196 



STKATIFIED EPITHELIUM. 



towards the surface, we find the component cells becoming more flattened and 

 larger, whilst at the same time undergoing a change in their chemical constitution, 

 so that at first the external part, and afterwards the whole of the protoplasm of 

 the cell, is converted into horny substance, even the nucleus being at last involved. 



The conversion into horny substance is in many instances preceded by the 

 deposit of a granular material within the cells, which is termed eleidin (Hanvier) or 

 keratohyalin (Waldeyer). In the epidermis and some other parts the cells which 

 contain this granular material form an almost complete layer between the soft, still 

 protoplasmic, deeper cells and the superficial horny stratum (fig. 224). The layer is 

 termed stratum granulosum and was described by Langerhans. 



The deeper protoplasmic cells of a stratified epithelium are continually multi- 

 plying by cell-division, and, as the new cells which are thus produced in the deeper 

 parts increase in size, they compress and push outwards those previously formed. 

 In this way cells which were at first deeply seated, become gradually shifted 

 towards the surface, undergoing meanwhile the chemical change above spoken of. 

 The older superficial cells are continually being removed by abrasion, while others 

 rise up to supply their place. 



Intercellular bridges and channels. The deeper layers of a stratified epi- 

 thelium are not closely applied to one another by their edges, but there exists a 

 system of intercellular channels, which are bridged across by fibres which run from 

 one cell to the other (see figs. 222, 224). "When the cells are isolated, the fibres are 



Fig. 223. EPITHELIUM-SCALES FROM THE INSIDK OF THE MOUTH ; MAGNIFIED 260 DIAMETERS. (Henle.) 



Fig. 224. PORTION OF EPIDERMIS FROM A SECTION OF THE SKIN OF THE FINGER, COLOURED WITH 



PICROCARMINE. (Ranvier.) 



a horny layer ; 6, its deepest part (stratum lucidum) with flakes of eleidin ; c, eleidin granules in cells 

 of stratum granulosum ; d, deeper cells of stratum mucosum with intercellular channels ; e, dentations 

 by which the deepest cells are fixed to the surface of the cutis vera. 



broken through and appear as spikes or dentations on the surface and edges of the 

 cells (fig. 225). Sometimes the intercellular channels become widened in con sequence 

 of an excess of fluid accumulating in them, but usually they are very narrow and 

 but little obvious. 



The spikes and ridges upon the deeper cells of a stratified epithelium were first noticed by 

 Max Schultze, who was of opinion that they were for the purpose of effecting, by indenting 

 with those on adjoining cells, a firmer connexion between the cells of the epithelium. The 

 true relations of the structures in question, and the intercellular channels which are bridged 

 across by them, were discovered by Bizzozero. The researches of J. Arnold and of Thoma 

 have shown that similar intercellular channels occur extensively in all varieties of epithelium. 

 The fibrils which bridge across the intercellular spaces are described by Eanvier as passing 



