TRANSITIONAL EPITHELIUM. 



197 



through the protoplasm of the cells. According to Ramon y Cajal, they are covered by a pro- 

 longation of cell- membrane. A similar view is taken by Ide, who describes the cell-membrane 

 as reticulated. A radiating system of fibrils has also been shown to occur in the flattened 

 epithelium cells which cover the posterior surface of the cornea, and in this case also the 

 fibrils traverse the intercellular spaces, passing from one cell into another. 



Stratified scaly epithelium occurs in one of its simplest and most typical forms 

 covering the anterior surface of the cornea of the eye (fig. 222). It is found also 

 lining the mouth, the chief part of the pharynx, and the oesophagus, and in the 

 female it lines the vagina and part of the 

 cervix uteri, but its most extensive distri- 

 bution is over the surface of the skin, where 

 it forms the epidermis. In many parts 

 of the epidermis the layers become very 

 numerous, and their arrangement some- 

 what complicated, as will be noticed in the 

 description of the skin. It may be remarked 

 that, in most of the situations where it is 

 found, stratified scaly epithelium is of 

 epiblastic origin, but this is by no means 



invariably the case and its occurrence depends much more upon the physiological 

 conditions of the parts which it covers. Thus, wherever a surface is liable to undergo 

 friction or abrasion, there we find a development of stratified scaly epithelium. 



Fig. 225. Two " PKICKLE-CELLS ' 



DEEPER PART OF THE EPIDERMIS. 



FROM THE 



(Ranvier. ) 



d, space around the nucleus, probably caused 

 by shrinking of the latter. 



Fig. 226. SECTION OF THE TRANSITIONAL EPITHELIUM LINING THE BLADDER. (E. A. S.) 

 a, superficial ; b, intermediate ; and c, deep layer of cells. 



Transitional epithe- 

 lium. Epithelium to which 

 the term transitional may be 

 applied, as being in a sense 

 intermediate between those 

 forms which consist of but a 

 single layer of cells and the 

 stratified which we have just 

 described, may be classed 

 under the three heads of 

 columnar, ciliated, and scaly 

 transitional, according to the 



Fig. 227. EPITHELIAL CELLS FROM THE BLADDER OF THE RABBIT. HIGHLY MAGNIFIED. (Klein.) 

 a, large flattened cell from the superficial layer, with two nuclei, and with strongly marked ridges and 

 intervening depressions on its under surface ; a', one of the same cells shown in profile ; b, pear-shaped 

 cell of the second layer showing tie manner in which it is adapted to a depression on the superficial cell. 



kind of cell in each which happens to be most prominent or superficial. The 



