Quain's elements of anatomy . theepithelium. Throughout the mucous membrane a number of fine elasticfibres are found, but in the deeper parts the elastic fibres are very largeand numerous {d). Along the posterior membranous part, they aremore abundant than elsewhere, and are there collected into distinct longi-tudinal bundles, which produce visible elevations or flutings of themucous membrane. These bundles are particularly strong and numerousopposite the bifurcation of the trachea. The epithelium consists of a layer of columnar ciliated cells, oftenvery irregular at their fixed end, where the
Quain's elements of anatomy . theepithelium. Throughout the mucous membrane a number of fine elasticfibres are found, but in the deeper parts the elastic fibres are very largeand numerous {d). Along the posterior membranous part, they aremore abundant than elsewhere, and are there collected into distinct longi-tudinal bundles, which produce visible elevations or flutings of themucous membrane. These bundles are particularly strong and numerousopposite the bifurcation of the trachea. The epithelium consists of a layer of columnar ciliated cells, oftenvery irregular at their fixed end, where they are impressed by smallercells, between which they penetrate to reach the basement cilia serve to drive the mucous secretion upwards towards thelarynx. Between these ciliated cells, are found others, some roundedothers elongated ; the latter being prolonged at one end towards thesmface, whilst the other end, which is not unfrequently forked, reachesto the subjacent membrane. A few lymph corpuscles are also found. 514 THE LUjSGS. amongst the epithelial cells, as in other epitheha. Some of the cellssecrete mucus, and hence goblet-cells are frequently found here (seep. 44). The trachea is provided with numerous small mucous glands. Thelargest are situated at the back part of the tube, either close upon theouter surface of the fibrous layer, or occupying little recesses formedbetween its meshes. Smaller glands are found between the cartilaginousrings, upon and within the fibrous membrane, and still smaller ones closebeneath the mucous membrane. They are compound glands, and theircavities are lined by a columnar or cubical epithelium : the excretoryducts pass through the muscular layer and the mucous membrane, onthe surface of which their orifices are perceptible. Vessels and Nerves.—The arteries of the trachea are principally derivedfrom the inferior thyroid. The larger branches run for some distance longitudi-nally, and then join a superficial capillary plexus with
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