EYE OF THE EISTGLISH SPARROW 399 



THE SCLERA 



The sclera of the bird eye differs in shape, thickness, and 

 structure from that of man. The curvature differs in various 

 portions (fig. 30). The posterior part is very symmetrical and 

 forms practically a segment of a hollow sphere whose center is at 

 the middle of the posterior surface of the lens. This segment 

 extends forward to near the region of the ora serrata where it 

 turns rather abruptly toward the lens. Near the equatorial 

 margin of the lens it unites with the corneal part of the sclera. 



The corneal part represents a segment of a much smaller 

 sphere, the ratio of which to that of the larger is 1: 2|. The 

 diameter of the base of the corneal segment, or at the junction 

 of the cornea and sclera is 1.8 mm. The diameter of the base 

 of the posterior spherical segment is 7.3 mm. These two bases 

 give a ratio of 1 :4. 



In a cross-section of the eye that part of the sclera which 

 unites these two spherical segments approximates a straight line. 

 A slight bend occurs, but instead of curving outward, it bends 

 inward with the convexity toward the vitreous body. The 

 shape of this portion of the sclera would therefore closely re- 

 semble a segment cut from a funnel whose larger diameter is 

 7.3 mm. and the smaller, 1.8 mm. The shape of the whole eye 

 is therefore quite different from that of man. Instead of being 

 made up of two adjacent spherical segments, the two spherical 

 segments are joined by a segment of a blunt cone. 



The sclera is thickest where it blends with the corneal portion 

 near the margin of the lens, where it measures about .14 mm. 

 From this region it diminishes in thickness both anteriorly and 

 posteriorly. In the center of the cornea it measures .071 mm. 

 The thinnest part of the sclera is slightly posterior to the equa- 

 tor. At this point it measures from .03 to .04 mm. Another 

 thickened region surrounds the optic nerve. This is thickest 

 near the nerve and diminishes rather abruptly to almost a uni- 

 form thickness over the whole posterior part of the eye. 



The sclera is pierced in several places by blood-vessels and 

 nerves, the most important of which are the vessels to the 



