RESPIRATORY SYSTEM. 225 



caliber — not to tliuiinish 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 contract the caliber 

 of the canal. I would say, then, that the trachea was made 

 muscular in order that it might have the power of increasing its 

 capacity for the passage of air, whenever the lungs were called 

 into extraordinary action : in addition to which, I think, that this 

 band may, in some degree, counteract any tendency certain posi- 

 tions of the head and neck have to alter its shape and diminish 

 its circumference. This opinion is corroborated by the circum- 

 stance, 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*. 



Membrane. — The trachea is lined by a soft, pale red membrane, 

 which, anteriorly, has a close adhesion to the rings themselves, and 

 presents a smooth polished internal surface; but which, poste- 

 riorly, is loosely attached to the muscular band, and puckered into 

 fourteen or fifteen longitudinal pliccB or folds, that extend with re- 

 gularity 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 enlargement of the caliber of the tube during 

 the contractions of that muscle : if this be plausible, I may ad- 

 duce the corrugation of the membrane as another proof that the 

 caliber of the trachea is susceptible of augmentation. This mem- 

 brane is continuous with that which clothes the rima glottidis ; 

 but it is paler than it, and not near so sensitive. Its arterial rami- 

 fications, also less abundant than upon the glottis, exhale a vapour 

 from its surface; independently of which, it is kept continually 

 lubricated by mucus, furnished from its numerous lacinia:, to de- 

 fend it from any thing acrimonious that may be contained in the 

 breath. 



BRONCHIAL TUBES.— The trachea having entered the 

 thorax, bifurcates into the two bronchial tubes:— of them, the 

 right is the more capacious canal, on account of having com- 

 munication with the larger division of the lungs ; the left the 



* In this opinion I find I am at variance witliGiRARD. Tiie French pro- 

 fessor ascribes to it the power of contracting the caliber of the trachea. "Cette 

 couche, bien evidemment inusculeuse, pent retrecir le calibre de la trachee, 

 en rapportant les extremites des segmens." Anat. Vet., p. 146 et 147, 

 torn. ii. 



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