6 VERTEBRATE PHOTORECEPTORS 



amount of light. Perhaps the most striking cases of large 

 corneas occur among the nocturnal lemurs where they occupy 

 approximately one-third of the perimeter of their bulbs 

 (Detwiler, 1940) (Figure 49). 



Uvea 



The tunica vasculosa (uvea) is made up of the choroid, 

 ciliary body, and iris. It is characterized by the presence 

 of abundant blood vessels and pigment cells. By reason of 

 the heavy pigmentation of this layer, the bulb is light-tight 

 except through the pupil. When the sclera is removed from 

 the eye, this middle heavily pigmented tunic resembles the 

 inside of a purplish grape skin, the pupil representing the 

 point where the stem is pulled out — hence the name uvea 

 or uveal tract. 



Choroid. The choroid, which forms the posterior part of 

 the uvea, is highly vascular, resembling somewhat in this 

 respect the chorion of the foetus — hence the name choroid. 

 The extreme vascularity of this region including the large 

 spaces occupied by the veins makes this structure resemble 

 erectile tissue. This layer is believed to share in the regula- 

 tion of intraocular pressure. The choroid is intimately bound 

 or attached to the epithelial pigment layer of the retina 

 (tapetum nigra). In retinal detachment, the split occurs 

 between the neural layers of the retina, and the epithelial 

 pigment layer so that the latter remains adherent to the 

 choroid. 



The choroid varies considerably in thickness in various 

 vertebrates. It shows also some noteworthy specializations. 

 In some teleost fishes a silvery membrane is found between 

 the suprachoroid and the vascular layer. This is formed of 

 specialized cells containing crystals of guanine. The mem- 

 brane extends into the iris and presents a characteristically 

 silver luster. In some cartilaginous fishes and in some mam- 

 mals there is present a reflecting layer known as the tapetum 

 lucidum. It is situated between the choriocapillaris and the 

 vessel layer in the posterior region of the eye, usually above 



