RETINAL STRUCTURE AND ANIMAL HABITS 67 



pigmentation is not specifically characteristic of nocturnal 

 eyes, as is sometimes assumed. The great variation in the 

 amount of epithelial pigment in the eyes of strictly diurnal 

 and nocturnal forms, as well as in those with a dual retina, 

 has been an obstacle to an adequate general explanation for 

 the function of the pigment in all forms. 



The amount of pigment varies greatly in animals with a 

 pure rod retina. For example, in the elasmobranch eye it 

 is entirely lacking; in the rat and the bat it is sparse. It is 

 entirely absent in the greater part of the retinas of Galago 

 and Nycticebus (lemurs); it is scant in the tapetal region 

 of the alligator and in the region of the choroidal tapetum 

 of mammals. Yet, in the nocturnal gecko, epithelial pig- 

 ment is not only abundant but extends down between the 

 rods so as to surround or enclose the external segments. 



In addition to possessing the above nocturnal character- 

 istics, the eyes of lemurs exhibit other noteworthy features 

 which are in great contrast to those typifying the diurnal 

 anthropoids and man. Chief amongst these are the very 

 large corneas, anterior chambers, and lenses. These features 

 are illustrated in Figures 48 and 49. The corneas are greatly 

 curved and in meridional section occupy more than one- 

 third of the perimeter of the bulb (particularly in Nycti- 

 cebus). The lens is also strikingly large, notably in Galago 

 where it is more than half as large as the vitreous (v. Table 2 

 and Figure 48) . As was pointed out in the first chapter, the 

 ciliary muscle in relation to the size of the lens is particularly 

 large in Nycticebus (Figure 9). 



The ratios given in Table 2 are not expressions of volumes, 

 but are merely surface area ratios (as determined by the 

 planimeter) of the various eye structures through the merid- 

 ional plane. Hence they can at best be approximate only. 

 However, the sections of the eyes shown in Figures 48, 49, 

 50, 52 bear out in general the great size contrast between 

 these various structures in lemurs and in the anthropoids 

 listed. 



