40 THE EPITHELIAL TISSUES 



epithelia. This is especially pronounced in the case of the epidermis 

 of the skin, which protects underlying tissues and still retains a 

 sensitiveness to external stimuli. If the upper superficial scaly layer 

 of dead cells continue to accumulate to considerable thicknesses, 

 there is an increased protection with an accompanying loss of 

 general sensitivity. This actually occurs in the callosities localized 

 at points of constant friction and pressure. The maintenance of a 

 protective layer of optimum thickness despite the wearing away of 

 the superficial layers is made possible through the continual replace- 

 ment of cells from the basal layers, where they are being produced 

 through normal mitotic activity. A lesion in this type of epithelium 

 is quickly repaired through the activity of these basal cell lasers. 

 P^pithelia of other types in other locations are also subject to con- 

 tact with substances causing gradual or abrupt destruction and loss 

 of cells. In the alimentary tract, dead cells of the columnar type are 

 replaced by mitotic activity of less differentiated cells intercalated 

 among them. In many places mitotic activity is rarely observed 

 under normal conditions. 



In healing of surfaces from which the epithelimn has been lost 

 through injury the newly formed cells of the bordering region appar- 

 ently migrate by a sliding movement to form a covering layer 

 prior to the regeneration of the tissue characteristic of the injured 

 region. Experiments have also shown changes in type in the case 

 of the pseudostratified ciliated epithelium of the trachea of the cat, 

 which, after repeated treatments with a formalin solution, became 

 a stratified squamous membrane. In the embryo cat the esophagus is 

 lined with a ciliated columnar epithelium, but in the adult this 

 region has a lining of stratified squamous, the type commonly 

 found in places subjected to external wear. 



SECRETORY EPITHELIAL CELLS AND GLANDULAR 

 ORGANIZATIONS. 



Probably all epithelial cells to some extent elaborate active 

 substances through biochemical processes of their protoplasm; 

 these processes being collectively spoken of as secretion. Although 

 protection and absorption are the major tasks of some epithelial 

 cells, there are others primarily concerned with this process of 

 secretion. In those cells active in secretion there occurs an accu- 

 mulation of very small secretion granules in the cytoplasm. The 

 origin of these granules is debated, but they appear first in the basal 



