30 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



four layers of polyhedral cells. This type of epithelium occurs in the bladder, 

 ureter, and pelvis of the kidney. 



Specialized Epithelium. Glandular epithelium forms the active secreting 

 agent in the glands; the cells are usually spheroidal, but may be polyhedral 

 from mutual pressure, or even columnar; their protoplasm is generally oc- 

 cupied by the materials which the gland secretes. Examples, of glandular 



FIG. 29. Stratified Columnar Ciliated Epithelium from the Human Trachea. 



(goblet) cell also is present. 



A mucous 



epithelium are to be found in the liver, figure 31, in the secreting tubes of 

 the kidney, and in the salivary, figure 32, and gastric glands. 



Ciliated epithelium consists of cells which are generally cylindrical in form, 

 figures 29, 30, but may be spheroidal or even squamous. 



This form of epithelium lines: a. The mucous membrane of the respira- 

 tory tract beginning just beyond the nasal aperture, and completely covers 

 the nasal passages, except the upper part to which the olfactory nerve is 



. 



- 



FIG. 30. Transitional Epithelium from the Human Bladder. (Bailey.) 



distributed, and also the sinuses and ducts in connection with it and the 

 lachrymal sac, the upper surface of the soft palate and the naso-pharynx, 

 the Eustachian tube and tympanum, the larynx, except over the vocal cords, 

 to the finest subdivisions of the bronchi. In part of this tract, however, 

 the epithelium is in several layers, of which only the most superficial is ciliated, 



