wise very delicate, very vascular, and imbued with black pigment. Its vessels are derived from a particular branch of the ophthalmic artery, different from two which belong to the choroid ; they descend on the folds of the black membrane and form ramifications there of great beauty when injected. This mem brane penetrates directly into the vitreous humour, as if a wedge had been driven into it ; it is in a vertical plane directed obliquely forward. The angle nearest the cornea in those species in which it is very broad, and all its anterior margin in those in which it is narrow, comes nearly to the inferior boundary of the capsule of the crystalline. In some species it approaches so near that it is difficult to say whether or not it is attached to it ; such is the case in the swan, the heron, the turkey, &c. according to Petit ; but there are other birds in which it remains at some distance, and in which it does not appear to attach itself except to some of the numerous plates which divide the vitreous humour into cells. In the swan, heron, and turkey, this membrane is broader in the direction parallel to the produced extre mity of the optic nerve than in the contrary direction. In the ostrich, cassowary, and owl the reverse is observed. It is folded like a sleeve in a direction perpendicular to the caudal termination of the optic nerve. The folds are rounded in most species ; in the ostrich and cassowary they are compressed and sharp, and so high perpendicular to the plane of the membrane that at first sight it resembles a black purse. The folds vary in number, there being sixteen in the swan, ten or twelve in the duck and vulture, fifteen in the ostrich, and seven in the grand duke or great horned owl.
The purpose for which the pecten exists in the eyes of birds does not appear to be fully ascertained. Petit says, " when a bird views an object with both eyes, the rays enter oblique ly in consequence of the situation of the cornea and crystalline lens, and proceed to the bottom of the eye; but as they enter in lines parallel to the membrane, they do not encounter it. The rays which enter the eye in lines perpen dicular to the plane of the cornea encounter this membrane, and are absorbed by it as well as those which come from the posterior side ; the subject is, however, a difficult one." Haller supposed that it was merely destined to afford a medium through which vessels might pass to carry blood to the crystalline. Cuvier says, " It is difficult to assign the real use of this membrane. Its position should cause part of the rays which come from objects at the side of the bird to fall upon it. Petit believed that it was destined to absorb these rays and prevent their disturbing distinct vision of objects placed in front. Others thought, and the opinion has been lately reiterated by Home, that it possesses muscular power, and that its use is to approach the lens to the retina when the bird wishes to see distant objects. Never
theless, muscular fibre cannot be detected in it, and the experiments intended to prove its muscularity after death are not absolutely con clusive; moreover, as it is attached to the side of the crystalline, it could move it only obliquely." The experiments and inferences contained in Sir E. Home's paper in the Phi losophical Transactions for 1796, do not appear to me worthy of any attention. A pecten in an imperfect or rudimentary state appears to exist in fishes and reptiles, and has been noticed by Haller, W. Siimmerring, and Dr. Knox. In the article Ayes of this work Mr. Owen has also described the pecten, and to that arti cle I refer the reader for additional information.
Of the choroid gland or choroid niusck.— The eyes of fishes present several remarkable peculiarities, to he accounted for perhaps from their occasional residence in the obscurity of the deep, and at other times near the surface, exposed to the full blaze of sunshine; they must also be frequently exposed to great pres sure at considerable depths. The sclerotic is not merely a fibrous membrane, but is strength ened by a cartilaginous cup, and sometimes even by one composed of bone; the cornea is generally flat or presenting little of lenticular character; the crystalline lens is spherical, and so dense that its central part is a hard solid; and the choroid presents the remarkable pecu liarity which I hare now to describe.
On cutting through the cartilaginous sclerotic, a fluid is found generally interposed between this and the choroid; at least it is so in the genus gadus, (cod, haddock, &c.) The external part of the choroid is formed by a most beau tiful membrane of a brilliant silver aspect, scarcely to be distinguished from that metal when rough and recently cleaned. On tearing this membrane away, the vascular choroid is exposed, and a red horse-shoe-shaped promi nent mass, encircling the entrance of the optic nerve, appears. This is the choroid gland or choroid muscle. The veins of the choroid, apparently commencing from the iris, ascend in tortuous inosculating branches, of enormous size compared with the dimensions of the part, and appear to terminate by entering this horse shoe-shaped organ, but this is not their distri bution, as it is not hollow. The area enclosed by the organ round the optic nerve does not exhibit the same extreme vascularity. On pulling away a delicate film which covers the organ, it appears composed of Mminr or plates divisible into fibres, which run transversely from within outwards, confined into a compact body by the delicate film just spoken of, and a concave depression in the structure beneath. The annexed plate, made from an accurate drawing of a careful dissection, represents the general form and vascularity remarkably well.