2 6 NORMAL HISTOLOGY. 



CHAPTER II. 



THE EPITHELIAL TISSUES. 



THE free surface of the skin and of the various mucous membranes 

 is covered by epithelium, which affords protection to the more 

 delicate parts lying beneath. In this tissue the intercellular con- 

 stituent is reduced to a minimum, being represented alone by the 

 scanty cement-substance between the cells; the latter, in consequence 

 of this relation, form practically an unbroken sheet. 



The epithelia are best grouped under two chief heads squamous 

 and columnar. The designation as tessellated or pavement is not 

 distinctive, since either variety may present a mosaic when viewed 

 from the free surface. These tissues may be classified in several 

 divisions as below indicated. 



VARIETIES OF EPITHELIUM. 



/ Squamous. II. Columnar. 



a. Simple consisting of a single layer a. Simple. 

 b. Stratified consisting of several layers b. Stratified. 



/// Modified. 

 a. Ciliated; b. Goblet; c. Pigmented. 



IV. Specialized, 

 a. Glandular epithelium ; b. Neuro-epithelium. 



The epithelium contains no .blood-vessels, the nutrition of the 

 tissue being maintained by the absorption of the nutritive juices 

 conveyed by means of the intercellular clefts within the cement- 

 substance. The nervous supply of epithelium is likewise ordinarily 

 very scanty, the existence of nerve-fibrils within the epithelium in 

 many localities being doubtful; in certain regions possessed of high 

 sensibility, as the corneal or tactile surfaces, the termination of nerve- 

 fibres among the epithelial elements may be regarded as definitely 

 established. The epithelial cells usually rest upon a basement- 

 membrane, or membrana propria, a modification of the subjacent 

 connective tissue of which it is part. 



The principal distributions of the various forms of epithelium 

 follow. 



