The spherical mass of vitreous humour, it has already been stated, is exactly fitted into and adheres to the inner surface of the retina. From the anterior termination of the retina to the posterior chamber of the aqueous humour, it is in contact with, and adhering to, the ciliary processes of the choroid. Where it is truncated or compressed on its anterior part to form the posterior chamber of the aqueous humour, it has the crystalline lens fitted into a depression in its centre, while a narrow circle of it appears between the circumference of the lens and the anterior extremities of the ciliary processes of the choroid, forming part of the boundaries of this chamber of aqueous humour.
If the eye be allowed to remain for a day or two in water in order to destroy by maceration the delicate connexions between the hyaloid membrane and the choroid, and then the vitreous humour with the lens attached care fully separated, the point of a fine blowpipe may be introduced under the surface of the hyaloid membrane at the circumference of the lens, and a series of cells encircling the lens inflated. This is the canal of Petit, or canal godronn. It is thus described by the dis coverer in the Histoire de l'Academie des Sciences for 1726. " I have discovered a small canal surrounding the crystalline, which I call the circular canal godronne; it can be seen only by inflating it, and when filled with air it forms itself into folds similar to the ornaments on silver plate, called for this reason Vaiselle godronne. It is formed by the doubling of the hyaloid membrane, which is contracted into cells at equal distances by little canals which traverse it, and which do not admit of the same degree of extension as the membrane, which is very feeble; it thus becomes godronni. If the crystalline be removed from its capsule without injuring the membrane which forms this canal, these godronni folds are not formed by inflation or only in a very slight degree, but the canal becomes larger. It is in man commonly a line and a quarter, a line and a half or two lines in breadth, and not larger in the ox." An nexed is a representation of this canal of Petit on a large scale.
As the nature of the connection between the choroid and the hyaloid membrane, the formation of the posterior chamber of the aqueous humour, and the structure of this canal of Petit, have been the subject of contro versy, I venture to introduce here an extract on this subject from the paper published by me in the Aledico-Chirurgieal Transactions.
" If the sclirotic, ehoroid, iris, and retina be removed one or two days after death, leaving the vitreous humour with the lens embedded on its anterior part, we observe a number of stria on the vitreous humour, converging towards the circumference of the lens, cor responding in number, size, and form to the ciliary processes, giving the same appearance collectively that the circle of ciliary processes or corpus ciliare does on the choroid, and nar rowed towards the nasal side as the corpus ("Marc is. This appearance has been noticed by most authors, but some describe it as arising merely from the marks left by the ciliary processes, while others consider these stria of the same nature as those productions of the choroid, and call them the ciliary pro cesses of the vitreous humour ; it is the corona ciliaris of Camper and Zinn. If we remove the black pigment with a camel-hair pencil, we leave those productions on the vitreous humour more distinctly marked than when covered by the colouring matter, and presenting all the characters above stated, commencing behind with a well-defined margin, and terminating anteriorly by attachment to the capsule of the lens, the furrows between them capable of receiving the ciliary processes of the choroid, and the folds calculated to be lodged in the corresponding furrows of these processes. The annexed figure is a representation of the vitreous humour of the human eye thus treated.
" If the cornea and iris be removed from a human eye within a few hours after death, a dark circle surrounding the lens, between it and the anterior extremities of the ciliary pro cesses, may be observed : this is the part of the corona ciliaris of the vitreous humour to which the ciliary processes of the choroid do not extend, which appears dark on account of its perfect transparency ; the converging stria are evident, even on this part where the ciliary processes are not insinuated, interrupting the view if we attempt to look into the bottom of the eye by the side of the lens. It is, in my opinion, therefore certain, that part of the vitreous humour enters into the formation of the posterior chamber of the aqueous humour. The demonstration of this fact is, however, attended with difficulty, because the flaccidity arising from even slight evaporation of the fluids of the eye permits the ends of the ciliary processes which present themselves in the posterior chamber of the aqueous humour to fall towards the circumference of the lens, and appear attached there. For myself I can say that having made the dissection in the way just pointed out, the eye of course in water, and beneath one of those globular vessels which I formerly described, I could see to the bottom of the eye through the space in front of the vitreous humour, between the ciliary processes and the margin of the lens ; this space is, however, perhaps larger in some individuals than in others. Each fold of the corona ciliaris
of the vitreous humour seems to consist of two layers of hyaloid membrane, capable of being separated one from the other byinflation, and ad mitting of communication with each other round the lens. It appears to me that the canal of Petit or canal godronni is formed in consequence of these folds receiving the injected air one from the other ; it is, however, generally described as being formed by the membrane of the vitreous humour splitting at the circumference of the lens, one layer going before and the other behind that body, the canal existing between these two layers and the capsule of the lens. That the capsule of the lens has no share in the formation of the canal of Petit, I conclude from filling this canal with air, and allowing the part to remain for some days in water, and then with great care removing the lens included in its capsule ; this I do not find, however, causes the air to escape from the cells, but leaves them presenting nearly the original appearance ; and after the air has escaped, I can pass a small probe all round in this canal, raising by this means the folds from the hyaloid membrane.. It is difficult, however, to pre serve the air in these folds for any length of time under water, because the tendency of the air to ascend causes the rupture of the membrane, by which it is allowed to escape. After the lens, included in its proper capsule, has been detached from its situation on the vitreous humour, the space it occupied pre sents the appearance of a circular depression, surrounded by those productions of the hyaloid membrane of which 1 have just spoken ; the vitreous humour remaining in every respect perfect, notwithstanding this abstraction of the lens." AI. Ribes, in the M6moires de la Societd Medicare d'Emulatien for 1816, describes the ciliary processes of the vitreous humour as follows. " At the anterior part of the vitreous humour, and at a short distance from the cir cumference of the crystalline, may be seen a ciliary body almost altogether similar to that of the choroid, and which has been named by anatomists corona ciliaris, but no writer has hitherto pointed out its structure, or the impor tant office it appears to perform. Each of these processes has a margin adherent to the vitreous humour, and encroaches a little on the circumference of the lens. It appears to me impossible to ascertain whether the surfaces are reticulated, but they are villous. The free margin is obviously fringed, and presents nearly the variety of appearance observed in the fringes of ciliary processes (of the choroid) of different animals examined by me, except that the summits are black ; the interval which separates each process of the vitreous humour is a species of depressed transparent gutter. The black colour of the free margins and the transparency of the space which separates each ciliary process adorns the anterior part of the vitreous humour with a circle remarkable for its agreeable effect, and which has been com pared to the disc of a radiated flower." Dr. Knox, in a communication made to the Royal Society of Edinburgh, at the same time that mine was made to the Medico-Chirurgical Society, describes the ciliary processes of the choroid as follows: " In whatever way, the membrane or assemblage of membranes pro ceeds forwards to be inserted into the circum ference of the capsule of the lens, forming in its passage numerous longitudinal folds, and small projecting fimbriated bodies, by which, in a natural state, the transparent humours are connected with the superjacent ciliary body (of the choroid); when examined with a good glass, these folds are remarkably distinct, and the whole bears the closest resemblance in its distribution to the true ciliary body and pro cesses. I have, therefore, ventured to call them the internal or transparent ciliary body, or the ciliary body of the hyaloid membrane, in contradistinction to that of the choroid." It must not be forgotten that these ciliary pro cesses of the hyaloid membrane were described by Mourn in his Treatise on the Eye, and are strongly marked in a coarsely executed plate. Ile considered that the retina was continued to the lens, and describes its course under the ciliary processes of the choroid ; thus " on ex amining the retina with still greater accuracy, it appears that it has exactly the same number of folds or doublings that the choroid coat has; for it enters double between the ciliary pro cesses, nearly in the same way that the pia mater enters into the coats of the brain. the furrows and doublings of the retina, which, if we are to use the favourite term ciliary, may be called its ciliary processes, make an impres sion on the anterior part of the vitreous hu mour." The structure alluded to was also observed by llovius nearly an hundred years before.