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JAMES ROLLIN SLONAKER 



at its anterior termination and again in a region around the 

 optic nerve. The variation of the thickness of the cartilage 

 layer at the anterior margin is as follows: dorsal, .032 mm.; 

 ventral, .044 mm.; anterior, .036 mm., and posterior, .036 mm. 

 Posteriorly from the margin it thins to from .020 to .024 mm. 

 It maintains this thickness over the posterior surface to near 

 the optic-nerve entrance. Here the cartilage becomes rather 

 abruptly thickened to from .052 to .056 mm. This thickened 

 portion forms a ring4ike area around the nerve. The value of 

 this thickened area is apparent. Owing to the very short and 

 thick optic nerve, movements of the eyeball would cause con- 

 siderable pressure in this region. This would injure the delicate 

 structures of the retina if the sclera were not stiffer to with- 



TABLE 1 



Showing the thickness of the different layers of the adult cornea, chorioid coat, and 

 sclera; also the axial and equatorial diameters of the whole eye 



stand this strain. The increased thickness of the cartilage at 

 the anterior margin is apparently to meet the extra strain pro- 

 duced by the muscles of accommodation. 



Imbedded in the fibrous portion of the sclera at the anterior 

 margin of the cartilaginous layer are a number of thin plates of 

 bone, (fig. 30, S) the scleratic bones, found in most birds' eyes. 

 The number, size, and arrangement vary in different species.' 

 In the sparrow there are fourteen of these plates which have a 

 common shape (fig. 31). They are roughly quadrilateral in 

 outline (C). Their thickness is greatest in the middle portion 

 and tapers off to a thin edge on their lateral sides. Figure 31, 

 D-D, shows three of these plates cut across at right angles to the 

 axis of vision. As can be readily seen, they overlap each other 



