STEUCTURE OF THE TEACHEA. 267 



of the latter, and encroach also for a short distance upon their inner 

 surface. 



Outside the transA'erse fibres are a few fasciculi having a longi- 

 tudinal direction. These are said to ai'ise by minute tendons of elastic 

 tissue, in part fi-om the inner surface of the end of the tracheal rings, 

 and in part fi'om the external fibrous membrane. 



Situated in the submucous tissue immediately beneath the mucous 

 membrane are numerous longitudinal fibres of elastic tissue. They 

 are found all round the tube, but are much more abundant along the 

 posterior membranous part, where they are principally collected into 

 distinct longitudinal bundles, which produce visible elevations or 

 flutings of the mucous membrane. These bundles are particularly 

 strong and numerous ojjposite the bifurcation of the trachea. The 

 elastic fibres serve to restore the windpipe to its ordinary size after it 

 has been stretched. 



A quantity of adipose tissue is often found in the submucous areolar 

 tissue. 



The trachea is provided with numerous small mucous glands. 

 The largest are situated at the back part of the tube, either close 

 upon the outer surface of the fibrous layer, or occupying little recesses 

 formed between its meshes. They are compound glands, and their 

 cavities are lined by a columnar or cubical epithelium : their excretory 

 ducts pass through the muscular layer and the mucous membrane, on 

 the surface of which multitudes of minute orifices are perceptible. 

 Similar but smaller glands are found between the cartilaginous rings, 

 upon and within the fibrous membrane, and still smaller ones close 

 beneath the mucous membrane. 



The mucous membrane is smooth and of a pale pinkish white 

 colour in health, although when congested or inflamed, it becomes in- 

 tensely purple or crimson. It contains a considerable amount of 

 lymphoid tissue, the reticulum of which is condensed immediately 

 imderneath the epithelium into a well-marked basement membrane, com- 

 posed of flattened cells which send processes up amongst the epithelium 

 cells. In the deeper parts of the mucous membrane a large number of 

 fine elastic fibres are found. The epithelium consists of more than one 

 layer of cells. The most superficial (fig. 24, p. 47), are columnar and cili- 

 ated, and send processes, which are often branched, downwards into the 

 subjacent tissue, to join, it is said, with processes from connective tissue 

 corpuscles. The cilia serve to drive the mucous secretion upwards towards 

 the lar}Tix. Between the smaller or attached ends of these ciliated 

 cells, are found elongated, often spindle-shaped cells; which commonly 

 are prolonged at one end towards the surface, whilst the other end, 

 which is not unfrequently forked, reaches to the subjacent membrane. 

 One or more layers of smaller, more irregularly-shaped cells occupy the 

 deeper part of the epithelium : amongst the epithelial cells a few 

 leucocytes are also found. The cells generally contain mucus, and 

 hence are readily converted into goblets (see p. 211). 



Vessels and Nerves. — The arteries of the trachea are principally 

 derived from the inferior thyroid. The larger branches run for some 

 distance longitudinally, and then form a superficial plexus with rounded 

 meshes. The vmns enter the adjacent plexuses of the thyroid- veins. 

 A rich plexus of Ijpnpltatics may readily be injected in the mucous 

 membrane and submucous tissue, but the lymphoid fohicles, so commoE 



