TRANSFORMATION OF EPITHELIUM. 519 
Note on the Transformation of Ciliated into 
Stratified Squamous Epithelium as a Result 
of the Application of Friction. 
= DY 
John Berry Haycraft, M.D., D.Sc., 
and 
E. W. Carlier, M.B., B.Sc., 
Physiological Laboratory, University of Edinburgh. 
With part of Plate XXXIII. 
In man, in the rabbit, and in some other animals, the trachea, 
as is well known, is built up of a series of cartilaginous rings, 
which are incomplete behind. Posteriorly, where the cartilage 
is wanting, involuntary muscular fibres are found. These are 
placed transversely, and form what is called the trachealis muscle, 
which is placed outside the cartilage. The mucous membrane 
forms a smooth cylindrical lining to the cartilaginous walls of 
the trachea, and is covered by ciliated epithelium. In the cat 
and the dog the rings of cartilage overlap posteriorly for a con- 
siderable distance, and the trachealis muscle, which is much 
developed in these animals, and placed outside the cartilage, 
evidently exerts a powerful action in varying the diameter of 
the tube. 
When the trachealis contracts the cartilages overlap to a 
considerable extent, the extremities which are over-ridden 
projecting into the cavity of the trachea. The projecting 
ends form a vertical ridge which can readily be seen on slitting 
open the trachea. This ridge is separated by a deep groove 
from the part of the wall formed by the over-riding cartilages. 
The mucous membrane lining the cavity of the trachea passes in 
between the overlapping ends of the cartilages into the groove, 
and in this situation it is subject to a certain amount of 
friction whenever the trachealis contracts or relaxes (fig. 5). 
