28 THE EPITHELIAL TISSUES 



preparations, but between them there exist numerous intermediate 

 forms. Those cells having the form resembling scales or irregular 

 discs are called squamous. In other cells the three dimensions are 

 approximately equal and these are called cuboidal epithelial cells, 

 although they are rarely perfect cubes. The columnar type has 

 elongate prismatic or roughly hexagonal cells that resemble irregular 

 columns. 



It is unfortunate that in the study of histological preparations the 

 field presented by the microscope is primarily two-dimensional. 

 The third dimensional aspect of the objects under study is depen- 

 dent upon the focus, and in the case of very thin sections it is neces- 

 sary to study a series of consecutive sections and develop the ability 

 to form a com])osite picture incorporating this aspect. As an 

 example of limitation imposed by studying the two dimensions 

 presented by histological preparations under the microscope, con- 

 sider the possible sections through a cell having the form of a 



SQUAMOUS CUBOIDAL COLUMNAR 



Fig. 5. — Diagram of types of simple epithelial cells. 



hexagonal column with a central bean-shaped nucleus. (Fig. 6.) 

 If a series of cuts pass through such a cell parallel with the long axis, 

 thin sections will be obtained which appear as rectangles, and not 

 all sections will have the nucleus represented. P\irthermore, if 

 the first cut through the region of the nucleus takes only a small 

 portion of its convex surface, such a section will appear to have a 

 spherical nucleus, wdiile sections through the body of the nucleus 

 will have an ovoid form. Xo one section so obtained gives any 

 clear evidence of the third dimensional aspect or the picture of 

 the cell as a whole. If another series of thin sections is obtained 

 using cuts at an angle of 45 degrees with the long axis, then there 

 will be a variation from trapezoids without nuclear portions to 

 rectangles containing roughly spherical nuclear portions. As the 

 angle of cutting approaches a right angle with the long axis, the 

 hexagonal structure of the cell appears in the sections, although 

 distorted. Sections from cuts made at right angles to the longi- 

 tidinal axis will show hexagons, some of which will have no miclear 

 portion, while others will have a spherical nucleus centrally situated. 

 Therefore, in order to visualize the complete structure of such a 



