THE MARSUPIAL EYE 



673 



top of it in a low glial ridge which projects a trifle into the vitreous. 

 The iris contains an unstriated sphincter near the pupil margin (as in 

 other mammals) ; but no marsupial is known to have a dilatator. Both 

 retinal layers are therefore heavily pigmented. The stroma is likewise 

 densely pigmented and is richly vascularized, often with many vessels 

 partially extruded from its anterior surface. 



In large eyes the ciliary body forms a broad zone with well-marked 

 orbicular and coronal regions; but in small eyes, whose lenses are enor- 

 mous, the ciliary body is reduced about as it is in snakes. In large eyes 



Fig. 196 — The marsupial eye. 



a, ventral half of left eyeball of a kangaroo, Macropus giganteus. xl. After Soemmerring. 



b, inner surface of segment of anterior uvea of a kangaroo, Macropus agilis. After Franz. 



c, iris-angle region of cuscus, Trichosurus vulpecula. Redrawn from Franz. 



cm- ciliary muscle; cp- ciliary process; cw- ciliary web; i- iris; Ic- limbus corneae; ot- ora 

 terminalis retina; p- pupil margin. 



the ciliary processes are regular, tall, and thin (Fig. 196b) ; and they 

 are about as numerous as in comparable placentals (e.g., 120 in Tricho- 

 surus) . Those of small eyes are low, tortuous, and not so readily counted. 

 A ciliary web can usually be made out. Though no marsupial has yet 

 been demonstrated to have any accommodation whatever, a small ciliary 

 muscle is always present. This may present itself as a meridional Briicke's 

 muscle with exactly the same relationship to the corneal margin as in 

 reptiles (Fig. 196c, cm). More often, apparently, it contains both 

 circular and meridional fibers. The circular ones occupy the anterior half 



