THE TRACHEA. 



1835 



Tunica propria 



.Submucous 

 layer 



second pieces of the sternum, where it divides into the two bronchi. The point of 

 division is usually on the right of the median line : sometimes so far as to lie behind 

 the right edge of the sternum. The trachea is a cylindrical tube, flattened behind. 

 The convexity is due to the so-called rings, which represent only about three-quarters 

 of a circle. The length is difficult to determine with accuracy on account of the elas- 

 ticity of the organ as well as of its variation. It may be said to be, on the average, 

 from 10.5-12 cm. (44^ in.) in man and from 9-11 cm. (3^-4^2 in.) in woman. 

 The isolated trachea can be stretched and compressed to a surprising extent, and 

 even in life the changes are considerable. The antero-posterior and the transverse 

 diameters are not very different, except just at the lower end, where the trachea 

 enlarges transversely. It is very plausibly stated by Lejars l that in life the windpipe 

 is more or less constricted by the 



FIG. 1559. 



.Epithelium 



tonic contraction of its muscles. 

 According to him, it grows con- 

 tinually smaller from above down- 

 ward. Braune and Stahel 2 be- 

 lieved that after death it is largest 

 in the middle. We have no doubt 

 whatever that, as a rule, the dead 

 trachea is enlarged transversely 

 at the lower end. Abey 3 gives 

 the following measurements for 

 the upper and lower ends : upper 

 transverse diameter 13.1 mm., 

 sagittal 1 6 mm. ; lower transverse 

 diameter 20.7 mm., sagittal 19.1 

 mm. The framework of the 

 trachea is so light that its shape 

 may be influenced by neighbor- 

 ing organs, such as the thyroid 

 body and the arch of the aorta. 

 Structure. The frame- 

 work of the anterior and lateral 

 walls of the trachea consists of 

 the so-called rings of hyaline car- 

 tilage, which form some three- 

 quarters of a circle. In the great 

 majority of cases there are from 

 sixteen to nineteen rings. It is 

 not rare to find twenty, but very 

 rare to find more. The rings are 

 from 25 mm. broad, usually 



measuring 3 or 4 mm. They are Transvere ; section of trachea , showing general arrangeme nt 

 plane externally and convex in- of its wall, x 80. 



ternally, becoming pointed at the 



ends. They are very irregular in many respects. Sometimes one end bifurcates, 

 the rings above and below ending prematurely. Occasionally bifurcation of the oppo- 

 site ends of alternate rings is observed. Rarely both ends of the same ring may 

 divide. The first ring, which is broader than the others, is occasionally fused with 

 the cricoid cartilage. A highly elastic fibrous sheath, continuous with the peri- 

 chondrium of the rings, envelops them, connects their posterior ends, and completes 

 the tube. The distance between the rings is less than their breadth, at times only 

 half as much. Involuntary muscular fibres of the trachcalis muscle lie between the 

 fibrous sheath and the lining mucous membrane. They are in the main disposed 

 transversely, some of them connecting the ends of the rings ; some bundles, however, 

 run longitudinally. 



Perichondrium 



1 Revue de Chirurgie, 1891. 



2 Archiv f. Anat. u. Phys., Anat. Abth., 1886. 



3 Der Bronchialbaum der Menschen, u. s. w., 1880. 



