166 SPECIAL HISTOLOGY. 



incomplete layer, 0'3'" thick, composed of transverse fibres; 

 on the outer aspect of which are isolated longitudinal bun- 

 dles, whose elements, 0*03'" long and 0*002 — 0*004'" wide, 

 are united into small fasciculi, 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. 277.) 



On the inner aspect of the cartilages and muscles, which, to 

 a certain extent, are to be regarded as one layer, we find a 

 stratum about 012"' thick, of more common, close, connective 

 tissue, and then the true mucous membrane. This consists of 

 two layers; an external composed of connective tissue, 0*12" 

 in thickness ; and an internal, yellow, of 009 — 0*1'", almost 

 wholly elastic, the plexiform fibres of which, 0001 5'" in dia- 

 meter, run longitudinally, and in places, especially on the 

 posterior wall, often constitute flattened fasciculi joined at 

 acute angles. The innermost portion of the elastic layer, 

 0*024 — 0*03'" thick, is, particularly in the posterior wall, as in 

 the larynx, frequently composed more of connective tissue 

 with fine elastic fibrils ; it may also be raised as a thin pel- 

 licle from the thicker elastic layer, and internally always pre- 

 sents a more homogeneous layer, 0*005'" 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 -^ — ?", 

 found especially on the anterior wall, within the mucous 

 membrane and immediately exterior to the elastic layer ; and 

 2. larger of i — 1'", 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 tesselated epithelium in their vesicles; whilst the 

 smaller, situated in the mucous membrane itself, and some 

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

 csecal follicles, present in their oval gland-vesicles, 0*02 — 03"' 

 in size, a very narrow cavity and walls of 0*006 — O'Ol'", a thick- 

 ness which may, perhaps, be referred altogether to the well 

 marked cylinder-epithelium. 



The blood-vessels of the trachea are very numerous, and, in 



