16 A TYPICAL VERTEBRATE EYE: THE HUMAN 



part of the retina throughout their extents. The innermost of the two 

 layers of the ciliary epithelium is unpigmented and is a forward con- 

 tinuation of the sensory retina, which drops sharply in thickness at the 

 ora terminalis (Fig. 3; Fig. 7g). 



At the anterior end of the ciliary body the uveal tract bends sharply 

 inward, away from the fibrous tunic, to form the iris (Figs. 3, 5, and 7). 

 This structure is an opaque disc of tissue with a hole, the pupil, in its 

 center. It is not flat, but bulged slightly forward by the lens which lies 

 behind it, so that the iris forms a low truncated cone when seen in profile. 

 The periphery of the iris is anchored to the inner aspect of the limbus 

 corneas by a connective-tissue meshwork, this region being known as the 

 iris- or filtration-angle (Fig. 5, mt, fa). It is important that this crevice 

 between iris and cornea remain wide, and not be squeezed shut or 

 blocked by material of any kind. This would lead to glaucoma, for the 

 only important exit-pathway for excess fluid, the canal of Schlemm (Fig. 

 3; Fig. 5, sc), lies shallowly embedded in the fibrous tunic at the iris 

 angle, separated from the aqueous only by a thin layer of the meshwork 

 tissue. 



In the iris, the uvea and retina are even more intimately associated 

 than in any part of the ciliary body. On the posterior surface of the iris 

 — that is, the surface directed toward the lens and the vitreous — the 

 relations of the pigmented and unpigmented layers of the double retinal 

 epithelium (here called the pars iridica retinae) are reversed; for here 

 it is the innermost or posterior layer, nearer the lens, which is pigmented. 

 The anterior or outer layer, toward the cornea, contains little or no pig- 

 ment (Fig. 7g). In blue eyes, the brown pigment of the retinal backing 

 of the iris is the only pigment the iris contains — the blue color of the iris 

 being caused by optical trickery similar to that which makes veins, con- 

 taining dark red blood, appear blue when seen through white skin. In 

 brown and black irides, there is more or less pigment also in the uveal 

 connective-tissue stroma of the iris (Fig. 5, is; Fig. 7a, s), which is much 

 like the chorioid in construction. Inasmuch as the usual color of the 

 mammalian iris is brown, and the human blue eye represents a failure to 

 develop stromal pigment, the blue eye may properly be considered an 

 abnormality — a developmental anomaly — despite its common occurrence. 

 This viewpoint is strengthened by the fact that blue eyes are recessive to 

 the darker colors in heredity. The reader is not advised, however, to 

 refer slightingly to the azure orbs of his inamorata! Perfect albinos 

 (which perhaps never occur in the human species) of course lack even 



