THE EPITHELIAL TISSUES. 



55 



Upon the free surface they are reduced to thin scales, closely 

 adhering to each other and their subjacent neighbors, but entirely 

 devoid of both cytoplasm and nucleus (Fig. 38). 



Stratified epithelium is found upon surfaces exposed to friction, 

 which it serves to protect against mechanical injury, and, in some 



FIG. 38. 



Stratified epithelium, oesophagus of the rabbit: a, karyokinetic figure in a cell of the deep 

 layer, demonstrating the fact that the cells multiply in this region ; b, larger flattened 

 cell nearer the surface ; c, horny layer made up of cells that have undergone keratoid 

 degeneration ; d, underlying fibrous tissue. In one place, near the centre of the figure, 

 six blood-corpuscles reveal the presence of a small vessel ; e, tangential section of a small 

 fibrous papilla extending into the epithelium and surrounded by young epithelial cells. 



cases, against desiccation. It forms the epidermis of the skin, 

 and lines the mouth, oesophagus, rectum, and vagina. In these situ- 

 ations the scaly or squamous cells of the surface are constantly 

 being removed by the attrition to which they are exposed, but are 

 as constantly replaced by fresh cells from the deeper layers of the 

 epithelium. Pressure and moderate friction stimulate the multi- 

 plication of the cells in the deepest layers of the tissue, so that 

 parts, e. g. of the skin which are especially subjected to such influ- 

 ences acquire a thicker epidermis (callus). 



Where the stratified epithelium consists of many layers of cells, 

 as is the case, for instance, upon the skin, there is a provision for 

 the nourishment of the growing cells which are somewhat removed 

 from the vascularized subjacent tissues. The cells of the deeper 

 layers are somewhat separated from each other, leaving a space 

 between them through which nutrient fluids can circulate. Across 

 this space numerous minute projections or " prickles," springing 

 from neighboring cells, join each other, forming connecting bridges 

 between the cells. When isolated, such cells appear covered with 

 these small spicules (" prickle-cells "), and their presence probably 



