174 SPECIAL HISTOLOGY. 



trabecule, as a connective medium between the numerous 

 capillaries. 



The epithelium of the air-cells is of the common tesselated 

 kind, without cilia, and composed of polygonal, pale, granular 

 cells., in morbid states, containing fat, 0*005 — 0-007'" in dia- 

 meter and 0-003 — 0-004'" in thickness, resting immediately 

 upon the fibrous membrane of the air-cells. A regular de- 

 tachment of this epithelium is not to be supposed, any more 

 than with that of the trachea and bronchia, whilst it is in- 

 dubitable, that by chance, or in diseases of the air-passages, its 

 isolated elements may become mixed with the bronchial mucus. 

 In Man, these cells are detached with remarkable readiness and 

 then lie free in the air-cells and finest ramifications of the 

 bronchia, although in almost every lung, at all events in some 

 of the alveola, they may still be seen in situ ; and in animals 

 recently killed, the observation of their disposition presents no 

 difficulty whatever. 



The interlobular connective tissue of the lungs, which is con- 

 tained sparingly, even between the secondary lobules, and be- 

 tween the primary, exists in inappreciably minute quantity, con- 

 sists of common connective tissue with fine elastic fibres and 

 contains, in the adult, a larger or smaller quantity of blackish 

 pigment, in the form of irregular, minute granules, aggrega- 

 tions of granules and also crystals, which, it may be said, 

 are never enclosed in cells. The walls of the alveola them- 

 selves, also, very frequently contain this pigment, which, when it 

 is deposited in smaller quantity and regularly, marks out very 

 distinctly the contours of the secondary lobules, and not un- 

 frequently also, to some extent, those of the primary. 



§ 178. 



Vessels and nerves of the lungs. — As regards their blood- 

 vessels, the lungs occupy a unique position, inasmuch as 

 they possess two complete vascular systems, for the most part 

 distinct from each other — that of the bronchial vessels, for the 

 nutrition of certain portions and that of the pulmonary vessels 

 for the fulfilment of their proper function. The branches 

 of the pulmonary artery follow pretty nearly the course of 

 the bronchial tubes, which are most usually placed below and 

 behind them, with this difference, that they divide dichoto- 



