268 ORGANS OF THE SENSES. 



the vitreous humour (104. B. h.) The retina terminates 

 with a thickened edge at the beginning of the ciliary pro- 

 cesses, and a similar structure is presented in most of the 

 chelonian reptiles. 



The eyes of birds are adapted in all their parts for the 

 rare medium through which they receive the rays of light, 

 and for sudden changes in the density of that medium, 

 and in the intensity of light, and for the varying distances 

 and directions of their objects of vision, and by their high 

 development and their magnitude they compensate for the 

 imperfect condition of most of the other organs of sense. 

 From the lateral position of the eyes and the great projection 

 of the cornea they command an extensive field of vision, 

 and from this circumstance and the great mobility of their 

 head and their long neck the large organs of vision of 

 birds are less moveable in their orbits than those of qua- 

 drupeds. They are the most remote in structure and 

 form from those of fishes, which accords with the difference 

 of density in the media of vision, and the fluids now 

 most abundant in their interior are the least refractive, 

 the aqueous and the vitreous, while the crystalline lens 

 is flattened in its form and reduced in the density of 

 its texture and in the space which it occupies in the 

 axis of the eye. To prevent the sphericity of the eye, 

 from the equal pressure of the contained fluids, and 

 to preserve a great convexity of the transparent cornea 

 especially in rapaceous birds, the anterior margin of the 

 sclerotic is strengthened by a circular series of quadrangular 

 moveable imbricated osseous plates, disposed between its 

 coats around the edge of the cornea as in many reptiles, 

 and which often give a conical or even a tubular form 

 to the fore-part of the eye, as seen in the large nocturnal 

 eyes of the long eared owls. The tough posterior mem- 

 branous part of the sclerotic, forms a great hemisphere 

 occupied almost entirely by the vitreous humour, and filling 

 the very large orbits of the cranium, and from the quantity 

 of space occupied by the thin and yielding aqueous and 

 vitreous humours in the eyes of birds, they are probably 

 more easily and quickly adjusted to the different distances 

 of surrounding objects, and to the varying density of the 

 medium through which they see. The thin convex dense 



