The source from which this fluid is derived has been the subject of controversy in con sequence of Nuck, a professor of anatomy at Leyden, having asserted that he had discovered certain ducts through which it was transmitted, and published a small treatise to that effect, which ducts were proved to be vesssels by a cotemporary writer, Chrouet, in which deci sion subsequent authors have concurred. In the present day this fluid is generally believed to be secreted by a membrane lining the cavity, as the fluid which lubricates the serous cavities is secreted by their lining membranes. Al though this is in all probability the fact, the circumstances are not exactly the same in both cases. In the serous cavities, merely as much fluid as moistens the surface is poured out, while in the chamber of the aqueous humour sufficient to distend the cavity is secreted. In the serous cavities the membrane from which they derive their name can be demonstrated ; in the chamber of aqueous humour this can scarcely be accomplished. I have resorted to various methods to enable me to demonstrate the existence of the membrane of the aqueous humour on the back of the elastic cornea, such as maceration, immersion in hot water, soaking in alcohol, and treating with acids, alkalis, and various salts, but without effect: In describing the structure of the cornea, I have shewn that the elastic cornea itself can not for a moment be considered the membrane in question, on account of its strength, thick ness, elasticity, and abrupt termination ; and I do not think that the demonstration of a serous membrane expanded on such a struc ture as transparent cartilage is to be expected, inasmuch as the demonstration of the synovial membrane on the cartilages of incrustation in the joints is attended with much difficulty. The pathological fact which tends most to prove the existence of such a membrane here, is, that in iritis, especially that of a syphilitic character, the aqueous humour appears often very muddy, especially in the inferior half of the chamber ; this, however, in the latter stages may be found to arise from a delicate speckled opacity on the back of the cornea, which re mains permanently, and injures vision con siderably. Analogy also favours the inference that the whole cavity of the chamber must be lined by serous membrane, inasmuch as all structures, of whatsoever nature they may be, in the serous or synovial cavities, are so covered or lined. This provision is so universal, that if such various structure, as the elastic cornea; iris, capsule of the lens, ciliary processes, and hyaloid membrane, which enter into the con struction of the chamber of aqueous humour, be exposed to the contact of the fluid without any intervening membrane, it constitutes an unexpected anomaly in the animal (economy. The consequences of inflammation greatly strengthen the conclusion that the cavity is lined by a membrane of the serous character. The slightest injuries or even small ulcers of the cornea are frequently accompanied by effu sion of purulent matter into the anterior chamber, from the extension of the inflam mation into that cavity, constituting the hy popion or onyx of the books; and the yellow masses which appear on the iris in syphilitic iritis, whether they are abscesses, or as they are called, globules of lymph, are efffisions beneath a delicate membrane, as vessels may be seen with a magnifying glass, ramifying over them. In iritis the rapidity with which adhesions are formed between the margin of the pupil and the capsule, proves that these two structures are covered by a membrane of this nature. In addition to all these facts the still more conclusive one is to be adduced, namely, that the membrane can without diffi culty be demonstrated on the back of the iris, as I have stated in speaking of that part of the organ, and as it is represented in fig. 127, where the fold of membrane stained with black pigment is seen turned down from that structure.
In the preceding pages I have availed my self of whatever valuable and appropriate facts in comparative anatomy I found calculated to illustrate or explain the structure of the human eye. There are, however, two organs in other animals which do not exist even in the most imperfect or rudimental state in the human subject—the peeten or tnarsupium uigrum in birds, and the choroid gland or choroid muscle in fishes.
Of the pecten.—This organ is called pecten from its folded form bearing some resemblance to a eoinb, and marsupium nigrum from its resemblance in the eye of the ostrich to a Ida( k purse, according to the anatomists of the Academy, who compiled the collection of memoirs on comparative anatomy. The organ is obviously a screen projected from the bottom of the eye forward toward the crys talline lens, and, consequently, received into a corresponding notch or wedge-shaped hollow in the vitreous humour; it appears to be of the same vascular structure as the choroid, and is deeply stained with the black pigment, which renders it perfectly opaque and imper vious to light. The annexed figure, from the
work of D. W. Siimmerring, represents it in the eye of the golden eagle.
It is composed of a delicate membrane, highly vascular, folded exactly like the plaits of a fan, and when removed with sharp scissors from the bottom of the eye, and its free margin cut along the edge so as to allow the folds to be pulled open, it may be spread out into a strip of continuous riband-shaped membrane, as seen in fig. 129, from a paper of Sir E. Home's in the Philosophical Transactions for 1822.
The first account I find of it is by Petit in the Mem. de l'Acad. Roy. 1735. Ile says it is a trapezium or trapezoid, five lines long at the base, and three lines and a half deep, com posed of parallel fibres, and that a fine trans parent filament runs from the anterior superior angle to the capsule of the crystalline lens, not easily seen on account of its transparency, and that sometimes the angle itself is attached to the capsule near its margin. Haller, in his work " Sur la formation du eceur dans le poulet," describes it as follows:—" It is a black membrane folded at very acute angles, as the paper of a fan, upon which transparent vessels are expanded; it generally resembles the ciliary processes. It originates from the sclerotic in the posterior part of the eye by a serrated line, pierces the choroid, retina, and vitreous humour to attach itself to the side of the capsule of the crystalline, very near the corona ciliaris. The posterior extremity is broad, and the anterior narrows till it becomes 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 Elementa Physiologic', 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. Ile says " 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 Minoru 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. Siimmerring says, that in the pecten of the golden eagle, of which fig. 128 is a representation, tbere are fourteen folds like ciliary processes, and that it adheres by a transparent filament to the capsule of the lens; that jn the great homed 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..