254 



NORMAL HISTOLOGY. 



FIG. 290. 



completely surrounds the alveolus and constitutes the septum be- 

 tween the adjoining air-sacs, at the same time supporting the capillary 

 vessels and the investing epithelium. In addition to the elastic fibres, 

 a very small quantity of fibrous tissue, with a few connective-tissue 

 cells, aids in the construction of the air-sac. 



The capillary net-work within the walls of the air-sacs is re- 

 markable for the closeness of its meshes, being one of the densest 

 vascular net-works within the body. The larger arterial stems 



take their course, in company with 

 the veins, bronchioles, nerves, and 

 lymphatics, within the thicker 

 tracts of interlobular connec- 

 tive tissue ; the smaller twigs 

 extend among the groups of in- 

 fundibula, embracing the openings 

 into the air-sacs with more or less 

 complete rings, from which pass 

 the capillaries enveloping the air- 

 sacs with net-works on all sides. 

 Between the adjoining alveoli lies 



Section of injected and inflated lung of cat : Only a Single layer of Capillary 

 -.air-sacs enclosed in dense capillary net-works yessels which hoWCVCr, are not 

 (6) ; c, larger interlobular branches of pul monary 



artery. confined to a single plane, but 



encroach alternately upon the 

 adjacent air-sacs as projecting arches or loops. 



While the interalveolar septa are reduced to a minimum, the two 

 layers of respiratory epithelium lining the adjoining air-sacs, the 

 scanty connective-tissue framework and the capillary net- 

 work constituting their entire bulk, the alveoli belonging to differ- 

 ent, although neighboring, infundibula are separated by distinct 

 connective-tissue partitions ; these increase in thickness as the 

 included divisions of pulmonary substance become larger, and reach 

 their greatest development in the fibrous envelopes ensheathing 

 and separating the lobes. 



Owing to the accumulation of the pigmented particles conveyed 

 by the lymphatics in the manner already described, the interinfun- 

 dibular and often also the interlobular connective tissue present dark 

 patches, the degree of discoloration varying from a few scattered 

 irregular points to an intense almost uniformly black area. The 

 presence of pigment within the connective tissue emphasizes the 

 outlines and boundaries of the lobules with diagrammatic 

 sharpness. 



The blood-vessels of the lungs enter at the hilus along with the 

 large divisions of the bronchus ; the smaller branches of the pulmo- 



