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 cesophagus 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. 
