I/O NORMAL HISTOLOGY. 



Still farther inward, and not distinctly separated 

 from the submucosa, lies the mucosa, composed of 

 fibrillar connective tissue containing an abundance of 

 variously shaped cells, and, for the most part, longi- 

 tudinally arranged elastic fibres. In some parts of 

 the mucosa the tissue resembles diffuse lymphotd 

 tissue. The connective tissue of the mucosa, in 

 some parts of the trachea, does not form an uniform 

 layer, but is arranged in more or less well-defined 

 longitudinal bundles, giving the surface a wavy or 

 folded appearance. 



Internally, the mucosa is bordered by a thin, 

 homogeneous membrane, the basal membrane, upon 

 which the epithelial cells lining the trachea rest. 

 The epithelial cells are usually arranged in about 

 three layers. Lying upon the basal membrane are 

 irregularly spheroidal, often somewhat elongated 

 cells ; upon these lie fusiform or pear-shaped cells, 

 while the surface is formed by a layer of pyramidal 

 or cylindrical ciliated cells. In the submucosa, and 

 sometimes extending outward along and between 

 the cartilaginous rings, lie racemose mucous glands, 

 whose excretory ducts, lined with cylindrical epi- 

 thelium, pass obliquely inward and terminate on the 

 surface in expanded orifices. The alveoli of the 

 glands are lined with a single layer of slightly granu- 

 lar, polyhedral cells. When the glands are in a con- 

 dition of functional activity, however, the cells 

 become larger, the outlines indistinct, their nuclei 



