SOFT TISSUES OF THE TRACHEA. 47 



a basement membrane, on which again is an epithelial layer of 

 ciliated cells, having a thickness of O06 0'075 of a millimeter. 



The submucous tissue, like the tissue of the mucous mem- 

 brane, is marked by the predominatingly longitudinal direction 

 pursued by its fibres, which are for the most part composed of 

 connective tissue, becoming progressively mingled with more 

 elastic fibres towards the outer surface. In the posterior por- 

 tion of the trachea, and extending between the anterior sur- 

 faces of the free extremities of the cartilage, there is a thick 

 (0'8 1'2 of a millimeter) transverse layer of organic muscular 

 fibres, inserted by means of small delicate tendons into the 

 perichondrium of the cartilaginous rings, or more rarely lost in 

 the mucous membrane. 



This muscular layer is intersected or segmented by strong 

 bands of connective tissue, several of which usually correspond 

 to each cartilaginous ring. These septa are accompanied by 

 vessels and nerves which supply the mucous membrane from 

 behind. Not unfrequently short longitudinal muscular fasciculi 

 lie on the outer side of the transverse muscular coat, which 

 arise from, and terminate in, the above-named septa, and serve 

 as clamps for the transverse fasciculi. Lastly, on the outer- 

 most surface there is a layer of longitudinal connective tissue 

 (fibrous layer). 



Similar but much stronger muscular fasciculi occur in the trachea 

 of the Dog and Cat, and when these contract they cause the rings to 

 become imbricated, or to overlap each other, so that when divided 

 horizontally the sections of two adjacent rings are obtained, separated 

 by elastic tissue, and arranged concentrically. 



In these animals, as well as in the Kabbit, Sheep, etc., the trans- 

 verse muscular layer presents the peculiarity of extending far beyond 

 the extremities of the cartilage, and of being inserted into their 

 external surfaces. It invests quite as much as one- third of the 

 circumference of each ring, and can contract so powerfully as to 

 bring their free extremities into contact, or even to make them 

 overlap. 



The trachea is abundantly supplied with acinous glands, 

 which form anteriorly and laterally a continuous layer, that 

 does not appear to be interrupted even over the most convex 



