30 THIRD LECTURE. 



retina. They were formerly called polyhedral pigment cells 

 (Fig- 35)- Their surface presents a delicate mosaic, as a rule, 

 of a hexagonal form, of 0.0135 to 0.0204 mm. The quantity 



of the pigment granules embedded in the 

 soft, homogeneous substance of the cell 

 body varies, so that the nucleus is some- 

 times visible, at others concealed. The 

 outer portion of the cell remains free 

 fig 35 -rigment epkhe- fr°m these melanine molecules, which 

 IS,? .fort&jlJS appear to form small crystals (Frisch). 

 ^nalone! 1 " ,alargerocta " The profile view shows, however, that 



our cell, far from constituting a flat struc- 

 ture, possesses, rather, a certain, sometimes a considerable, 

 height, which, at least, equals the diameter. It is thus in the 

 lower vertebrate animals ; here their body sends downwards 

 a number of filamentous and spiny processes. The latter, 

 at first, still contain pigment molecules, and surround in a 

 sheath-like manner the rods and cones, the marvellous termi- 

 nal apparatus of the retina. This is less frequent in mam- 

 malial animals. These pigmented cells extend beyond the 

 limits of the specially nervous portion of the retina, over the 

 so-called ora serrata, where they become smaller, more rich 

 in pigment, and are in thin layers. In this manner they then 

 cover the ciliary processes and the posterior surface of the 

 iris. 



Certain of the mammalia (the carnivora and ruminantia) 

 have a bright, glistening place in the interior of the eye, 

 called the tapetum. Our retinal epithelium is here unpig- 

 mented. In albinos this is the case throughout, and the 

 cells present the appearance of a delicate pavement epithe- 

 lium, as, for instance, in the white rabbit. 



Many mucous membranes present material, and sometimes 

 very considerable, layers of pavement epithelium. Among 

 these are the conjunctiva of the eye, the mucous membrane 

 of the nostrils and of the anus, the mouth and pharynx, the 

 oesophagus, the urinary passages, and the vagina. 



In the most superficial layers of the conjunctiva (Fig. 36, a) 



