224 RESPIRATORY SYSTEM. 



tlie windpipe, have all a close resemblance to one another: if 

 there beany disparity between them worthy of notice, it consists 

 in those that form the superior part of the pipe being somewhat 

 larger and broader than those nearest to the bronchial tubes*. 

 A ling is not uniform in its breadth, in consequence of having 

 waving or scolloped borders ; the advantage of which is, that a 

 sort of dove-tailed connexion is effected which materially contri- 

 butes to the compactness and strength of the entire structure. 

 Its front and sides measure, in the broadest places, half an inch in 

 breadth, and nearly a quarter of an inch in thickness — evidently 

 made so substantial to resist external injury; whereas its poste- 

 rior or unexposed parts grow suddenly thin and yielding, and taper 

 to the extremities ; which instead of meeting and uniting, pass 

 one over the other, and thus form a shield of defence behind, while 

 they admit of a certain dilatation and contraction of the internal 

 dimensions of the tube. These attenuated ends are joined toge- 

 ther by a ligamentous expansion, mingled with a quantity of cel- 

 lular membrane. The rings are likewise attached to one another 

 by narrow ligamentary bands, strong and elastic ; which after they 

 have been drawn apart in certain positions of the head and neck, 

 have the power to approximate them : when the pipe is removed 

 from the body, and suspended by the uppermost ring, these liga- 

 ments counteract the tendency its weight has to separate the rings, 

 and still maintain them in apposition. The lowermost ten or 

 twelve pieces of cartilage appear on examination but ill to deserve 

 the name of rings ; indeed they are little more than semi-annular, 

 the deficiencies in them behind being made good by intermediate 

 moveable pieces of cartilage. These pieces, whose breadth in- 

 creases as we descend, are let into the vacuities in such manner 

 as to overlap the terminations of the segments, and they are con- 

 fined and concealed by the same sort of ligamentary and cellular 

 investment as was before noticed. 



Muscle. — Where the outward extremity of the ring suddenly 

 turns inward and degenerates into a thin flexible flap on either side, 

 a band of muscular fibres is fixed and stretched across the canal, 

 dividing it into two unequal semi-elliptical passages: — the ante- 

 rior one is the proper air channel ; the posterior or smaller one is 

 filled with a fine reticular membrane connecting the band to the 

 posterior part of the ring, and preventing it, in action, from en- 

 croaching upon the main conduit. This self-acting band appears 

 to me to have been added to the tube to enable it to enlarge its 



* Now and then we find, at the upper part of the tube, two or three or 

 more of these rings accreted together: it gives rise to some prominence 

 thereabouts generally, and may often be detected by taction in the living 

 animal. 



