Ixxiv 



cartilaginous, compressible, and elastic; they are 104 in num- 

 ber in the female, (I did not count them in the male speci- 

 men) ; sixty-four are above the tracheal opening, six corres- 

 pond to the open portion of the tube, and there are thirty-four 

 between this and the division into the right and left bronchi ; 

 in each of the latter are six semicircular cartilages, and three 

 rudimental fragments ; beyond these, the air-tubes abruptly 

 become membranous and muscular, and no cartilages are con- 

 tinued into the lungs. The form of the tracheal tube is 

 somewhat transversely elliptical, but is indented posteriorly 

 through its whole extent ; that is, each ring is curved poste- 

 riorly, so as to be convex towards the canal, and concave to- 

 wards the spine ; this general indentation is increased by the 

 slightest pressure ; the oesophagus is closely connected to it, 

 and when distended, during the deglutition of any large sub- 

 stance, would appear to derive some accommodation from 

 this structure. This posterior indentation is much increased in 

 depth opposite the tracheal opening, and thus accounts for 

 the corresponding vertical prominence internally already de- 

 scribed ; in this particular situation, the cartilages are also 

 somewhat differently modified, as we shall notice presently. 

 A yellowish elastic structure extends the whole length of this 

 posterior depression on the trachea (fig. 5) ; this increases 

 in strength inferiorly, and terminates at the division into the 

 two bronchi ; beneath this elastic ligament, opposite the tra- 

 cheal opening, and a little above and below it, short, but 

 strong, transverse muscular bands pass across the groove, and 

 are inserted into the cartilages at either side (fig. 5) ; these 

 fibres, by approximating this attachment, will tend to pre- 

 serve, and even protrude the keel-like projection within, and 

 thus enable it the better to support the approximated ends of 

 the lateral cartilages in front ; the longitudinal elastic liga- 

 mentary tissue admits of the extension of the trachea, and can 

 restore it to its state of rest, on the subsidence of the extend- 

 ing force. 



