900 



THE LUNGS. 



also upon portions of foetal or adult lung injected with mercury. In the 

 lungs of some animals, as of the lion, cat, and dog, they are very large, 

 and are distinctly visible on the surface of the organ. In the adult human 

 lung their most common diameter is about -y^^th of an inch, but it varies 

 from j^Qth to T V ta of an inch ; they are larger on the surface than in the 

 interior, and largest towards the thin edges of the organ : they are also 

 said to be very large at the apex of the lung. Their dimensions go on 

 increasing from birth to old age, and they are larger in men than in women. 

 In the infant their diameter is usually under -^^th of an inch. 



The small bronchial tube entering each lobule divides and subdivides 

 from four to nine times, according to the size of the lobule ; its branches, 

 which diverge at less and less acute angles, at first diminish at each subdi- 

 vision, but afterwards continue stationary in size, being from ^th to ^o ta 

 of an inch in diameter. They lose at last their cylindrical form, and are 

 converted into irregular lobular passages, beset, at first sparingly, but after- 

 wards closely and on all sides, with numerous little recesses or dilatations, 

 and ultimately terminate near the surface of the lobule in a group of similar 

 recesses. These small recesses, whether seated along the course or at the 



Fig. 628. Fig. 628. SEMIDIAGRAMMATIC REPRESENTA- 



TION OP Two SMALL LOBULI FROM NEAR THE 

 SURFACE OF THE LUNG OF A NEW-BORN CHILD 

 (from Kolliker). 2 f 



a, exterior of the two lobuli or infundibula ; 

 6, pulmonary vesicles or alveoli on these and on 

 c, the smallest bronchial ramifications. 



extremity of an air passage, are the air- 

 cells or alveoli ; and each group of alveoli 

 with the comparatively large passage 

 between them constitutes an infundi- 

 bulum, so called from the manner in 

 which it dilates from the extremity of 

 the bronchial tube. The arrangement of 

 these finest air -passages and air-cells 

 closely resembles, though on a smaller 

 scale, the reticulated structure of the 

 tortoise's lung, in which large open 



passages lead in all directions to clusters of wide alveoli, separated from 

 each other by intervening septa of various depths. 



At the point where the small bronchial tubes lose their cylindrical 

 character, and become covered on all sides with the cells, their structural 

 elements also undergo a change. The muscular layer disappears, the longi- 

 tudinal elastic bundles are broken up into an interlacement of areolar and 

 elastic tissue, which surrounds the tubes and forms the basis of their walls. 

 The mucous membrane becomes exceedingly delicate, consisting merely of a 

 thin transparent membrane, covered by a stratum of squamous instead of 

 ciliated cylindrical epithelium. 



The walls of the alveoli, their orifices, and the margins of the septa, are 

 supported and strengthened by scattered and coiled elastic fibres, in addition 

 to which, according to Moleschott, Gerlach, and Hirschmann, there is like- 

 wise an intermixture of muscular fibres. It was stated by Rainey, and 

 corroborated by Todd and Bowman, and it is still maintained by Henle, 

 Luschka, and others, that the alveoli are destitute of all epithelium. The 



