CHAP. IL] RESPIRATION. 561 



of homogeneous nature containing imbedded in itself a large 

 number of elastic fibres and fibrils with a few connective tissue 

 corpuscles, and a network of capillaries so close set that the 

 membrane seems to be merely elastic material filling up the 

 meshes of the network. On the outside, this capillary membrane, 

 if we may so call it, is continuous with the looser ordinary 

 connective tissue, still however containing abundant elastic 

 elements, which carries the small arteries and veins going to and 

 coming from the capillary network, and which unites the infun- 

 clibula and bronchioles into lobules. On the inside lies the 

 attenuated epithelium, all the cells of which are flat and some of 

 which are mere nucleus-less plates. The muscular fibres have 

 either wholly disappeared or, according to some observers, persist 

 as a few straggling fibres spreading over the infundibulum. The 

 terminal portion of the pulmonary passage is a sac, whose walls 

 are reduced to almost the greatest possible thinness consistent 

 with their retaining very great elastic power. 



The bronchial passages of medium size are essentially elastic 

 muscular tubes, capable like the arteries of varying their calibre, 

 but unless their muscular fibres are thrown into unusually power- 

 ful contractions, remaining always fairly open; the smaller ones 

 however, those which are devoid of cartilage, may perhaps close 

 by collapse. These passages are lined by mucous membrane, the 

 cells of which are well formed and active, some secreting mucus, 

 and others by their cilia driving that mucus onwards towards 

 the trachea. The air which passes into the lungs is frequently 

 laden with impurities, these are entangled in the mucus of the 

 passages, especially the smaller ones, and so are either carried 

 upwards in tne mucus, or as we shall see otherwise disposed of. 



The larger passages are open flexible tubes becoming more 

 rigidly open, and less susceptible to change in calibre by muscular 

 contraction the larger they are. 



322. The lungs are well provided with lymphatics. The 

 reticular tissue underlying the epithelium of the mucous membrane 

 is here and there developed into masses of true adenoid tissue 

 crowded with leucocytes, that is to say, into more or less completely 

 differentiated lymphatic follicles, and similar follicles are met with 

 in deeper parts. Among the flat polygonal epithelioid plates which 

 form the surface of the pleural membrane investing the lung are 

 numerous stomata ( 290); and during the rhythmie movements 

 of the lungs in breathing the lymph or serous fluid of the pleural 

 cavity is continually being pumped into the lymphatic vessels of 

 the lungs. These lymphatic vessels, arising from lymph-spaces in 

 all parts of the lungs including the connective tissue around the 

 alveoli, and running in the connective tissue binding together 

 infundibula, bronchial tubes and blood vessels into lobules, and the 

 lobules into lobes, find their way at last, after traversing several 

 lymphatic (bronchial) glands to the roots of the lungs, whence 



