THE BRONCHIAL TUBES. 
509 
I think that from this connexion between the two glands, a 
glimmering of light breaks upon us; and that we have, in the 
later period of foetal life, when alone the thymus gland is fully 
developed, the beginning of the first essay at the preparation for 
those important functions in which the stomach and bowels are 
to be afterwards employed. 
The Bronchial Tabes. 
We had in our last lecture followed the trachea through the 
duplicatures of the mediastinum to the base of the heart, and 
there, beneath the curvature of the posterior aorta, it separated 
into two tubes, corresponding with the two divisions of the 
lungs; these are the bronchial tubes. The right is somewhat 
larger than the left, because, as we shall presently see, the right 
lung is larger than the left. These main trunks plunge deep into 
the body and substance of the lungs. They presently subdi¬ 
vide, and the subdivision is continued in every direction, until 
branches from the trachea penetrate every assignable portion 
and part of the lungs. These are still air-passages, they are 
carrying on this fluid to its destination for the accomplishment of 
a vital purpose. 
Their Construction .—It is still necessary that they should be 
pervious, or that their calibre should not be long obliterated. 
They are now exposed to a new pressure; a pressure alternately 
applied and removed,—to which they are compelled to yield, but 
against which they must, to a certain extent, struggle, and in 
consequence of that struggle regain their former calibre the mo¬ 
ment the pressure is removed. The lungs in which they arc 
embedded alternately contract and expand, and these tubes must 
contract and expand again with the lungs. The perfect carti¬ 
laginous ring of the trachea would not be sufficiently yielding, 
or fold up sufficiently close. Yet there must be resistance and 
elasticity. We have it, and we preserve the cartilage while we 
have it. The ring remains, but it is divided into five or six seg- 
ments connected together. The ligamentous muscular band 
which connected each ring with its neighbours is no longer 
found; there is, however, a thin, yet strong and elastic substance. 
The lungs being compressed, the rings yield, and the segments 
overlap each other and they fold up and occupy little space; but 
elasticity, although latent, is still at work, and as the pressure is 
removed they start again and resume their previous form and 
calibre. It is a beautiful contrivance, equalled only by the struc¬ 
ture of the trachea; and both exquisitely adapted to the situation 
in which they are placed, and the functions they have to 
discharge. 
We shall have reason, however, to suspect, by and by, 
