136 ORGANS OF RESPIRATION. 



pulmonary artery, and opposite the fifth dorsal vertebra. The 

 bronchia then divide and subdivide very minutely through the 

 lungs. 



Very dissimilar structures enter into the composition of the 

 trachea; they are cartilage, ligamentous fibre, muscle and a mu- 

 cous membrane. 



The Cartilage preserves fhe cylindrical shape of the trachea, 

 and consists in from sixteen to twenty distinct rings, which are 

 deficient in the posterior third of their circumference. Each 

 ring is about two lines broad, and half a line thick, and is kept 

 apart from the one above 'and below it by a small interstice : 

 sometimes, however, they run into each other. 



There is an almost uniform similitude between these rings; the 

 principal departure from which is observed in the first being ra- 

 ther broader in front than the others, and in the last ring, which, 

 by its corset-like shape in front, contributes to the beginning. of 

 each bronchu . 



The rings of the Bronchia are, like those of the trachea, defi- 

 cient in their posterior third, and the oame arrangement holds 



during their primitive ramifications in the lungs. But as they 

 subdivide more and more, the cartilages do not succeed each 

 other so closely, and are smaller segments of circles; they also 

 are not regularly deficient at the posterior third ; but, in place 

 of the latter, the bronchial ramification is surrounded by car- 

 tilage, consisting in several pieces put end to end. The pieces 

 become, after awhile, more and more scattered and smaller, 

 till they finally disappear, and the bronchus is simply mem- 

 branous. At the orifice of each branch of the bronchia, there 

 is a semi-lunar cartilage, forming rather more than one half of 

 its circumference, and having its concave edge upwards. The 

 whole arrangement resembles somewhat the pasteboard to an 

 eared bonnet, and is evidently to keep the orifice open. 



The Ligamentous Structure of the trachea and of the bron- 

 chia is very evident between the proximate margins of the car- 

 tilaginous rings, and fills up the intervals between them so as to 

 make the tube perfect. This tissue may be traced over the sur- 

 faces of the rings, forming their perichondrium, so that they may 

 be considered as imbedded in it. It does not exist satisfactorily, 



