Chap, in.] EPITHELIUM. 23 



portant function of columnar epithelial cells viz., 

 the formation of mucus. In mucus-secreting 

 glands all the epithelial cells have this function 

 permanently, but in ordinary columnar epithelium 

 only a comparatively small number of the cells, 

 as a rule, undergo this change, and then only 

 temporarily ; for a cell subject to it at one time may 

 shortly afterwards resume the original shape and 

 aspect of an ordinary protoplasmic, cylindrical, or 

 conical epithelial cell, and vice versa. If ciliated 

 cells undergo this change, the cilia are generally first 

 detached. 



It can be shown that in this change of an ordinary 

 columnar epithelial cell into a goblet cell, the inter- 

 stitial substance of the cell reticulum increases in 

 amount, the meshes enlarging and distending the body 

 of the cell. The interstitial substance probably under- 

 goes the change into mucin. 



26. Prickle Cells (Fig. 14). Amongst the middle 

 and deeper layers of the stratified pavement epithelium, 

 such as is present in the epidermis and on the surface 

 of the oral cavity and pharynx, we meet with a close, 

 more or less distinct and regular striation, extending 

 from the margin of one cell to that of each of its 

 neighbours, by means of fine transverse short fibrils 

 which, passing from protoplasm to protoplasm, connect 

 the surfaces of the cells, 



27. Pigmented epithelial cells i.e., epithelial cells 

 filled with black pigment particles (crystals) are found 

 on the internal surface of the choroid and iris of 

 the eyeball. 



In coloured skins, and in coloured patches of skin 

 and mucous membrane, such as occur in man and 

 animals, there is found pigment in the shape of 

 granules lodged in the protoplasm of the deeper 

 epithelial cells, as well as in branched cells situated 

 between the epithelial cells of the deeper layers. Minute 



