HISTOLOGY 



141 



cells are longer than they are wide and are arranged side by side 

 (Fig. 77 D). Stratified columnar epithelium is not abundant, but a 

 modified type is found lining the trachea (Fig. 77 E). Columnar 

 epithelial cells are sometimes modified by having their free sur- 

 faces, that is, the surfaces exposed to the cavity that they line, 

 covered with cilia, which are fine, hair-like cj^oplasmic processes 



Fig. 77. — Epithelial tissues. A. Simple squumous epithelium from human 

 mouth. B. Simple squamous epithelium (endothelium) from peritoneum. 

 C. Stratified squamous epithelium from lining of nasal canal. D. Simple 

 columnar epithelium from mucous membrane of digestive tract. E. Pseudo- 

 stratified ciliated columnar epithelium from lining of trachea. One cell is 

 shown secreting a drop of mucus. F. Simple cihated columnar epithelium. 

 G. Glandular epithelium from the pancreas. //. Goblet cell with drop of 



mucus. 



(A, B, and C from drawings by D. F. Robertson.) 



(Fig. 77 F). The cilia are vibratile and, by their motion, act to keep 

 the surfaces clean. The epithelium of the air passages in higher 

 vertebrates and the roof of the frog's mouth offer examples of this 

 variation. 



In the retina and in other places, epithelial cells are sometimes 

 characterized by pigment granules and known as pigmented epi- 



