EYE OF THE ENGLISH SPARROW 



405 



tions, shows that those at the anterior and posterior portions 

 of the eye are smaller than the others. The anterior one is also 

 smaller than the posterior. This is no doubt due to the fact 

 that the eye of the sparrow is not symmetrical. Taking the 

 axis of vision as a median line, a horizontal section of the eye 

 shows that the two sides are not mirror-like repetitions of each 

 other and that the lens is asymmetrically placed. Owing to the 

 lateral location of the eyes in the head, such asymmetry tends to 

 assist binocular vision in the bird, since that part of the retina 

 which functions in distinct vision is more temporal. The axes 

 of V sion ^yill thus be more nearly parallel. 



TABLE 2 



Showing the dimensions of the sclerotic bones at different locations on the eye; also 



the distance they overlap each other at these locations. All 



measurements are in millimeters 



Anterior margin. 

 Dorsal margin . . . 

 Posterial margin 

 Ventral margin. . 



ANTEHIOPOSTE- 



RIOR 



DIMENSION 



1.022 

 1.408 

 1.280 

 1 . 536 



EQUATORIAL 

 DIMENSION 



1.600 



1.856 

 1.664 

 1.729 



DISTANCE EACH 

 OVERLAPS 



0.512 

 0.586 

 0.512 

 0.640 



THE CORNEA 



The cornea consists of three principal parts: 1, the anterior 

 layer, or epithelium; 2, the middle portion, or substantia pro- 

 pria, and 3, the posterior endothelial cells, or membrane of 

 Descemet (plate 11, fig. 66). 



The epithelium covering the front of the cornea is continuous 

 with the lining of the lids and constitutes the conjunctiva. It 

 consists of stratified epithelium. The innermost layer consists 

 of short and broad columnar cells, The external layer is com- 

 posed of very much flattened cells. Between these two layers 

 are found the transitional forms ranging from polygonal to the 

 flattened type. The thickness of this epithelial layer in the 

 center of the cornea is .014 mm. 



The middle portion, the substantia propria, forms the main 

 thickness of the cornea. It is the modified portion of the con- 



