136 



ANATOMY OF VERTEBRATES. 



56 



compared with the brain and with the entire head, fig. 56, being 

 analogous, in this respect, to the eyes of some of the flying 

 insects. Their form is admirably adapted to promote the objects 

 above named. The anterior segment of the eye is more promi- 

 nent than in any other class of animals, and is in many Birds 



prolonged into a tubular form, 

 terminated by a very convex cor- 

 nea, fig. 137, 6? ; the Owl fur- 

 nishes the best example of the 

 disproportion between the ante- 

 rior and posterior divisions of the 

 globe, the axis of the anterior 

 portion being twice as great as 

 that of the other. This gives 

 room for a greater proportion of 

 aqueous fluid, and by removing 

 the crystalline lens from the 

 retina, causes a greater converg- 

 ence of the rays of light, by which 

 the nocturnal bird is enabled to 

 discern the objects placed near it, 

 and to see with a weaker light. 

 The anterior division of the eye 

 is least convex in the Swimming 

 Birds. The antero-posterior dia- 

 meter is to the transverse as 19 to 26 in the Swan, and as 17 to 

 20 in the Duck. 



The sclerotic coat, fig. 57, b, is divisible into three layers. 

 It is thin, flexible, and somewhat elastic posteriorly, where it 

 presents a bluish shining appearance, but anteriorly its form is 

 maintained by a circle of osseous plates or scales, ib. a, fig. 26, 17, 

 interposed between the exterior and middle layers. These plates 

 A ary from thirteen to twenty in number, and are situated imme- 

 diately behind the cornea, with their edges overlapping each 

 other. They are in general thin, and of an oblong quadrate 

 figure, becoming elongated from before backward in proportion 

 as the bird possesses the power of changing the convexity of 

 the cornea. In the Owls they extend from the cornea over the 

 long anterior division of the eye to the posterior hemisphere, 

 which they also contribute to form. The figure of the eye is 

 thus maintained, notwithstanding its want of sphericity. 



The bony plates are capable of a degree of motion upon each 

 other, which is, however, restrained within certain limits by the 



Cerebral nerves, eyes, &c. in situ, of a Goose. 



