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THE RESPIRATORY TRACT. 



whole inner surface of the larynx, except that, over the vocal 

 bands, delicate stratified epithelium is seen, which is most marked 

 in the middle portions. Here, at the boundary of the connective 

 tissue, a narrow, so-called hyaline layer is also noticed, which is 

 found in no other portion of the mucosa of the larynx. 



The epithelial cover passes, with numerous prolongations, 

 into the connective tissue, forming acinous glands. These appear 

 as extended, flat layers underneath the mucosa covering the vocal 

 bands ; while they constitute more or less globular formations 

 in other portions. The largest mucous glands are found, as a 

 rule, at the lower portion of the laryngeal surface of the epiglot- 

 tis and along the posterior wall of the larynx. 



The blood-vessels in the mucosa of the larynx are expanded 

 into several flat layers, and in some places we can distinguish three 

 such layers, whose meshes become narrower near the surface. 

 The largest veins are found in the mucosa covering the posterior 

 wall. The lymphatics also produce two indistinctly marked lay- 

 ers, the meshes of the upper layer being narrower than those of 

 the lower. The nerves are said to be supplied with numerous 

 ganglionic elements. 



(3) The trachea is a rigid, elastic tube, composed of incomplete 

 cartilaginous rings, which, at the posterior wall, are closed by a 

 layer of fibrous connective tissue abundantly supplied with elas- 

 tic substance. The cartilage of the tracheal rings is hyaline, but 

 in the middle portions fibrous, similar to the laryngeal cartilages 5 

 the fibrous structure being more marked in adults than in chil- 

 dren. The mucosa is in close connection with the perichondrium, 

 and has a nearly uniform composition of delicate, fibrous con- 

 nective tissue, scantily supplied with lymph-tissue, but with an 

 ample supply of blood-vessels, among which wide capillaries and 

 veins are the most marked. Nearer the surface a delicate layer 

 of smooth muscle-fibers is found, arranged usually in a circular 

 direction. In the posterior wall, at the places where the cartilage 

 is lacking, the circular muscles produce a broad layer, uniting 

 the blunt ends of the cartilage, and on the peripheral portion of 

 the posterior wall large longitudinal muscle-bundles are found. 

 The mucosa of the trachea is destitute of papill3, but lies in folds 

 along the inner circumference. If these are well marked in the 

 anterior portion, then the posterior portion, where the cartilage 

 is absent, is without folds. The innermost layer of the con- 

 nective tissue produces a delicate basement-membrane, which, 

 however, is not invariably present in all trachea. (See Fig. 317.) 



