bird's-eye views. 



11 



could be gained of them, were they merely represented at 

 the isolated points where they cross the vertical plane ; and 

 they are therefore introduced somewhat artificially. The sac- 

 rifice of theoretical accuracy is more than compensated for 

 by increased perspicuity. 



Recollecting that the "eyeball" — as we shall continue to 



call it for convenience sake — is filled with fluid that presses 



equally in every direction, we cannot at first make out how its 



Yi^,2* peculiar shape 



is maintained' 

 But the reason 

 why the ball 

 does not as- 

 sume a spheri- 

 cal shape really 

 is plain enough 

 when we come 

 to dissect its 

 coats. They 

 are partly Sony. 

 They are splin- 

 ted, as it were, 

 with the bones 



(A, h) that are packed alongside each other all around the 

 circumference of one part of the ball. 



The large discoidal segment of a bird's eye is mostly 

 made up of a membrane called the sclerotic, from its hard, 

 dense structure. It is a thick, strong, tough membrane, of 

 a glistening livid, or grayish blue color. Three sclerotic 

 coats or layers, difiering from each other a little in texture, 

 may be demonstrated by careful dissection, though on super- 

 Fig. 2.* Vertical antero-posterior section through middle of eyeball, a, optic nerve; 

 b, sclerotic, outer coat; c, sclerotic, middle and inner coats; d, choroid; e, hyaloid; /, 

 marsupiiim ; g, cornea ; h, h, bony plates between layers of sclerotic ; i, i, cornigations 

 of choroid, foniiing the ciliary processes ; k, k, canal of Petit ; I, I, iris ; m, anterior 

 chamber; n, capsule of lens; o, lens ; p, posterior chamber. Neither the retina, nor the 

 peculiar sheathing of the optic nerve, is shown. The nerve, the marsupium, and the 

 ciliary processes, do not wholly fall within a vertical section through the middle of the 

 eye, and cannot be represented in this figure except artificially. 



