EPITHELIUM. 



271 



fattened tlian tliose of the deeper layers, which are more spher- 

 ical in form. They contain protoplasm, a nucleus, and a number 

 of granules. The superficial cells are evontually cast off, the 

 deeper ones taking their place, and the latter become more and 

 more flattened as they approach the surface. Consisting thus of 

 layers or strata of cells, the name stratified epithelium has been 

 applied to this tissue; it is found in the alimentary mucous 

 membrane, from tLe mouth to the first half of the stomach, in 

 the bladder, the rectum, the entrances to the nose, the eye, 

 urethra, and vagina, and in a dried and desquamating form it 

 constitutes the epidermis or scarf skin ; it is non- vascular, and 

 for the most part non-sensitive. 



Endothelium. — This resembles epithelium in structure, differ- 

 ing in its situation, as above described, and in the fact that it is 

 arranged as a single layer of flattened cells. 



Columnar or cylindrical epithelium. — In this variety the cells 

 are cylindrical in form, and arranged side by side, with their long 

 axes perpendicular to the subjacent surface. Their contents 

 resemble those of the squamous kind, but the nucleus is more 

 distinct. This form of epithelium is found on the mucous 

 membrane of the intestines and pyloric half of the stomach, and 

 it is continued into the ducts of the glands which open on the 

 surface of the membrane. The cells covering the villi of the 

 intestine have expanded free extremities, which may appear 

 slightly striped. 



Spheroidal or Polyhedral epithelium is found in the follicles 

 and ducts of glands, hence the name glandular epithelium. The 

 cells of this variety are spheroidal, or it may be irregularly 

 polyhedral in shape. A variety known as transitorial occurs in 

 ihe mucous membrane of the 

 urinary bladder, the cells being 

 transitional in form between 

 those of columnar and scaly 

 iinds, and arranged in a small 

 number of layers. 



Ciliated epithelium consists 

 of columnar cells, provided at 

 their free extremities with delicate hair-like processes or cilia, 

 which wave to and fro in a marked manner. The motion is 

 apparently independent of the direct influence of the nervous 

 system, since it is observable for some time after the death of 



ciliated epithelium, highly magnified. 

 a, Nucleated cells ; 6. Cilia. 



