536 THE TRACHEA. 



tached above to the larynx, and continued below into the 

 bronchi. It has the cartilaginous portion deposited in it, 

 and forms a sheath for each particular ring, and supplying 

 the deficiency behind, where the rings cease. This tissue 

 is regarded as belonging to the yellow elastic, and restores 

 the trachea, when elongated, to its natural length. 



The elastic tissue presents the form of longitudinal bands, 

 is found between the mucous and muscular coats, at the 

 posterior portion of the trachea, and descends into the 

 bronchi. The muscular fibres are attached to the ends of 

 the cartilaginous rings behind fill up their deficiency, 

 are about half a line in thickness, and run transversely. 

 They are exposed by dissecting off the fibrous coat, when 

 they are seen to be thin and pale. Their function is to 

 diminish the size of the trachea, and assist in expelling 

 the mucus during expiration. 



The mucous membrane extends from the larynx, lines the 

 trachea, and is traced downward through the bronchial 

 tubes in all their ramifications, as far as the air-cells of the 

 lungs. This membrane is thin, delicate, and pale, and 

 presents numerous foramina, the orifices of mucous glands. 

 These glands are found most abundantly on the posterior 

 surface of the trachea, situated in the muscular coat, be- 

 tween the muscular and fibrous, in the substance of th'e 

 fibrous, and between the latter and the mucous. 



They mostly present the form of small ovoid bodies, but 

 have occasionally attained a much larger size. 



The blood-vessels supplying the trachea come principally 

 from the superior and inferior thyroid arteries. The veins 

 are superficial and deep, and enter the adjoining veins. 

 The nerves come from the par vagum. 



