epithelium be- 

 comes hypertrophied and proliferates. This is what gives ' 

 rise to the false membranes in croup, and to the characteris- 

 tic features of pneumonia; the alveoli are then entirely 

 obliterated and transformed into a compact and resisting 

 tissue, for which reason this state is known by the name of 

 hepatisation. This epithelium has also the chief share in 

 producing tubercle, and some other less common transforma- 

 tions, as cancer of the lung. 



In cases of infarctus of the lung, especially when pro- 

 duced artificially in the dog, the epithelium may easily be 

 seen to have undergone a certain hypertrophy in the pul- 

 monary alveoli, infiltrated with blood, some of its cells falling 

 into the alveolus, and mixing with the blood globules (Vul- 

 pian). 



2. This epithelium is supported by a membrane, which 

 forms a sort of shell to the alveolus. This consists of a con- 

 nective tissue, which is nearly amorphous and full of plasmatio 

 cells, and it has a large number of elastic fibres, forming a 

 close network, the meshes of which are extremely minute. 

 Sometimes the elastic fibres are found at a greater distance 

 from each other, and, by dividing them, they may be made 

 perfectly distinct in a preparation. These elastic elements, 

 formed of fibres whose outline is strongly marked with nu- 

 merous bifurcations, are of great importance in a physiological 

 point of view; as, for instance, in spu ta, these resist decay 

 for a long time, and are often the only part of a necrosed and 

 worn-out lung, which preserves the characteristic features that 

 can be recognized by the microscope. In some animals this 

 membrane is composed, in part, of smooth muscular fibres : it 

 is not easy to decide, by anatomical examination, whether 

 the case is the same in man. 1 We shall inquire later whether 



air by the mouth, and, after having absorbed a part of the oxygen, 

 gives off carbonic acid by the anus. Leydig could discover no 

 intestinal epithelium in this fish, in -which the respiration is partly 

 intestinal; but Schmidt, by the aid of nitrate of silver, ascertained 

 that the surface in question has a complete epithelial covering: 

 here, too, the different cells are intermingled without any order, 

 being sometimes of equal size and tolerably regular in arrange- 

 ment, and at others grouped in such a manner that several small 

 cells appear surrounded by smaller ones. 



1 " The muscular fibres appear in the large bronchi under the 



