THE VERTEBRATE EYE 



and lateral to the optic disc. In this region of the mam- 

 maUan eye, the pigment of the epithelial pigment layer is 

 either sparse or lacking, hence incident light falls directly 

 upon the tapetum and is reflected. Due to the presence of an 

 overlying layer of doubly refracting crystals and to inter- 

 ference phenomena caused by the stratified formation and 

 surface irregularities, an 

 iridescent effect is pro- 

 duced. The eye of the 

 animal thus shines with 

 a peculiar luster in sub- 

 dued light. The tapetum 

 of carnivors is made up 

 primarily of brick-like 

 cells and is called the 

 tapetum cellulosum. In 

 herbivors it is made up of 

 fibers — hence the name 

 tapetum fibrosum. A 

 very prominent fibrous 

 tapetum is present in the eyes of the nocturnal lemurs (e.g. 

 Galago and Nycticebus, Figure 7), but in the nocturnal 

 monkey (Nyctipithecus) it is absent. Nocturnality does not 

 necessarily imply the presence of a tapetum, for in the gecko, 

 which is nocturnal in its habits, it is absent. It is absent also 

 in the eyes of apes and man. 



Ciliary Body. In front of the ora serrata (Figure 2), the 

 uveal tract becomes thickened and modified to form the 

 ciliary body which runs around the eye as an asymmetrical 

 girdle. In section it is essentially triangular in shape, the 

 apex of which is continuous with' the choroid. It forms the 

 attachment for the suspensory ligament of the lens and gives 

 origin, also, to the iris. The inner (vitreal) surface of the 

 ciliary body presents two zones. The posterior two-thirds 

 is deeply pigmented and relatively smooth. This is the orbic- 

 ulus ciliaris or pars plana. The anterior third bears, in man, 

 from 70 to 80 radially arranged ridges — the ciliary processes. 



Fig. 7. Photomicrograph showing fibrous 

 choroidal tapetum of the lemur, Nycticebus 

 tardigradus. X 300. 



