CHAP. IL] RESPIRATION. 559 



The trachea consists of a ciliated mucous membrane, resting 

 on a coat of connective tissue, strengthened with hoops or imperfect 

 rings of cartilage and provided with a certain amount of plain 

 muscular tissue. A vertical section of the mucous membrane 

 shews an epithelium consisting of three or more layers of cells, 

 those in the uppermost layer being columnar ciliated cells ( 93), 

 and those in the lower layers small rounded cells, the cell- 

 substance being scanty in proportion to the nucleus ; it is supposed 

 that some of these small cells may at times develope into ciliated 

 cells in order to replace loss. Among ciliated cells are seen a certain 

 number of goblet cells ( 261). Beneath the epithelium runs a 

 fairly distinct basement membrane, and below this in turn is seen 

 some fine reticular tissue, like that in the small intestine ( 259), 

 containing in its meshes a certain number of leucocytes. Mixed 

 up with the reticular tissue, which in different animals varies much 

 in the amount present, are seen a certain but variable number of 

 fine elastic fibres. These structures constitute together the mucous 

 membrane, below which is a somewhat conspicuous layer of elastic 

 fibres, arranged more or less in a network, but running distinctly 

 longitudinally and forming a longitudinal elastic layer separating 

 the mucous membrane above from the loose submucous connective 

 tissue below. In this submucous tissue are placed a number of 

 small mucous or albuminous glands, like those of the oesophagus, 

 the ducts of which passing through the elastic layer, reticular 

 tissue and epithelium, open into the canal of the trachea. The 

 outer part of this submucous tissue forms a somewhat denser coat 

 of connective tissue, in which are lodged hoops of hyaline cartilage, 

 that is to say, rings which are imperfect behind. Stretching 

 transversely between the ends of each hoop of cartilage are several 

 bundles of plain muscular fibres, completing the ring as it were 

 by a muscular band; a few longitudinally disposed muscular 

 bundles may also be seen outside the transverse bundles. These 

 two sets of muscular fibres may be taken as being the remains 

 of the original complete double muscular coat of the alimentary 

 canal, almost obliterated by the introduction of the cartilaginous 

 hoops. 



The main purpose served by these several structures is to 

 provide a wide flexible elastic tube, the. bore of which remains 

 large and open and the lining smooth during the bending of the 

 tube. The mucous fluid secreted by the goblet cells and small 

 glands helps to arrest solid particles carried in by the inspired air, 

 while the cilia are continually driving that mucus, with the 

 particles entangled in it, upwards to the larynx and so into the 

 mouth. The elastic layer adapts the mucous membrane to the 

 variations in the length of the tube during its bending, and so 

 keeps it smooth. The transverse muscles by contracting can some- 

 what narrow the bore, when required ; but their effect in this 

 direction can be slight only. 



