THE LUNGS. 



167 



the mucous membrane, are especially characterised by the cir- 

 cumstance, that the larger branches run chiefly in a longi- 

 tudinal direction, whilst 

 the superficial capillary 

 plexus, which is frequently 

 met with above the elas- 

 tic elements, close beneath 

 the homogeneous layer, 

 more commonly forms 

 rounded-angular meshes. 

 The trachea is abundantly 

 furnished with lymphatics; 

 and in one case I found 

 their commencement in 

 the mucous membrane, in 

 the form of wide-meshed plexuses 0*003 — O'OOl"' broad, of 

 thin-walled vessels, from which, here and there, isolated, caecal 

 processes were given off (fig. 235). The nerves also of the tra- 

 chea are numerous, and present the same conditions as those 

 of the larynx. 



§ 175. 



Lungs. — The lungs are two large, compound, racemose 

 glands, in which are to be distinguished : 1 . a special serous 

 coat — the pleura ; 2. the secreting parenchyma, consisting of 

 the ramifications of the two bronchi, with their terminations, 

 the air-cells, and numerous vessels and nerves; and 3. an 

 interstitial tissue interposed between these parts and uniting 

 them into larger and smaller lobules. 



The pleurae, in their structure, entirely correspond with the 

 peritonaeum, as in which, the parietal layer is the thicker, and 

 consist of connective tissue abounding in finer or coarser 

 elastic elements, with a tesselated epithelium, to which consti- 

 tuents, on the walls of the thorax, as on the exterior of the 

 pericardium, a more purely fibrous lamella is superadded. 

 Vessels are seen most abundantly in the pleura pulmonalis, 

 where, arising from the bronchial and pulmonary arteries, they 

 ramify in the subserous tissue ; whilst the parietal lamella is 



Fig. 235. Commencement of the lymphatics in the tracheal mucous membrane of 

 Man, x 350 diam. 



