488 
MR. YOUATT S VETERINARY LECTURES. 
suddenly alters its form to adapt itself to the narrow triangular 
aperture through which it is to pass. It becomes a narrower 
ellipse. When it has entered the chest, it preserves the same 
cartilaginous structure; for if it has not the pressure of the ex¬ 
ternal muscles or of accidental violence to resist, it is exposed to 
the pressure of the lungs in the act of inflation, and, like every 
other thoracic viscus, it shares in the pressure of the diaphragm 
and the intercostals in the act of expiration. Having entered the 
chest, it passes, as you will see, a little to the right, leaving the 
oesophagus on the left. It gradually separates from the dorsal 
vertebra, with which it became connected when it had left the 
neck ; it passes through the duplicature of the mediastinum to 
the base of the heart; and, beneath the curvature of the posterior 
aorta, it divides. The divisions are the bronchial tubes. 
When the trachea has entered the thorax, and begins to de¬ 
tach itself from the dorsal vertebrae, a provision is made to supply 
that support which it had previously received from the bones of 
the neck and spine. The rings are as perfect as before, and are 
continued round the trachea, and overlap each other, and are 
connected together by dense ligamentous substance; but a new 
piece of cartilage now begins to present itself; it first appears 
behind, or rather above, the tenth ring from the bottom—it 
spreads over the union between the posterior ends of the rings, 
and is attached to a portion of these rings by the same kind of 
dense cellular ligamentous substance, extending a little ante¬ 
riorly on each side ; it holds them in closer and firmer con- 
tact with each other; it stands in the stead of both the 
transverse muscle which here begins to disappear, and the 
support of the cervical and dorsal vertebrae ; and it prevents 
separation of the rings, when the trachea is distended by an 
additional quantity of air during violent exercise. It spreads 
down upon and defends the commencement of the bronchial 
tubes. There are beside three smaller plates of cartilage, not only 
covering the bifurcation of the trachea, but reaching a con¬ 
siderable way down the two main divisions of the bronchiae. 
Anteriorly, or now interiorly, the last ring has a central triangular 
projection, which also covers and defends the bifurcation of the 
trachea. 
The Lining Membrane .—The membrane lining the trachea is 
a continuation of that of the larynx. It is paler than that of the 
larynx, and does not possess its acute sensibility; yet inflam¬ 
mation of it, or the presence of foreign bodies, is sometimes pain¬ 
ful enough. It is covered by numerous follicular glands which 
secrete a thick but not very abundant fluid. That portion of it 
which covers the transverse muscle is comparatively loosely at- 
