28 



NORMAL HISTOLOGY. 



FIG. 16. 



A far more usual arrangement is as several layers, constituting 



the stratified squamous variety. The isolated cells of such epi- 

 thelium differ greatly in form, size, and 

 appearance according to the layer from 

 which they are taken. The cells com- 

 posing the deepest stratum are not scaly, 

 but irregularly columnar, resting, with 

 slightly expanded bases, upon the sub- 

 jacent membrana propria. The irregular 

 borders of these cells join with neighboring 

 elements in such a manner that minute 

 intercellular clefts are formed; these are 

 occupied by the yielding cement-sub- 

 stance, and allow the passage of the 

 nutrient juices, as well as of the migratory 

 leucocytes, or wandering cells. The 



nuclei of the columnar elements are oval, and often situated nearer 



the outer ends of the cells. 



Passing from the basement-membrane towards the free surface, 



the form of the cells undergoes a radical change. The pronounced 



columnar type belongs to the 



deepest layer alone ; the cells next FIG. 18. 



become irregularly polyhedral, 



FIG. 17. 



Squamous epithelium from frog's 

 skin, viewed from the free surface. 



Stratified squamous epithelium in section, from 

 the cornea : the deepest cells are columnar ; the 

 superficial are scaly plates. 



Isolated cells of stratified squamous epithelium : 

 a, surface-cell ; b and c, cells from middle layers ; 

 d, from deepest stratum. 



then gradually expand in the direction parallel to the free surface, and 

 become, finally, converted into the large thin scales so characteristic 

 of the outer layers of stratified squamous epithelium. 



The cells constituting the middle strata are irregularly polyhedral, 

 and not infrequently seem to be mutually connected by means of 

 delicate processes, which bridge the intervening intercellular clefts 

 and establish a direct continuity between the neighboring cells ; when 

 such elements are isolated, the delicate threads are broken and the 

 disassociated cells appear as if beset with minute spines : these con- 



