li 
Ixxv 
The tracheal cartilages present some difference in size and 
form in different portions of the tube; their diameter gradu- 
ally, but. slightly, increases from above downwards, and a 
very trifling swell is observable in the region of the opening. 
The upper and middle rings present flattened surfaces, and their _ 
edges, above and below, are connected by elastic membrane; 
those below the opening, though forming larger circles, are 
small, round, and weak, and have an oblique direction down- 
wards and forwards; this portion of the tube is very elastic; 
the four or five last cartilages are deficient posteriorly, in a 
narrow, angular interstice, which is closed by lining mem- 
brane, weak muscular fibres, and elastic tissue; from the last 
ring no internal ridge or bar proceeds, but from the angle be- 
tween the bronchi the mucous membrane rises mesially in a 
prominent falx-like fold, which is increased or diminished in 
depth and tension in proportion as the trachea and bronchial 
tubes are shortened and contracted, or elongated and distended ; 
the bronchial cartilages are little more than semicircles, their 
extremities being somewhat thick and expanded ; the two su- 
perior present, externally and posteriorly, small projecting 
processes, into which some fibres from the long cervical mus- 
cles are inserted ; the posterior wall of each bronchus is mus- 
eulo-membranous; although there is no inferior larynx, yet 
each bronchial opening,—which is of an oval form, the long 
axis from before backwards, or, during life, from above down- 
wards,—is capable of great alteration in size, figure, and ten- 
sion of its lateral boundaries; these changes can be effected 
by the varying degrees of inspiration and expiration, by exten- 
sion and flexion of the neck, and also by the action of the, 
tracheal muscles. 
The six cartilages which are concerned in. the tracheal 
opening, deserve particular notice; they are peculiarly elastic, 
and appear composed, each, of two symmetrical portions, one 
on either side; each lateral portion is crescentie (fig. 6); 
the anterior cornu, flat, thin, and yery moveable, is enveloped 
, 
: 
, 
