270 GENERAL ORNITHOLOGY part ii 



in the figure ; but some birds' eyes are much more tubular in front, 

 — owls', for example. The eyeball being hollow and filled with 

 fluids which jjress in all directions, it is hard to see at first how 

 such a peculiar shape is maintained. But the sclerotic coat is very 

 dense, almost gristly in some cases ; and it is reinforced by a circlet 

 of hones, the sderotals, h, h; see also Fig. 62, where the circlet is 

 shown. These are packed alongside eacli other all around the cir- 

 cumference of one part of the sclerotic, like a set of splints. The 

 large discoidal segment of a bird's eye is mostly composed of the 

 membrane called from its hardness the sclerotic, — thick, tough, and 

 strong, of a glistening livid colour. Three sclerotic coats or layers 

 may be demonstrated by careful dissection ; in the figure b is the 

 outer, c the combined middle and inner ones, — much exaggerated 

 as to their distinctness. The bony plates lie between the outer and 

 middle coats anterior to the greatest girth of the eyeball, extending 

 from the rim of the disk nearly or quite to the edge of the cornea. 

 They are a dozen to twenty in number, of oblong squarish shape, 

 tapering toward the cornea, around which they are thus circularly 

 disposed ; they are pretty closely bound together, but the circlet as 

 a whole enjoys some little motion back and forward with the vary- 

 ing convexity of the cornea, g. This last is the thin transparent 

 membrane completing the eyeball in front, like the crystal over 

 the face of a watch. It is very protuberant in birds, — even a 

 hemisphere, or almost tubular. Its structure is not pecvdiar in 

 birds ; but it is remarkable in this class of creatures not only for 

 its convexity, but for the wide range of the variability in convexity 

 which increased or diminished pressure of the contained humours 

 may effect, and its collapse in death. 



The sclerotic coat is lined with the choroid membrane, d, loosely 

 woven of cellular tissue, replete with blood-vessels, and painted 

 pitcli-black with a heavy deposit of pigment-cells. It lines the 

 whole globe as far forward as the edge of the sclerotal bones, where 

 it splits in two layers. The inner choroid layer turns away from 

 the wall of the eye, toward the interior, and in so reflecting becomes 

 plaited, as a bag is puckered by pulling the strings. These pleats 

 converge upon the rim of the delicate capsule enclosing the lens of 

 the eye, n, and there adhere, forming the ciliary processes, i, i. The 

 outer layer also starts away from the circumference of the sclerotic 

 wall, as if to pass directly across the cavity, but ends in the iris. 

 Around the circumference of the iris, Avhere sclerotic, corneal, and 

 choroid coats come together, is a circular band of fibres, the ciliary 

 ligament ; and on the outer surface of the choroid is a similar band 

 of circular and radiating contractile fibres, the ciliary muscle. These 

 ciliary structures are supposed to be the agents of the accommodat- 

 ing faculty of the eye, acting upon the lens to alter its shape or its 



