390 RESPIRATION. [CHAP. XXIX. 



mammalia), their superficial cells have their inequalities mutually 

 adapted to each other, and even their walls fused together, so that 

 the lobules would not remain distinct, were it not that their air-cells 

 do not communicate across the interval. 



To convey a correct, and at the same time a simple, idea of the 

 constitution of the pulmonary lobules, we must regard each as an 

 elementary lung, perfect in itself as an arrangement of a respiratory 

 membrane adapted to the aeration of the blood. First, the ter- 

 minal bronchial tube pertaining to each lobule, loses its epithelium 

 and muscular tunic at about one-eighth of an inch distant from the 

 last air-cell to which it leads, and is thus reduced to basement 

 tissue and yellow elastic fibres, which become blended into a single 

 coat, the only membrane composing the tubes beyond, and the air- 

 cells. The terminal bronchial tube, thus simplified, ramifies 

 within the lobule, and its branches may be conveniently distin- 

 guished from the bronchial tubes under the name of lobular pas- 

 sages* The lobular passages are wider than the terminal bronchia, 

 and are remarkable (being honeycombed on their interior) for 

 presenting a series of sacculi or cells on their wall. These are the 

 pulmonary air-cells. They form a number of bulgings of the wall, 

 and are separated from one another by septa projecting inwards from 

 the wall towards the axis of the passage (fig. 206).. They each open 

 separately into the lobular passages, and do not communicate with 

 each other except through the passages. The terminations of the 

 several lobular passages are air-cells coming up to the surface of the 

 lobule, but some of the air-cells placed laterally on the passages, 

 also contribute to form the surface of the lobule. The air-cells 

 thus surround and terminate each lobular passage and the 

 lobule consists of a number of lobular passages, associated by their 

 dependence on a single terminal bronchial tube, and each clothed 

 as it were, on its sides and at its end, with a honey-comb of air- 

 cells, with orifices open towards the cavity of the passages. Con- 

 tiguous cells of the same passage are separated by a simple septum 

 or process of the wall, while the contiguous cells of neighbouring 

 passages are separated by a septum, likewise simple, formed by the 

 union of the walls of each. Where the septa spring from the wall 

 of the passage, or in the angles where neighbouring cells unite, the 

 wall is strengthened by a greater thickness of elastic tissue, which 

 often has the form of arching bands of considerable strength in these 

 situations, as well as around the orifices of the cells. Thus the 

 lobular passages and the air-cells are formed of one tissue, and 

 * Dr. W. Addison, Phil. Trans. 1840. 





