STRUCTURE OF THE LUNGS OF MAMMALS. 59 



membranes, the alveolar septa. This, however, though common, 

 is by no means constant in the case of the walls of adjacent 

 alveoli belonging to different infundibular or alveolar passages. 

 These are usually separated by uniform thin layers of loose 

 fibrous interstitial connective tissue. Stronger septa of the 

 same kind in the lungs of all Mammals divide off polyhedric 

 portions of the alveolar parenchyma which are known as 

 lobuli. As a general rule the size of these diminishes with 

 that of the animal : in Man their diameter varies from 0'5 to 

 1 centimeter, and their irregularly polygonal boundaries, with 

 four to six sides, are faintly seen as it were in transverse 

 section through the pleura, as well as in sections made 

 through the substance of the lung. The interstitial connec- 

 tive tissue of the parenchyma of the lung is firmly attached 

 on the one hand to the pleura, and on the other is directly 

 continuous with the investing and connecting loose adven- 

 titious fibrous tissue of the bronchia, vessels and nerves. 



The fundamental layer of the alveolar wall is composed of a 

 transparent structureless membrane, that only here and there, 

 and especially in the thicker parts, exhibits a distinctly fibrous 

 character ; whilst sparingly distributed through it are elongated 

 oval connective-tissue corpuscles without any noticeable sur- 

 rounding granular area. This transparent layer is traversed 

 by numerous elastic fibres, the peculiar arrangement of which 

 principally confers upon the pulmonary tissue its characteristic 

 appearance under the microscope. The elastic tissue occurs in 

 greatest abundance in the main trunks of each infundibular 

 system. In this situation annular bundles of strong elastic 

 fibres are found, not always forming complete circles, which 

 surround and in great measure compose the free thickened 

 borders of the strong septa between the lateral groups of alve- 

 oli ; these are especially well developed as the septa of bifur- 

 cation of the whole infundibulum, and also constitute the 

 invariably somewhat constricted orifices of the lateral and 

 terminal infundibula. From these compact bands of strong 

 elastic fibres thinner fasciculi are given off, which serve on the 

 one hand to support the borders and angles where several al- 

 veoli meet, and on the other form a border to the rounded poly- 

 gonal orifices of the several alveoli, whether these open directly 



