574 SPECIAL HISTOLOGY. 



whilst the smaller, situated in the mucous membrane itself, and some 

 of which are in the strictest sense simple, or only bifurcated coecal folli- 

 cles, present in their oval gland-vesicles, 0-02-0-03 of a line in size, 

 a very narrow cavity and walls of 0-006-0-01 of a line, a thickness 

 which may, perhaps, be referred altogether to the well-marked cylinder- 

 epithelium. 



The bloodvessels of the trachea are very numerous, and, in the mucous 

 membrane, are especially characterized by the circumstance, that the 



larger branches run chiefly 



Fig - 235- in a longitudinal direction, 



whilst the superficial capillary 

 plexus, which is frequently 

 met with above the elastic 

 elements, close beneath the 

 homogeneous layer, more com- 

 monly forms rounded-angular 

 meshes. The trachea is abun- 

 dantly furnished with lym- 

 phatics; and in one case I 

 found their commencement in 

 the mucous membrane, in the 

 form of wide-meshed plexuses 



0'003-0-001 of a line broad, of thin-walled vessels, from which, here 

 and there, isolated, caecal processes were given off (Fig. 235). The 

 nerves also of the trachea are numerous, and present the same condi- 

 tions 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 ramifica- 

 tions 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 pleurce, in their structure, entirely correspond with the peri- 

 toneum, as in which, the parietal layer is the thicker, and consist of 

 connective tissue abounding in finer or coarser elastic elements, with a 

 tessellated epithelium, to which constituents, 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 more scantily 

 supplied by the intercostal and mammary arteries. Luschka found nerves 



FIG. 235. Commencement of the lymphatics in the tracheal mucous membrane of Man, 

 magnified 350 diameters. 



