520 JOHN BERRY HAYCRAFT AND E. W. CARLIER. 
The mucous membrane at sc will be apt to come in contact 
with that lining the opposite side of the groove p. These 
surfaces will rub against each other, to some slight extent at 
least. In the more usual condition, in which the trachealis is 
not contracted to its utmost, the mucous membrane at Bc will 
form the sides of the groove, and the part at B will rub against 
that at c. It is obvious that the greatest amount of friction 
will take place between B and c. 
The interior of the trachea is lined by a stratified and ciliated 
epithelium placed upon a bed of connective tissue (fig. 7). Super- 
ficially a layer of long columnar cells is found, each cell ter- 
minating in a tuft of cilia; below these are cells less elongated 
in shape, and below these again are rounded cells. 
If the epithelium be traced from p round the groove to B, 
we see that in the region p, where the friction will only occa- 
sionally occur, the epithelium is still columnar in character 
but devoid of cilia; as we near c, however, where the friction 
is more common, the cells become sub-columnar (fig. 8), till 
when c is reached they are transitional in character, like those 
of the urinary bladder. In this region the superficial cells are 
somewhat elongated in shape and placed in a direction parallel 
with the surface, while the lower ones are vertical. 
At s, where the friction is greatest, genuine stratified 
squamous epithelium, similar to that lining the buccal cavity, 
is seen (fig. 9). 
Here the superficial cells form a badly differentiated horny 
layer. They are flattened out, forming horizontal squames ; 
their nuclei are well marked and stain fairly deeply. Below 
these are large rounded cells with larger nuclei; and below 
these, and resting on the subjacent connective tissue, are two 
or three layers of smaller cells arranged vertically. 
It is seen, then, that in this part of the trachea we have a form 
of epithelium found in other situations covering surfaces or lining 
cavities exposed to friction, and that in this situation, in the 
trachea, friction between the mucous surfaces is found to occur. 
No two sections give quite the same appearances; in one, 
a portion near the bottom of the groove is transitional, while 
