SQUAMOUS EPITHELIUM 21 



Such is the condition of the epithelial cells which compose the 

 parenchyma of the liver. The liver cell may therefore be con- 

 sidered as exhibiting the typical struc- 

 ture of spheroidal epithelium. These 

 cells are polyhedral in shape ; in profile, 

 polygonal. They consist of soft, finely 

 granular protoplasm, whose surface is 

 slightly condensed to form an indistinct 

 limiting membrane. Within the cell a 

 distinct spherical nucleus sometimes 



two nuclei may be observed. The nu- FIG. 18. POLYHEDRAL EPITHE- 



cleus is deeply chromatic. Small fat LIUM ' FROM A SECTTON OF 



r J . . THE HUMAN LIVER. 



globules or vacuoles, pigment, and secre- ^ 



The central blood capillary 

 tory granules (zymogen, glycogen, etc.) contains one leucocyte, and its 



are found within the cytoplasm. Such wal1 contains the nucleus of 

 cells are often surrounded by a network L" dlt^ "'" 

 of lymphatic spaces, nutrient canals, or 



secretory canaliculi, and from these minute canals, still finer off- 

 shoots penetrate the cytoplasm of the cell. 



SQTTAMOUS EPITHELIUM (pavement epithelium). The cells 

 of this variety vary much, according to their location. Those 

 lying within the body (endothelium) are of soft consistence, highly 

 elastic, and form minute plate-like masses of protoplasm, joined 

 edge to edge by cement substance, to form a continuous yet ex- 

 tremely thin membrane. Within each cell is a large oval nucleus, 

 which, like the cell body, is much flattened. The nucleus, how- 

 ever, forms the thickest portion of the cell, and is usually found 

 near its center. 



In those cells which are exposed upon the free surface of the 

 body, such as the superficial cells of the epidermis or of the mucous 

 membrane of the mouth or pharynx, the cell cytoplasm becomes 

 changed in consistence as it changes its relative position in the 

 cell layer. A peculiar horny material, known as "keratin," is 

 developed within the superficial squamous cells of stratified epi- 

 thelium, which obscures their nucleus and changes the cell proto- 

 plasm into a firm horny substance. By the action of alkalis, such 

 cells may be softened and the nucleus again brought into view. 

 These cells, when seen " on the flat " their broad surfaces pre- 

 senting are irregularly polygonal in outline, and have serrated 

 margins and sharp angles. When viewed in profile, however 

 viz., their edges presenting as frequently occurs in transections 



