AIR-CELLS OF THE LUNGS. 



2 7 a 



enters n distinct pnlmonaiy lobule, ^vithin which it undergoes still 

 farther division, and at last ends in the small recesses named air-cells, 

 ah'coli ov pulmonary vesicles (b). ' .' - 



Within the lungs the air-tubes are not flattened behind like the 

 bronchi and trachea, but form completely cyhndrical tubes. Hence, 

 although they contain the same elements as the larger air-passages, 

 reduced gradually to a state of greater and greater tenuity, they possess 

 certain peculiarities of structure. Thus, the cartilages no longer appear 

 as imperfect rings running only upon the front and lateral surfaces of 

 the air-tube, but are disposed over all sides of the tube in the form of 

 iiTcgularly shaped plates and incomplete rings of various sizes. These 

 are most developed at the points of division of the bronchia, where they 

 form a sharp concave ridge projecting inwards into the tube. They 

 may be traced, becoming rarer and rarer and more reduced in size, as 

 far as bronchia only one-half a line in diameter, beyond which the 

 tubes are entirely membranous. The filrrous coat extends to the 

 smallest tubes, becoming thinner by degrees and degenerating into 

 areolar tissue. The rducoiis memhrane, which extends throughout the 

 whole system of air passages, is also thinner than in the trachea and 

 bronchus, but it retains its ciliated columnar epithelium (fig. 182, il). 

 The longitudinal bundles of elastic fibres (fig. 182, c, in transverse 

 section) are very distinct in both the large and small bronchia, 

 and may be followed by dissection as far as the tube can be 

 laid open, and by the microscope into the smallest tubes. The mus- 

 cular fibres, which in the trachea and bronchi are confined to the back 



Fig. 183. — Portion of the Fig. 183. 



Outer Surface of the 

 Cow's Lung (from Kolliker 

 after Hartiiig). Magnified 

 30 Diameters. 



a, pulmonary vesicles filled 

 artificially with wax ; h, the 

 margins of the smallest lobules. 



part of the tube, sur- 

 round the bronchial 

 tubes with a continuous 

 layer of annular fibres, 

 lying inside the carti- 

 laginous plates (fig. 

 182, h) ; they are 

 found, however, beyond 

 the place where the car- 

 tilages cease to exist, and appear as irregular annular fasciculi even 

 in the smallest tubes. 



The air-cells are grouped around the terminations of each lobular 

 bronchial tube, and, in the natural state, are always filled with air. 

 They are readily seen on the surface and in a section of a lung, which 

 has been inflated with air and dried ; also upon portions of foetal or 

 adult lung injected with mercury or wax (fig. 183, a, a). 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 Too^h of an inch, 



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