54 NORMAL HISTOLOGY. 



and the adjoining bony cavities, the mucous membrane of the uterus 

 and the Fallopian tubes, the vasa efferentia of the testis and a part 

 of the epididymus, the ventricles of the brain (except the fifth), the 

 central canal of the spinal cord, and the ducts of some glands. 



The possession of cilia, which are very motile organs, presents 

 a marked departure in specialization from the usual metabolic func- 

 tions of epithelium. Ciliated epithelium rarely exercises a secretory 

 function, its stock of energy being utilized to produce motion instead 

 of chemical change. But there are secreting varieties of epithelium 

 possessing a " cuticle " which appears to be morphologically anal- 

 ogous to the cilia, but in which the fibrils are less highly developed, 

 probably not motile, and, therefore, functionally not the equiva- 



FJG. 37. 



Cuticularized epithelium, intestine of dog. (Paneth.) Rodded cuticle of the free ends of 

 columnar cells. In most specimens of ciliated epithelium from human tissues, where no 

 special care has been taken to preserve the cilia, the ciliated border presents the appear- 

 ances shown in Fig. 37. 



lents of cilia. This cuticle is highly developed in the cells cover- 

 ing the mucous membrane of the intestine (Fig. 37). 



5. Stratified Epithelium. In the varieties of epithelium hitherto 

 considered the cells are, in the main, disposed upon some surface 

 in a single layer, some, at least, of the cells usually extending from 

 the bottom of the layer to its surface. 



Stratified epithelium is distinguished from these by being of 

 greater depth and consisting of several layers of cells. The epithe- 

 lium lining the cheek or the oesophagus may be taken as a typical 

 example of this variety. 



The most deeply situated cells are small and nearly filled by the 

 round or oval nucleus. They undergo frequent division, and as 

 they multiply some of them are crowded toward the surface. For 

 a time these increase in size through a growth of their cytoplasm. 

 But as they are pushed nearer to the surface and farther from the 

 sources of nutrition in the vascular tissues underlying the epithe- 

 lium, they become flattened and their bodies lose their cytoplasmic 

 character, being converted into a dry, horny substance, keratin. 



