354 JAMES ROLLIN SLONAKER 



The axes of vision in the eye of a dead bird form an angle of 

 about 65 degrees with the median plane. This is due to the 

 lateral position of the eyes in the head (fig. 29, AF, plate 6). 

 The angle formed by the axes of vision of the two eyes is 130°. 

 With this wide angle it does not seem possible for binocular 

 vision to exist along the axes of vision. In life the ability to 

 reduce this angle is very marked. Owing to the very short 

 nerve and the close fitting orbit, I do not think it possible for 

 binocular vision to occur in which the fovea of each eye is in- 

 volved. The experiments bearing on this will be described 

 later. 



Fig. 1 Diagram showing the comparative size of the human and the English 

 sparrow eye. X 2." 



As in all vertebrates, the eye of the sparrow consists of three 

 concentric layers which contain three refracting media. The 

 three layers are: 1) the sclerotic, or supporting structure; 2) the 

 chorioid, or vascular tunic, and 3) the retina, or nervous layer. 

 The refracting media are: 1) the aqueous humor in front of the 

 lens; 2) the crystalline lens, and 3) the vitreous humor behind 

 the lens. Before dealing with these structures the following 

 accessory parts wdll be described: a) the eyelids; b) the lacrimal 

 apparatus; c) the extrinsic muscles; d) the blood supply to the 

 eye; e) the nerve supply to the eye. 



