148 



ANATOMY AND PHYSIOLOGY OF 



quarter of an inch in thickness — evidently 

 made so substantial to resist external in- 

 jury ; whereas its posterior 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 dila- 

 tation and contraction of the internal 

 dimensions of the tube. These attenuated 

 ends are joined together by a ligamentous 

 expansion, mingled with a quantity of cel- 

 lular membrane. The rings are likewise 

 attached to one another by narrow ligamen- 

 tary bands, sti'ong and elastic ; which, after 

 they have been drawn apart in certain posi- 

 tions of the head and neck, have the power 

 to approximate them ; when the pipe is re- 

 moved from the body, and suspended by 

 the uppermost ring, these ligaments coun- 

 teract the tendency its weight has to sepa- 

 rate 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 

 deficiences 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 

 confined 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 de- 

 generates 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 

 anterior 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 

 i in action, from encroaching upon the main 

 conduit. This self-acting band appears to 

 me to have been added to the tube to enable 

 it to enlarg^e its caliber — not to diminish it, 

 as a superficial view of these parts might 



lead one to imagine ; for, in consequence of 

 the passage being naturally elliptical, and 

 the muscle being extended across its long 

 diameter, the contraction of its sides will 

 give the tube a circular figure, by increasing 

 the curvature of the ring anteriorly, and 

 thereby, in effect, will expand and not con- 

 tract the caliber of the canal. I would say, 

 then, that the trachea was made muscular 

 in order that it might have the power of in- 

 creasing its capacity for the passage of air, 

 whenever the lungs were called into extra- 

 ordinary action : in addition to which, I 

 think that this band may, in some degree, 

 counteract any tendency certain positions of 

 the head and neck have to alter its shape 

 and diminish its circumference. This 

 opinion is corroborated by the circumstance, 

 that the muscle grows slender and pale as 

 we approach the lower end of the pipe, 

 where the canal itself is nearly circular, and 

 where it is placed in the least moveable 

 part of the neck.* 



Me^nbrane. — The trachea is lined by a 

 soft, pale red membrane, which anteriorly 

 has a close adhesion to the rings them- 

 selves, and presents a smooth, polished in- 

 ternal surface ; but which, posteriorly, is 

 loosely attached to the muscular band, and 

 puckered into fourteen or fifteen longitu- 

 dinal plicw or folds, that extend with regu- 

 larity from one end of the tube to the 

 other. These folds were evidently made to 

 allow of the contraction and elongation of 

 this muscular band ; for I cannot myself 

 assign any reason why they should exist in 

 its relaxed state, unless this fulness of 

 membrane be given to admit of enlarge- 

 naent of the caliber of the tube during the 

 contractions of that muscle ; if this be 

 plausible, I may adduce the corrugation of 

 the membrane as another proof that the 

 caliber of the trachea is susceptible of aug- 

 mentation. This membrane is continuous 

 with that which clothes the rima glottidis ; 



* In this opinion, sajs Mr. Pcrcivall, I find I am at 

 variance with Girard. The French professor ascribes to 

 it the power of cont ractim/ the caliber of the trachea. " Cette 

 couche, bien evidenimcnt musculeuse, pent retrecir le cal- 

 ibre de la trachee, en rapportant les extremites des seg- 

 mens." — Anat. F£^,p.l46 et 147, torn. ii. 



