ENDOTHELIUM AND EPITHELIAL TISSUES. 63 



mous type which have become impregnated with melanin, a dark 

 pigment that gives coloration to the structure. The pigmeiited 

 epithelium of the retina is the best illustration. 



Transitional epithelium. This occurs in the urinary tract and is 

 illustrated by modifications of squamous and columnar cells, where 

 the one kind merges into the other. It occurs in the pelvis of the 

 kidney, ureter, bladder, and urethra. The cells are round, spindle- 

 shaped, cuboidal, or pear-shaped, and often exhibit one or more 

 slender processes. 



Specialized epithelium. This form of epithelial tissue consists 

 of cells so specialized as to engage in the elaboration of secretions ; 

 or to perform some special function. There are two varieties 

 glandular epithelium and neuro-epithelium. 



Glandular epithelium. The terms cuboidal and secretory also 

 apply to this tissue, the former arising from the general shape of 

 the cells, the latter from their functional character. It occurs in 

 the intestinal, gastric, and salivary glands, the pancreas, and liver. 



Neuro-epitlielium. This comprises highly specialized cells which 

 aid in nerve sensation. They are found at the terminations of the 

 nerves of special sense. The cells are generally elongated and con- 

 tain an inner nuclear part and an outer part directed toward the 

 periphery, which is often provided with hair-like processes. The 

 rods and cones of the retina and the olfactory and taste cells are 

 illustrations. 



The following ten characteristics of epithelial cells should be 

 carefully considered: (1) The cells are superficially disposed; (2) 

 they are united by cement; (3) they contain no blood-ves- 

 sel? : (4) they vary greatly in shape ; (5) they perform various func- 

 tions, those of protection and secretion being the more common; 

 (Cl) the cells multiply by karyokinesis ; (7) they are nourished by 

 absorption; (8) they have eccentric nuclei; (9) they rest upon a 

 ba?emenl membrane, or membrana propria; (10) the cells contain 

 mucin, melanin, etc. It should be borne in mind that all these char- 

 acteristics are not universally present. 



Laboratory exercise No. 16. Study of epithelial cells. Collect upon 

 the end of the tongue a quantity of saliva and apply the same to the 

 center of a slide. Cover and examine with high power. Observe the 



