388 RESPIRATION. [CHAP. xxix. 



substance more or less distinctly mapped out by areolar tissue and 

 termed lobules. 



Coats of the Bronchia. All these become gradually thinner as 

 they approach the air-cells. The cartilaginous pieces, which are 

 irregular in shape and position in the lobal bronchia, become reduced 

 to mere flakes, and finally cease in those of one-sixth or one-tenth 

 of an inch in diameter (fig. 204) . The last are seen mostly where 

 Fig 203 branchings occur. The muscular fibres of the 



trachea are continued down even to the terminal 

 bronchia, but instead of filling up only the gap 

 in the cartilaginous framework, they form a 

 uniform layer encircling the canal, but exces- 

 sively thin. The fibres are here arranged in 

 anastomosing bundles (fig. 203). Within the 

 muscular layer is that of the longitudinal elastic 

 fibres, here disposed as an even layer, and re- 

 smaii bronchial tube laid presenting the submucous areolar tissue. The 

 ciliated epithelium and the basement membrane 

 of the mucous tissue both descend into the 

 terminal bronchia. On the exterior of the 

 bronchia is some areolar tissue separating them 

 from the neighbouring masses of air-cells, and associated with the 

 arteries, veins, lymphatics and nerves belonging to the bronchial wall. 

 The bronchial arteries are usually two, coming from the aorta, 

 but irregular. They supply the coats of the bronchia, and have 

 corresponding veins. Their capillaries anastomose with those of 

 the pulmonary artery where the terminal bronchia become lobular 

 passages. The distribution and actions of the pulmonary nerves 

 have been already discussed (pp.120, 124-7). 



Ultimate Pulmonary Tissue. Lobules. In some parts of the 

 exterior of the lungs, particularly near the borders, and in some 

 animals throughout, may be noticed a sort of mapping out of the 

 pulmonary substance into small polyhedral masses separated by 

 areolar tissue, and having a very irregular shape. These are the 

 lobules of the lungs. They can only be made out in certain situ- 

 ations, even by dissection, for it does not appear that the whole 

 human lung is thus subdivided by areolar septa. Nevertheless, it 

 seems certain that each terminal twig of the bronchus is in relation 

 with only its own proper set of air-cells, and that such sets of cells 

 do not communicate except through the medium of the bronchia. 

 In this sense lobules exist everywhere, even when not isolated by 

 areolar tissue, and in this sense we shall use the term, as con- 



