THE LUNGS. 



573 



terior wall of the tube, only an incomplete layer, 0-3 of a line thick, 

 composed of transverse fibres ; on the outer aspect of which are isolated 

 longitudinal bundles, whose elements, 0-03 of a line long and 0-002-0-004 

 of a line wide, are united into small fasci- 

 culi, which arise by delicate minute tendons 

 of elastic tissue, in part from the inner sur- 

 face of the ends of the tracheal rings ; in 

 part, particularly the longitudinal bundles, 

 from the external fibrous membrane. ( Vid. 

 Mikr. Anat. II. 2, Fig. 27T.) 



On the inner aspect of the cartilages 

 and muscles, which, to a certain extent, are 

 to be regarded as one layer, we find a stra- 

 tum about 0-12 of aline thick, of more com- 

 mon, close, connective tissue, and then the 

 true mucous membrane. This consists of 

 two layers ; an external composed of con- 

 nective tissue, 0-12 of a line in thickness ; 

 and an internal, yellow, of 0-09-0-1 of a 

 line, almost wholly elastic, the plexiform 

 fibres of which, 0-0015 of a line in diame- 

 ter, run longitudinally^ and in places, espe- 

 cially on the posterior wall, often constitute 

 flattened fasciculi joined at acute angles. 

 The innermost portion of the elastic layer, 

 0-024-0-03 of aline thick, is, particularly 

 in the posterior wall, as in the larynx, fre- 

 quently composed more of connective tissue 

 with fine elastic fibrils; it may also be raised 

 as a thin pellicle from the thicker elastic 



layer, and internally always presents a more homogeneous layer, 0-005 of 

 a line thick. Upon this lies the ciliated epithelium, which is laminated 

 and differs in no respect from that of the larynx. Numerous glands 

 exist in the mucous membrane; these are : 1, smaller ones of 1-10-1-4 of 

 a line, found especially on the anterior wall, within the mucous mem- 

 brane and immediately exterior to the elastic layer ; and 2, larger, of 

 J-l line, which occur more in the posterior wall, externally to the 

 muscles and the whole mucous membrane, or between the cartilages. 

 These glands differ from those of the larynx only inasmuch as the larger 

 of them alone have the usual tessellated epithelium in their vesicles ; 



FIG. 234. Vertical section through the anterior wall of the human trachea, magnified 

 45 diam.: a, fibrous coat; 6. c, d, cartilage; 6, external layer, with flattened cells; d, inter- 

 nal layer, with elongated elements ; e, submucous connective tissue ; /, portion of a mucous 

 gland ; g, elastic longitudinal fibrous layer ; /z, epithelium, on which the cilia are not visible ; 

 i, glandular orifice. 



