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 caecal 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 

 ^^^- -'^- 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 li/7n- 

 pJiatics ; and in one case I 

 found their commencement in 

 the mucous membrane, in the 

 form of wide-meshed plexuses 

 O-OOo-O-OOl of a line broad, of thin-walled vessels, from which, here 

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

 nei'ves 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 yierves ; 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 j^eri- 

 to7ieum, 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 tlie lymphatics in the tracheal mucons membrane of Man, 

 magnified 350 diameters. 



