96 DIFFERENT KINDS OF EPITHELIAL TISSUE. 



nail and horn ; another may be supple and elastic, 

 like the epidermis, or soft and moist, like the epithe- 

 lial tissue of mucous membranes and internal passages, 

 while some forms of epithehal tissue are semi-fluid, 

 or more or less viscid, of the consistence of mucus, 

 §34. 



139. Different kinds of Epithelial Tissue. — The 

 student would scarcely believe that the soft, moist 

 epithelium of a mucous membrane was in any way 

 related to the hard dry tissue of which nail, horn, and 

 hair, § 118, consist, or to the hard calcified texture 

 of shell, dentine, § 120, or enamel, § 121 ; but if he 

 were to examine these textures at an early period of 

 their development he would be convinced of their 

 very close relationship, and would find that the formed 

 material was produced in the same manner in them 

 all. It may be truly said that one thing can scarcely 

 differ more from another than the soft, moist epithe- 

 lium of a papilla of skin or mucous membrane does 

 from the firm cuticular tissue of horn or hair, and 

 yet under modified conditions the former may become 

 so altered as to constitiite a tissue which any one 

 would admit was closely allied to the latter structures. 

 The fibre-like cells constituting certain forms of hair, 

 horn, and nail are very different from other forms of 

 epithelial tissue, but, as is well known, well-developed 

 horns are occasionally produced on the skin, and the 

 horny material consists but of modified epidermis. 

 The long drawn out cells or fibres of enamel and 

 dentine are probably modified forms of epithelium, 

 the formed organic matter of which has been gra- 

 dually impregnated with calcareous particles, § 11 9. 



140. Difference in function discliarg:etl by EpithcUal 

 Tissues. — Nor do epithelial textures difier from one 

 another less remarkably in structure and physical 

 properties than they do in fimction. The cell which 

 secretes bile, or urine, or gastric juice, § 124, would 

 seem to be very far removed from the epithelial cell 



