THE RESPIRATORY SYSTEM 141 
Ungulata, they have only been seen once within the 
tracheal tube. 
The Bronchi. 
The bronchi will be discussed as a separate part of the 
respiratory system in so far as possible since they present 
a very decided difference in anatomy between mammals 
and birds and because the incidence of their disease is 
other than mil be found for the lungs. However, distinc- 
tion has been made between changes in the grosser tubes 
and those in the finer bronchioles, especially because 
capillary bronchitis so-called is really a pneumonitis in 
which the mucosa of the larger passages need not par- 
ticipate. The mammalian tubes are not really greatly dif- 
ferent in their construction, passing through ever smaller 
branchings which give an increasing square area of tube 
capacity and more extensive mucous surface. The avian 
main bronchus breaks up very shortly after entering the 
lung into a varying number of spaces lined with low 
epithelium lying upon a fibrous support and without 
cartilage. These- spaces then open into secondary air 
spaces of a size visible to the naked eye which are in turn 
surrounded by microscopic alveoli. The largest spaces, 
first mentioned, continue to grow smaller toward the 
lower part of the lung where they usually communicate 
with one or other of the various ostia of air sacs. Bron- 
chial diseases in birds must therefore be limited at the 
place where the bronchi lose the cartilaginous rings since 
below tliis the surface functionates as pulmonary tissue. 
Bronchitis. 
The accompanying list. Table 8, will show the distri- 
bution of bronchitis not accompanying pneumonia or due 
to mycosis. It is striking that carnivorous animals are 
more prone to bronchitis than any other order (the 
struthious birds are too few to be important). There is a 
very decided preponderance of mammalian cases over 
avian, there being not only more cases but proportion- 
