610 THE LUNGS. 



eter of these tubes can be diminished, and, as suggested by 

 Dr. Physic, the expulsion of mucus greatly facilitated. 

 These bronchial cartilages, as they proceed, become smaller 

 and smaller till reduced to simple lines, patches, or grains, 

 when they are finally lost and the tube becomes wholly 

 membranous. 



The cartilages of the bronchi are all connected by a 

 continuation of the same elastic fibrous tissue, that belongs 

 to the rings of the trachea. The extent of the muscular 

 coat of the bronchi is not exactly determined ; some are 

 disposed to think it ceases at the last bronchial cartilage, 

 while others carry it somewhat beyond this point upon the 

 membranous portion towards the cells. The mucous mem- 

 brane of the bronchi is a continuation of the same that lines 

 the trachea and larynx, and is traced on into the air cells 

 of the lungs. It is very vascular, and is seen to present a 

 number of longitudinal folds. This membrane abounds 

 with mucous follicles, whose orifices, upon its surface, are 

 so numerous as to present the cribriform appearance. At 

 the beginning of the bronchi the mucous coat is found 

 firm, thick, and red. As it proceeds it becomes thinner 

 and paler, till in the membranous terminations it appears 

 transparent and of great tenuity. 



The Air Cells. These cells, which are, as just stated, 

 the ultimate coecal terminations of the bronchi, form 

 clusters of cells, constituting the several lobules. Their 

 precise form and arrangement still remain a matter of 

 doubt, for while on the one hand they are regarded by 

 Keisessen and his followers to be round, and related to 

 each other after the manner of the fruit on a bunch of 

 grapes, each grape being connected by a separate pedi- 

 cle to one common stock, so it was thought that the 

 cells of the lobules had no communication directly, but 

 only by their ducts, which lead to a common bronchial 

 tube ; Dr. Horner's experiments seem to show conclusively, 

 on the other hand, that the cells of each lobule communi- 

 cate directly the one with the other, but not with the cells 

 of different lobules; and the terminating bronchial branches, 



