EPITHELl UM. :\ 1 



I. EPITHELIAL TISSUE. 



Epithelium is made up entirely of closely approximated 

 cells consisting of cell protoplasm and nucleus. The intercellu- 

 lar substance is reduced to a minimum, and is seen only as a 

 cement substance joining the cells with one another. A true 

 cell membrane is usually wanting, only a slightly denser outer 

 sheath being present in the protoplasm. The classification of 

 epithelial tissue depends largely upon the function which it has 

 to fill. It covers the outer surface of the body, and lines the 

 body spaces. In addition to this, epithelial tissue has the power 

 of secretion and absorption, and in such a case it is called 

 glandular epithelium (glandular tissue). Finally it is in some 

 instances capable of receiving certain stimuli from the outer 

 world, and transmitting them to the nervous tissue. Such a 

 tissue forms the so-called sensory epithelium. 



Fm. 20. 



Nucleus** 



Cell- 

 boundary 



- - - -Nucleus 



_l -Cell 



boundary 



Diagram of flat epithelium. I. Seen from above. II. Seen from the side after trans- 

 verse section on the line m : (a) cell boundaries as straight lines; (6) cell boundaries as 

 wavy lines. 



With regard to the form of the cells, epithelium may be 

 flat or cylindrical. Flat epithelium consists of more or less 

 regularly polygonal cells, whose depth is very inconsiderable in 

 comparison with the other two dimensions. Looked at on the 

 surface, the cell boundaries are made up of straight or zigzag 

 lines. The spherical or oval nucleus lies usually more or less 

 in the middle of the cell. Figure 20 shows the flat epithelium 

 viewed from above and from the side. We notice that the cell 

 in the neighborhood of the nucleus contains more protoplasm 

 and is thicker at this point (Figs. 20, 21). 



