24 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



(2) Columnar. Surface cells columnar. 



(a) Non-ciliated. Portions of male urethra. 



(b) Ciliated. Trachea, bronchi, etc. 



II. Epithelia not in the form of membranes, but in solid masses or cords, 



usually glandular, 

 (i) Cells spheroidal, ova. 

 (.2) Cells polyhedral, liver, suprarenal, etc. 



Epithelia, classified mainly as to function. 



I. Protective. Skin, mouth, alimentary canal. 



1. Cornified. Skin, nails, hair. 



2. Cuticular border. Columnar cells of intestine. 



II. Glandular. 



1. Secretory. Cells of salivary glands, pancreas, etc. 



2. Execretory. Cells of kidney. 



3. Absorptive. Cells of alimentary canal. 



III. Sensory Epithelium. Cells of olfactory membrane, organ of Corti, 



taste buds, etc. 



IV. Reproductive. Sex cells. 



V. Pigmented. Pigmented layer of retina. 

 VI. Ciliated. Trachea, uterus, Fallopian tube, etc. 



Only a few of the more important of the above-mentioned types of epithe- 

 lium will be described here. 



Simple Epithelium. Simple Squamous. This form of epithelium 

 is found arranged in a single layer of flattened cells, for example, the lining of 

 the alveoli of the lungs and of the descending arm of Henle's loop of the kidney 

 tubule. Aside from endothelium as mesothelium it has very limited dis- 

 tribution in man. Endothelium and mesothelium are typical simple squamous 



FIG. 20. The Endothelium of a Small Blood-vessel. Silver-nitrate stain. X 35. 



epithelia. They consist of much flattened cells with clear or slightly granular 

 protoplasm and oval bulging nuclei, the edges of the cells are peculiarly wavy 

 or serrated. 



The presence of endothelium in any locality may be demonstrated by stain- 

 ing with silver nitrate, which brings into view the intercellular cement sub- 



