EYE OF THE ENGLISH SPARROW 401 



pecten and the ciliary vessels, the cihary nerves, and the optic 

 nerve. The blood-vessels and small nerves are described else- 

 where. The optic nerve enters the eye posterior to and slightly 

 below the center of vision. After piercing the sclera, it runs 

 downward and forward just within the sclera, but on the outside 

 of the retina (figs. 16 to 21, plate 4). The sclera is considerably 

 thinner immediately over the downwardly directed nerve, thus 

 forming a trough-like depression in which the nerve lies. Ex- 

 ternally this extension of the nerve between the sclera and 

 chorioid coat is not noticed. Internally, this very much 

 elongated optic disc is covered by the pecten and cannot readily 

 be seen. 



The sclera separates readily from the chorioid coat at all 

 points except w^here the blood-vessels and nerves pass through 

 into the chorioid coat. When these are cut the sclera may be 

 readily removed without injury to the chorioid. 



The sclerotic layer of the sparrow eye is composed of closely 

 arranged connective-tissue fibers, cartilage, and bone. 



The fibrous portion of the sclerotic forms the outer portion of 

 this layer and completely envelops the other portions. Its 

 thickness varies at different places. Over the posterior part of 

 the eyeball it is thinnest and varies in thickness from .004 to 

 .010 mm. It increases gradually in thickness toward the an- 

 terior portion of the eye and attains its greatest thickness a 

 short distance posterior to its junction with the cornea where it 

 measures .119 mm. From this point on it becomes transparent 

 and forms the substantia propria, or the main part of the cornea. 

 In the center of the cornea this layer measures .054 mm. By 

 consulting table 1 the thickness of this layer at various places 

 may be seen. 



The whole of the sclera, from the region of the lens backward, 

 is reinforced or stiffened by a layer of cartilage. This forms the 

 inner part of this tunic and in many places constitutes almost its 

 entire thickness. This cartilage layer ends rather bluntly just 

 about opposite the equator of the lens. Here it is overlapped 

 by the scleral, or bony plates. The thickness of the cartilagi- 

 nous part varies in different portions. It is in general thicker 



