THE LUNGS. 



175 



Fig. 239. 



mously with greater frequency and consequently, diminish 

 more rapidly in diameter. Ultimately, a twig goes to each 

 secondary lobule, which then subdivides into still finer ramus- 

 cules, 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 (infundibula), 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 trabecule, anasto- 

 mosing also occasionally, though not regularly, with each other, 

 or with branches of other lobular arteries and finally, terminate 

 in the capillary plexus of the air-cells. This plexus, which is 

 one of the closest 

 existing in Man, as 

 estimated in moist 

 preparations, presents 

 rounded or oval meshes 

 0-002 — 0-008'" wide, 

 and vessels of 0003 

 — 0-005"' in diameter. 

 It lies in the wall of 

 the air cells, at a dis- 

 tance of about 0-001'" 

 from the epithelium, 

 in the middle of the 

 fibrous tissue and is 

 continuous, not only 

 over all the alveola of one of the smallest lobules, but also, at 

 all events in the adult, is partially in connexion with the plexuses 

 of the contiguous lobules. The pulmonary veins arise from the 

 above-described capillary plexus, with roots which lie more 

 superficial than the arteries and more externally, on the 

 smallest lobules, then run deeply between them and unite with 

 other lobular veins into larger trunks, which proceed, in part 

 with the arteries and bronchial tubes, in part more isolated by 

 themselves, through the pulmonary parenchyma. 



Fig. 239. Capillary plexus of the human air-cell, x 60 diam. 



