STKl( TIKE OF THE ELEMENTARY TISSUES. 27 



der be teazed, and examined under the microscope, cells of a great variety 

 of forms may be made out (Fig. 23). Each cell contains a large nucleus, 

 and the larger and superficial cells often possess two. 



Special Epithelium in Organs of Special Sense. In addition 

 to -the above kinds of epithelium, certain highly specialized forms of epi- 

 thelial cells are found in the organs of smell, sight, 'and hearing, viz., 



FIG. 22. FIG. 23. 



FIG. 22. Epithelium of the bladder; a, one of the cells of the first row; 6, a cell of the second 

 row: c, cells in situ, of first, second, and deepest layers. (Obersteiner.) 



FIG. 23. Transitional epithelial cells from a scraping of the mucous membrane of the bladder of 

 the rabbit. (V. D. Harris.) 



olfactory cells, retinal rods and cones, auditory cells; they will be de- 

 scribed in the chapters which deal with their functions. 



Functions of Epithelium. According to function, epithelial cells 

 may be classified as: 



(1.) Protective, e.g., in the skin, mouth, blood-vessels, etc. 



(2.) Protective and moving ciliated epithelium. 



(3.) Secreting glandular epithelium; or, Secreting formed elements 

 epithelium of testicle secreting spermatozoa. 



(4.) Protective and secreting, e.g., epithelium of intestine. 



(5.) Sensorial, e.g., olfactory cells, rods and cones of retina, organ of 

 Corti. 



Epithelium forms a continuous smooth investment over the whole 

 body, being thickened into a hard, horny tissue at the points most ex- 

 posed to pressure, and developing various appendages, such as hairs and 

 nails, whose structure and functions will be considered in a future chapter. 

 Epithelium lines also the sensorial surfaces of the eye, ear, nose, and 

 mouth, and thus serves as the medium through which all impressions 

 from the external world touch, smell, taste, sight, hearing reach the 

 delicate nerve-endings, whence they are conveyed to the brain. 



The ciliated epithelium which lines the air-passages serves not only 

 as a protective investment, but also by the movements of its cilia is en- 

 abled to propel fluids and minute particles of solid matter so as to aid 

 their expulsion from the body. In the case of the Fallopian tube, this 



