MAIIMALS 469 



to find anchorage in the hmbal region of the sclera (the pectinate 

 ligament) ; these are of varying develoj^ment in different sjiecies and 

 are only vestigial in man. 



The pigmentation of the iris is much more drab and uniform than in manj' 

 other classes of Vertebrates. Except in albinotic individuals it is derived merely 

 from melanin-containing chromatophores, and depending on their number and 

 the density of pigment within them, the iris is a varying shade of bro\^^^. tending 

 to yellow when the jDigment is scarce and blue (as often in man) for reasons of 

 optical transmission when the stromal pigment is sufficiently sjDarse. As a rule 

 the pigment is plentiful and the eye dark brown or almost black, and since the 

 chromatophores He superficial to and between the vessels, the latter are visually 

 completely obscured ; only in albino types can the vascular pattern be made 



Figs. 602 to 60fi. — Pupillary Appendages in Placentals. 





Fig. 602. — The horse. Fig. 603.— The gazelle. 





'\,""- ««■ 



Fig. 604.— The goat. Fig. 60.").- The camel. Fig. 606.— The hyrax. 



out. Occasionally and \ery rarely this simple j^igmentary scheme is complicated 

 by the presence of other pigments and iridocytes, a circumstance which gives 

 rise to the green lustre of the eyes of some Carnivores, .such as the cat. and some 

 Prosimians. In animals provided Mith a choroidal tapetum, representative 

 elements of this structure are found in the iris — fibrous elements in Herbivora, 

 cellular in Carnivora (W'olfrum. 1926). 



The jiuiDillary margin is occasionally marked by special appendages 

 the purpose of which is presumably to diminish glare. These may be 

 of two types. ^ The first, the corpora nigra (grape-seed bodies or 

 FLOCCULi of Kieser, 1803), are immobile and are formed by a prolifera- 

 tion of the pigmented epithelium as highly vascularized cystic pro- 

 trusions of the marginal sinus. ^ They occur among the higher Ungulates 

 (Figs. 602 to 605). In the Ec[uidfe (horse, etc.) they are relatively 

 simple, being confined to the upper edge of the pupil (Fig. 557) ; in 



^ For literature, see Bayer and Frohner (1900), Johnson (1901), Lange (1901), 

 Stein (1902), Zietzschmann (1905), Rirhter (1909-11), Schneider (1930^, Rohen 

 (1951-52). 



^ The embryonic persistence of the primary optic vesicle between the two layers 

 of epithelium at the pupillary margin. 



