530 STRUCTURE OF LUNG. [BOOK 11. 



Each infundibulum is surrounded by connective tissue carrying- 

 blood vessels and lymphatics. A number of infundibula with their 

 respective bronchioles are bound together by connective tissue 

 carrying larger blood vessels to form a lobule, the bronchioles 

 joining to form the lobular bronchia. A number of lobules are 

 bound together with interlobular bronchia and still larger blood 

 vessels to form a lobe, and several lobes join to form the lung. 

 When a lung is inflated, and when as after death the blood vessels 

 are for the most part emptied of blood, the infundibula with their 

 alveoli form by far the greater part of the bulk of the lung. Hence 

 a section taken through a hardened and prepared inflated lung 

 seems to be made up almost wholly of a number of polygonal or 

 frequently hexagonal spaces, which are sections of alveoli, and 

 among which are seen sections in various planes of bronchia, small 

 and large, and of blood vessels; here and there the section may 

 disclose the opening of a bronchiole into an infundibulum, and 

 the division of one of the lobular bronchia into a number of 

 bronchioles. 



318. The infundibulum repeats in structure as we have 

 said the lung of the newt or the frog. A septum or wall between 

 two contiguous alveoli consists of a thin median basis of connective 

 tissue, crowded with a close-set capillary network, and covered on 

 each side with an epithelium. The connective tissue is richly pro- 

 vided with fine elastic fibres, but the ordinary gelatiniferous fibrillse 

 are imperfectly developed, the blood vessels being to a large extent 

 imbedded as it were in a homogeneous matrix. The septum, 

 especially towards its summit, is often so thin that the capillary is 

 exposed to the air on both sides. The cells of the epithelium, 

 which is much better shewn in the lung of a young animal, and 

 indeed is in the adult very difficult to see, are for the most part 

 transformed into small flat transparent plates from which the 

 nuclei have disappeared ; their outlines may be distinctly shewn 

 by silver nitrate treatment but otherwise are often very indistinct. 

 Between these clear flat plates there occur small groups of cells 

 distinguished by possessing nuclei, and by their cell-substance being 

 granular and staining with the ordinary reagents. These granular 

 cells, which are thicker than the clear plates, are placed in groups 

 in the meshes of the capillary networks, so that the capillaries 

 themselves are covered only by the thin nucleus-less plates. 



The wall of the infundibulum which forms the bases of the 

 several alveoli has a similar structure, and is lined with an 

 epithelium of similar character, the chief difference between the 

 sides and the base of an alveolus being that while the blood in the 

 capillaries of the latter is exposed to the air of the alveolus on one 

 side only, that of the former is often exposed on both sides of even 

 the same capillary. 



319. In describing the bronchial passages we had perhaps 

 better begin with the trachea. 



