144 MANUAL OF HISTOLOGY. 



nuclei of 0-0056 mm. in size, or less. All these cells display under high 

 microscopic powers a very peculiar structure (fig. 1 36). Their whole surface 

 namely is covered with prominent ridges and spines (a), by mean of which 

 neighbouring cells are attached to one another, " like two brushes whose 

 bristles are pressed in among one another " (SclmUze). 



In the outer layers, finally, the epithelial cells (fig. 136) appear as thin 

 scaly structures, without either grooves or pro- 

 minences, and of considerable size (0-0425- 

 0-0750 mm.), with more or less oval and 

 homogeneous nuclei of from 0'0090 to 

 0-0114 mm. Here the body of the cell con- 

 tains a few granules usually in the vicinity 

 of the nucleus. 



But the cell has also changed in its 

 physical condition. Instead of the softness 

 of former days, it now manifests a greater 



Fig. 137. Epithelial cells from the , , n , . , 



uppermost layers of the human or less degree ol hardness and brittleness ; 



moufch - it has become horny, as the saying is : it is 



also destitute of soft protoplasm. 



Apart from the differences in thickness which the whole bed may 

 show (being, according to Henle, 0-2 mm. on the palate, and on the gums, 

 behind the teeth, between the papillae, 0'4 mm.), the cells of the locality 

 just mentioned seem to differ but slightly. 



The persistence of epithelium, already considered in speaking of the 

 simplest pavement cells of closed cavities, appears to be the same in the 

 urinary apparatus ; in the thickly laminated coatings of other mucosse it 

 is well known not to obtain. Here we have to do with a tissue under- 

 going rapid repair, in that a certain quantity of the most superficial cells 

 is rubbed off continually, forming a regular constituent of the mucus of 

 the part, whilst the deeper cells advance to the surface, and a process of 

 cell-formation takes place in the undermost strata in order to cover the 

 loss of the desquamating cells of the surface. The multinuclear epithelial 

 cells which may be observed, by no means unfrequently, in deep parts 

 of the strata, are evidence in favour of such a process of cell-formation, 

 That the obliteration of the spines and ridges in senescent cells prepares 

 them for separation, is very probable. 



REMARKS. -M. Schultze in yirchow's Archiv, bd. 30, s. 260. 



89. 



A modification of the pavement epithelia we have just been discussing 

 is found in the eye, in the so-called "polyhedral pigment cells" of the uvea. 

 These are epithelial cells, partly laminated to a small extent, and partly 

 not, and moderately flattened, which occur in the eye in the form of a 

 delicate mosaic. They have peculiar contents as a rule, made up of 

 numerous elementary granules of the black colouring matter, melanin, 

 already described (p. 52). 



These cells are met with on the internal surface of the choroidea in an 

 unbroken but single layer, "which becomes suddenly laminated in the 

 vicinity of the ora serrata of the retina, at the same time that the 

 individual elements decrease in size. Thus arranged, they are found 

 covering the ciliary processes, and in the human eye the posterior surface 

 of the iris as far as the edge of the pupil. 



The granules of black pigment are sometimes elongated, sometimes 



