THE EPITHELIAL TISSUES. 



53 



the deep ends of the columnar cells, so that they are not strictly 

 prismatic in form. In cross-section, or when viewed in a direction 

 parallel to their long axes, the cells have a polygonal form due to 

 the lateral pressure they exert upon each other (Fig. 33). 



The nuclei of the columnar cells are oval, situated nearer the 

 base of the cell than its superficial end with their long axes parallel 

 to those of the cells themselves, and are vesicular in structure with 

 a distinctly reticular arrangement of the chromatin filaments. 



Columnar epithelium is found chiefly upon the free surfaces of 

 mucous membranes, but also occurs in some of the secreting glands. 

 The minute structure of the cells varies somewhat in different situ- 

 ations, but the consideration of these minutiae must be deferred 

 until a description of the structure of the different organs is under- 

 taken in a subsequent chapter. 



4. Ciliated Epithelium (Figs. 34, 35, 36). Ciliated epithelium 



FIG. 34. 



Fro. 35. 



FIG. 36. 



Ciliated epithelium. (Frenzel.) 



Fig. 34. Cubical cells with long cilia (hb). The nuclei of the cells are obscured by the gran- 

 ular cytoplasm. 



Fig. 35. Columnar cells. The rodded margin, fs, corresponds to the cuticle in Fig. 37. 



Fig. 36 Diagram illustrating variations in the structure of the ciliated ends of cells. The 

 rodded portion, ok to nk, corresponds to the cuticle of other varieties of epithelium, 

 though the latter do not possess the knobbed ends of the rods represented in this figure ; 

 hb, cilia. 



is merely a variety of either columnar or cubical epithelium in 

 which the free ends of the cells are beset with delicate hair-like 

 processes, which execute lashing movements in some one direction. 

 It is found lining the trachea and bronchi, the cilia here serving to 

 propel toward the larynx such particles of dust as are brought into 

 the respiratory passages by the currents of air during respiration. 

 Ciliated epithelium also occurs on the lining membranes of the nose 



