904 



EYE. 



adherent to the capsule of the lens by an inser- 

 tion a little narrower. This insertion appears 

 to be effected by the intervention of the hyaloid 

 membrane, to which this fan is attached. I 

 have not had time to establish this con- 

 nexion to my satisfaction, and I still entertain 

 doubts respecting it. I have seen a red artery 

 accompany this feather-like production and run 

 to the crystalline. It would be very convenient 

 for physiology that this folded membrane 

 should prove muscular ; we should then have 

 the organ sought after, which would retract the 

 crystalline to the bottom of the eye." In the 

 F.lcmenta Physiologia?, t. v. p. 390, he says it 

 originates from the entrance of the optic nerve, 

 but that you may remove the retina and leave 

 the pecten. He says again, " it advances for- 

 ward to the posterior part of the capsule, to 

 which it sometimes adheres by a thread, and 

 sometimes the lens is merely drawn toward it." 

 An artery and vein is supplied to each fold, 

 and perhaps to the capsule of the lens. In 

 the Opera Minora he says that there are two 

 red vessels to each fold in the kite, and no cord 

 runs to the lens ; that in the heron a branch of 

 artery runs to each fold, and it adheres so 

 closely to the lens that it cannot be ascertained 

 whether a red vessel runs from it to the lens 

 or not; that in the duck it is contracted toward 

 the lens, and adheres to it by a thread contain- 

 ing a red vessel. He also says that in the 

 wild duck it arises from the margin of the 

 linea alba, which terminates the entrance of 

 the optic nerve, contains numerous vessels, 

 and adheres to the lens ; and in the pie it is 

 large and adheres to the lens, so as to pull it. 

 D. W. Sommerring says, that in the pecten 

 of the golden eagle, of which Jig. 128 is 

 a representation, there are fourteen folds like 

 ciliary processes, and that it adheres by a 

 transparent filament to the capsule of the lens ; 

 that in the great horned owl it is short and 

 thick, with eight folds, and adhering to the lens 

 by an hyaloid filament, although at a great 

 distance from it; and that in the macaw it is 

 longer than broad, has seven folds, and adheres 

 to the lens. In the ostrich he says it is shaped 

 like a patella at its base, which is white, oval, 

 and thick ; eight lines long and five broad, 

 distinctly separate from the choroid, above 

 which it rises, the retina being interposed. 

 From the longer diameter of this patella (or 

 base) a white plane or lamina projects even up 

 to the lens, and sends out on each side seven 

 small plaits, the lower ones partly double, the 

 upper ones simple, black, and delicate. This 

 conical body, something like a black purse, 

 tapers toward the lens, and by its apex is 

 attached to the capsule by a short semi-pellucid 

 ligament. The white substance of the base 

 and partition of the pecten should not be con- 

 founded with the medullary part of the optic 

 nerve, which, emerging on all sides from be- 

 neath the base, expands into a great, ample, 

 and tender retina, terminating behind the 

 ciliary processes with a defined margin. Cuvier, 

 in his Lectures on Comparative Anatomy, says, 

 " It appears of the same nature as the choroid, 

 although it has no connexion with it ; it is like- 



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 aAer death are not absolutely con- 

 clusive ; moreover, as it is attached to the side 



