510 MR. youatt’s veterinary lectures. 
that the bronchial tubes are not quite destitute of muscular 
apparatus and power, partly under the control of the will, but 
more connected with the energies of organic life. 
Number of Segments .—In the larger divisions of the bronchias 
we can trace in each ring five or six of these segments. As the 
tubes decrease in calibre the cartilage is thinner and the segments 
are fewer. Here are plainly five; here we can trace only three. A 
little farther on, and we can perceive from the yielding nature of 
the thin cartilaginous ring, that it is divided into segments, but 
we cannot clearly ascertain the number, certainly not more than 
three: a little farther and the cartilage can no longer be traced, 
but the impression remains on the tube; it is there, but so thin 
and flexible as not to be distinguished from the lining mem¬ 
brane. 
Termination of the Bronchia .—And so we trace the bronchiae 
until they almost elude our sight: yet we can pretty satisfactorily 
follow any individual ramification until it arrives at one of the 
lobuli into which the substance of the lung is divided. We are 
sure that it enters there, otherwise the function of respiration 
could not be performed, but we have no ocular demonstration of 
the fact, nor of the real termination of these air-passages. There 
are some very pretty theories about this, and to a certain extent 
they probably are true, but we have no demonstration ; and 
therefore, gentlemen, at the fragile membrane which covers the 
lobuli, we will, if you please, leave the bronchial tubes. 
I should state that so far as we can fairly trace these tubes 
they are lined by a continuation of the membrane of the trachea. 
This membrane is subject to inflammation, which I shall here¬ 
after describe. 
Bloodvessels .—The bronchial artery rises from the posterior 
aorta and from the posterior face of the curvature of that vessel, 
by the side of the oesophagean artery, and sometimes from that 
artery itself. It advances in a serpentine direction towards the 
left bronchia, and there it divides and sends a nearly equal 
branch to the right and to the left. These arteries penetrate 
into the substance of the lungs, and ramify on the bronchial 
tubes through the whole of their course. The bronchial vein 
follows the course of the artery through the lungs, and pours 
itself into the vena azygos at its curvature, and close to the 
oesophagean vein. Sometimes these veins unite before falling 
into the azygos. 
Before I proceed to the structure of the lungs generally, and 
particularly of these lobules at which I have arrived, I must 
look around me and consider the cavity into which I have 
entered. The structure and contents of the thorax will form the 
subject of our next lecture. 
