150 PRACTICAL ORNITHOLOGY. 



the anterior edge of the sclerotica as to seem continuous with 

 it ; but some have supposed it to be inserted as if into a groove. 



The Sclerotic Bones, represented by Fig. 8, are in this eye 

 fifteen in number, and, aUhough not precisely equal in size, 

 arranged in a particular manner. The lowermost, a, overlaps 

 those next to it with both its margins ; the next on the right 

 hand overlaps by its right margin its successor, and all the rest 

 lie in the same relative position, to the number in all of nine, 

 exclusive of the first, until we come to that marked c, which 

 is analogous to a, being overlapped by those on each side of it. 

 From a to c, there are only four, exclusive of these two, which 

 overlap it in the same manner as the nine on the other side, 

 but in the contrary direction. The bone opposite to a is h, and 

 were the arrangement symmetrical, ought to be the central ; 

 but the two key-bones of the arch, a and c, are not opposite to 

 each other. 



Let us now make a vertical section of the eyeball, and take 

 note of the appearances disclosed, Fig. 9. Here we observe 

 first the external coat, the sclerotica, a ; within it a delicate layer, 

 the choroid coa^, covered with a dusky substance, the pifjmentum 

 nigrum, b ; anteriorly, and commencing at the posterior edge 

 of the sclerotic bones, a zone having a plaited or fibrous ap- 

 pearance, c, which at its anterior margin is attached to the 

 lens, d, a round, considerably flattened, transparent body ; then 

 another zone coming off near the base of the cornea, or the iris, 

 e, having a circular hole in its centre, the pupil ; and, lastly, 

 the anterior transparent coat of the eye, or the cornea, f. 

 The optic nerve is seen at g, penetrating the sclerotic coat, en- 

 tering an oblique sheath, and reappearing internally in a nar- 

 row slit, h ; from the side of which rises an elongated plaited 

 membrane, /, named the pecten. The optic nerve, on entering 

 the eye, expands into a very delicate pulpy layer named the 

 retina, or net, which how^ever is not a very appropriate term, 

 it being not a piece of net-work, but a delicate pulpy substance. 

 This internal cavity of the eyeball is filled with fluid con- 

 tained in a filmy transparent membrane. The space behind 

 the lens, d, is occupied by a fluid named the vitreous, ©r glassy ; 

 and that anterior to the lens is filled by another named the 



