" Let us contrast this account of the matter with the common one. The retina, a mem brane of such delicacy, is described as being extended between the vitreous humour and choroid, from the optic nerve to the ciliary processes, being merely laid between them, without any connection, and the medullary fibres in contact with a coloured mucus re tained in its situation by its consistence alone. This account is totally at variance with the general laws of the animal economy ; in no instance have we parts, so dissimilar in nature, in actual contact: wherever contact without connection exists, each surface is covered by a membrane, from which a fluid is secreted ; and wherever parts are united, it is by the medium of cellular membrane, of which se rous membrane may be considered as a mo dification. If the retina be merely in contact with the vitreous humour and choroid, we argue from analogy, that a cavity lined by serous membrane exists both on its internal and external surface : but this is not the fact. In the eye a distinction of parts was necessary, but to accomplish this a serous membrane was not required ; it is only demanded where great precision in the motion of parts was indis pensable, as in the head, thorax, and abdo men; a single membrane, with the interpo sition of cellular substance, answers the pur pose here. By this explanation we surmount another difficulty, the unphilosophical idea of the colouring matter being laid on the choroid, and retained in its situation by its viscidity, is discarded ; as it follows, if this account be correct, that it is secreted into the interstices of fine cellular membrane here, as it is upon the ciliary processes, back of the iris, and pecten, under the conjunctiva, round the cornea, and in the edge of the membrana nictitans and sheath of the optic nerve in many animals. Dissections are recorded where fluids have been found collected between the choroid and retina, by which the structure of the latter membrane was destroyed ; the ex planation here given is as sufficient to account for the existence of this fluid, as that which attributes it to the increased secretion of a serous membrane." The membrane is represented as it exists in the eye of the sheep, in the annexed figure, from my paper in the Medico-Chirurgical Transactions.
Mr. Dalrymple, in his valuable work on the anatomy of the eye, takes a different view of the arrangement of this part of the retina : he says :—" From observations made on the human eye, in connection with other expe riments on the eyes of animal, I am induced to consider it as a double reflected serous mem brane. I was first led to take up this opinion in the year 1827, by the accidental observation of a very delicate membrane, which lined and was adherent to the entire choroid. Having minutely injected the eye of a sheep, I made a vertical transverse section through the sclero tic, choroid, and retina, which last membrane, with Jacob's tunic, properly so called, and the vitreous body I removed. I then placed the remaining portion of the eye in dilute spirits. of wine, intending to preserve it for the ex hibition of the tapetum, which in this instance was remarkably beautiful. A few minutes after its immersion the tapetum lost to a con siderable extent its brilliant hue, and I re moved it from the glass to wash from its sur face some deposit, which I thought might have obscured its polish. In doing this, how ever, I detached a delicate membrane, mi nutely filled with injection, and this membrane it was which on being placed in the spirit, became slightly opaque and produced the effect alluded to ; for the tapetum thus denuded in stantly recovered, and still retains its bril liancy."
The inference that the membrane in ques tion is a double reflected serous membrane is certainly more in conformity with analogy than the assumption that it is a single layer, but this uniformity in nature's operations has been too much insisted upon. I have above stated my reasons for considering it a single layer, and not a double serous membrane; and I should be inclined to think that the layer which Mr. Dal rymple found adhering to the choroid was the membrane itself, which had not come away with the retina and vitreous humour, as I have found sometimes to happen, did not Mr. Dal rymple further state that he has " in his pos session a preparation, which does most dis tinctly chew the double portions of this mem brane; one lining the choroid, the other reflected over the pulpy structure of the retina." Mr. Jones, in the work formerly alluded to, gives the annexed representation of the mem brane as it appears when highly magnified. Fig. 113 is a representation of the membrane by Mr. Bauer, magnified fifty diameters, from the Philosophical Transactions for 1822.
In the centre of the retina, and consequently in the axis of vision, about an eighth of an inch from the entrance of the optic nerve, a very remarkable condition of structure exists. This is a small point destitute of cerebral or medullary fibres, appearing like a hole in the membrane, and hence called the foramen of Summerring, from the distinguished anatomist who discovered it. This point is surrounded by a yellow margin, and the retina is here also ,puckered into a peculiar form of fold. Slim merring, in the Commentationes Societatis Gottingenses, gives the following account of the discovery. " On the 27th of January, 1791, while I examined the eyes of a very fine and healthy young man, a few hours previously drowned in the Rhine, being per fectly fresh, transparent, and full, and sup ported in an appropriate fluid, with the in tention of exhibiting a perfect specimen of the retina to my pupils in the anatomical theatre, I so clearly detected in the posterior part of the retina, which was expanded without a single fold, on account of the perfect state of the eye, a round yellow spot, that I vas convinced it was a natural appearance, and not a colour produced by any method of preparation. In examining this spot more accurately, I perceived in its centre a little hole occupying the situation of the true centre of the retina. With the same care I examined the other eye and found it exactly similar. I then communicated the discovery to my pupils in the public demonstrations." " In this precise spot, or in the very centre of the re tina, is found an actual deficiency of the me dullary layer, or a real hole perfectly round, with a defined margin a fourth of a line in diameter." " The transparent vitreous humour and black pigment are so clearly seen through this hole, that there can be no doubt that it is n real nnortnr. which be' , mg situatedin the centre of the retina may be appro priately termed the foramen centrale. Sur rounding this foramen centrale the remark able yellow colour resembling that of gum gutter is so disposed that it appears much deeper toward the margin, and totally dis appears at a distance of a line. This colour varies much according to the age of the individual, being very faint in infants, much deeper at puberty, on account of the thickness and whiteness of the retina at that period, appearing of a deep yellow brownish or crocus colour. In more ad vanced age the colour is less intense, prin cipally on account of the diminished whiteness of the retina, which also appears extenuated at that period. Even the . .