THE EYE AND ITS APPENDAGES. 349 



packed with deeply-colored particles that its real constitution is 

 masked. This stratum is composed of two layers, an anterior and 

 a posterior. The anterior or outer layer is formed of radially- 

 arranged spindle-cells which pass without interruption from the 

 ciliary border of the iris to the pupillary margin ; at the ciliary 

 border the cells change their form and arrangement, becoming 

 polyhedral and circularly disposed and continuous with the low 

 pigmented elements constituting the corresponding layer of the 

 ciliary processes. 



The posterior layer presents a thicker zone (30-35 M) of pig- 

 mented cells, in which the colored particles are so densely packed 

 that the cell-boundaries and the nuclei are completely masked, the 

 entire layer appearing as one continuous mass of pigment. 



The pigment-layer covers the entire pupillary margin, and 

 often ends as a somewhat thickened free edge slightly in advance of 

 the plane of the iris ; at this border, which represents the free 

 anterior lip of the embryonic secondary optic cup, both strata 

 of the pigment-layer become continuous. 



The posterior surface of the pigment-layer is covered by a very 

 delicate cuticular membrane, the membrana limitans iridis, 

 which is continued from the similar structure extending over the cili- 

 ary processes ; it appears first at the ora serrata as a new formation, 

 since a true membrana limitans interna, in the sense of a distinct 

 cuticle, does not exist over the retina proper. 



The marked variation in the color of the iris is largely dependent 

 on the amount and position of its pigment. In blue eyes the 

 stroma of the iris is entirely free from pigment, the latter being 

 confined to the posterior pigment-layer, from which position it is 

 seen through the superimposed iridal strata. With the darker color 

 of the iris its stroma-cells also acquire pigment ; in light gray 

 eyes this is small in amount, in brown eyes greater, while in the 

 darkest eyes the colored particles are very numerous and sometimes 

 appear as almost continuous pigmented areas ; in albino eyes, on the 

 other hand, even the retinal portion of the iris is devoid of pigment. 



The nerves of the iris, derived from the intra-muscular ciliary 

 plexus, enter the more superficial part of the stroma-layer as med- 

 ullated fibres. Within the iris the nerve-fibres soon lose their med- 

 ullary sheath and form one or two irregular net- works, the most 

 constant of which is a circular plexus in the vicinity of the sphinc- 

 ter muscle ; from this net-work pale fibres are distributed to the 

 substance of the latter muscle. The principal plexus lies anterior 

 to the plane of the chief vascular net- work, the posterior zone of the 

 iris being poorly supplied with nerves. 



The irido-corneal angle, marking the junction of the cornea, the 



