4 VERTEBRATE PHOTORECEPTORS 



Sclera 

 The tunica fibrosa or outer layer of the eye comprises the 

 cornea and sclera. The sclera forms the posterior five-sixths 

 of the protective outer tunic. It is a tough membrane made 

 up in man and in higher animals of dense fibrous connective 

 tissue — mostly of collagenous fibers, but containing also 

 some elastic fibers (Figure 4). Where the optic nerve fas- 



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Fig. 4. Photomicrogiapli showing 

 fibrous sclera of the monkey eye 

 (Macaca mulatta). X 115. 



Fig. 5. Photomicrograph showing 

 cartilaginous sclera of the hawk eye. 

 X 115. 



ciculi leave the eye, this tunic is perforated and this region 

 is known as the lamina cribrosa. The sclera is pierced also 

 by nerves and blood vessels entering and leaving the bulb. 

 In fishes, amphibians, reptiles, and birds the sclera is made 

 up partly or entirely of hyaline cartilage. In fishes the 

 cartilage is present usually in the form of laterally situ- 

 ated plaques. In amphibians, reptiles, and birds, the car- 

 tilage is continuous around the eye (Figure 5) . In some forms 

 the sclera is made up largely of bone (e.g. whale, and sword- 

 fish), and in other forms, varying degrees of calcification of 



