580 SPECIAL HISTOLOGY. 



bid states, containing fat, 0-005-0-007 of a line in diameter, and 

 0'003-0'004 of a line in thickness, resting immediately upon the 

 fibrous membrane of the air-cells. A regular detachment of this 

 epithelium is not to be supposed, any more than with that of the 

 trachea and bronchia?, whilst it is indubitable, 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 ramifi- 

 cations of the bronchice, although in almost every lung, at all events in 

 some of the alveoles, 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 contained 

 sparingly, even between the secondary lobules, and between the primary, 

 exists in inappreciably minute quantity, consists 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, aggregations of granules, arid also crystals, which, it may be 

 said, are never enclosed in cells. The walls of the alveolce themselves, 

 also, very frequently contain this pigment, which, when it is deposited 

 in smaller quantity and regularly, marks out very distinctly the con- 

 tours of the secondary lobules, and not unfrequently, 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 por- 

 tions, 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- 

 inously with greater frequency, and consequently diminish more rapidly 

 in diameter. Ultimately, a twig goes to each secondary lobule, which 

 then subdivides into still finer ramuscules, in general corresponding in 

 number with the smallest lobules, and supplying the individual air-cells. 

 The course of these finest lobular arteries, as they may be termed, is 

 very easily traced in injected, inflated, and dried preparations ; and it 

 is apparent, that whilst traversing the uniting tissue between the 

 lobules (infimdibula), they supply not one lobule alone, but always two 

 or even three of them with finer twigs. These penetrate from without, 

 upon and between the air-cells, divide repeatedly while running in the 

 larger elastic trabeculw, anastomosing also occasionally, though not 



