218 STRUCTURE OF THE TRACHEA. 



stitute the trachealis muscle, and are attached to the inner surfaces of the cartilages 

 by means of elastic tendons at a little distance from their free-ends (Munniks, 1697). 

 The arrangement varies in different animals thus, in the cat, dog, rabbit, and rat 

 the muscular fibres are attached to the external surfaces of the cartilages, while in 

 the pig, sheep, and ox they are attached to their internal sm-faces^Stirling).] 

 Some muscular fibres are arranged longitiidinally external to the transverse fibres 

 (Kramer). The function of these muscular fibres is to prevent too great distension 

 when there is great pressure within the air-passages. 



The muCOUS membrane consists of a basis of very fine connective-tissue con- 

 taining much adenoid-tissue with numerous lymph-corpuscles. It also contains 

 numerous elastic fibres, arranged chiefly in a longitudinal direction under the base- 

 ment membrane. They are also abundant in the deep layers of the posterior part 

 of the membrane opposite the intervals between the cartilages. A small quantity of 

 loose sub-mucous connective-tissue contaiuing the large blood-vessels, glands, and 

 lymphatics unites the mucous membrane to the perichondrium of the cartilages. 

 The epithelium consists of a layer of columnar ciliated cells with several layers of 

 immature cells under them. [The superficial layer of cells is columnar and 

 ciliated (Fig. 97, b), while those lying under them present a variety of forms, and 

 below all is a layer of somewhat flattened squames, c, resting on the basement 

 membrane, d. These squames constitute a layer quite distinct from the basement 

 membrane, and they form the layer described by De'bove. They are active germi- 

 nating cells, and play a most important part in connection with the regeneration 

 )of the epithelium, after the superficial layers have been shed, in such conditions as 

 bronchitis (v. Drasch, Hamilton). Not unfrequently a little viscid mucus (a) lies 

 on the free-ends of the cilia. In the intermediate layer, the cells are more or less 

 pyriform or battledore-shaped (Hamilton), with their long tapering process inserted 

 amongst the deepest layer of squames. According to Drasch, this long process is 

 attached to one of these cells and is an outgrowth from it, the whole constituting 

 a "foot-cell."] 



Underneath the epithelial is the homogeneous basement membrane, through 

 which tine canals pass connecting the cement of the epithelium with spaces in 

 the mucosa. [This membrane is well marked in the human trachea, where 

 it plays an important part in many pathological conditions, e.g., bronchitis. 

 It is stained bright red with picrocarmine.] The cilia act so as to carry 

 any secretion towards the larynx. Goblet cells exist between the ciliated 

 columnar cells. Numerous small compound tubular mucous glands occur in 

 the mucous membrane, chiefly between the cartilages. Their ducts open on 

 the surface by means of a slightly funnel-shaped aperture into which the 

 ciliated epithelium is prolonged for a short distance. [The acini of some of these 

 glands lie outside the trachealis muscle. The acini are lined by cubical or 

 columnar secretory epithelium. In some animals (dog) these cells are clear, and 

 present the usual characters of a mucous-secreting gland ; in man, some of the cells 

 maybe clear, and others "granular," but the appearance of the cells depends 

 upon the physiological state of activity.] These glands secrete the mucus, which 

 entangles particles inspired with the air, and is carried towards the larynx by 

 ciliary action. [Numerous lymphatics exist in the mucous and sub-mucous coat, 

 and not unfrequently small aggregations of adenoid tissue occur (especially in the 

 cat) in the mucous coat, usually around the ducts of the glands. They are com- 

 parable to the solitary follicles of the alimentary tract. The blood-vessels are 

 not so numerous as in some other mucous membranes. [A plexus of nerves con- 

 taining numerous ganglionic cells at the nodes exists on the posterior surface of the 

 trachealis muscle. The fibres are derived from the vagus, recurrent laryngeal, and 

 sympathetic (C. Frankenhauser, W. Stirling, Kandarazi).] 



[The mucous membrane of the trachea and extra-pulmonary bronchi, 

 therefore, consists of the following layers from within outwards : 



