cnormo, where It corresponds to this fora men, sometimes appears a little deeper coloured." In the paper above alluded to, published in the Medico-Chirurgical Transactions, I have given the result of some careful quiries into the structure of this part, from which the following observations are tracted. " Sommerring describes it as a hole in the retina with a yellow margin, mentioning as accidental a fold which occupies the situation of this hole and tends to conceal it, and thus accounting for its remaining so long unnoticed. This appearance is so constant and remarkable, that its existence may be very rationally considered essential to correct vision, and it therefore becomes an interesting object of speculation. The circumstances which it seems important to ascertain, are, ther it is actually a hole in the retina with a yellow margin ; whether, in addition to this hole, the retina it folded or puckered in at this part; or whether the appearance of a hole arises from a deficiency of the medullary layer of the retina without any orifice in its vascular layer. Both Siimmerring himself and many others seem to consider that the fold is accidental and the consequences of changes occurring after death. It is here necessary to call to mind what those changes are with respect to the retina. If the eye had become flaccid previous to dissection, the retina on being exposed presents an gular surface, arising from a number of folds diverging from the optic nerve as from a centre, and evidently produced by the loss of support from the partial evaporation of the fluid of the vitreous humour. These folds, however, never observe any regular form, or preserve precise situations, and may be obliterated by changing the position of the eye in the water. They disappear altogether after the part has remained some time in water, in consequence of the vitreous humour becoming again 'distended from imbibing the fluid in which it is mersed. It however requires no very great care or experience to distinguish between those accidental folds and the peculiar one in tion. If the examination be made from out, removing the sclerotic and choroid behind, the retina appears to be forced or drawn at this point into the vitreous humour to the depth of about a twelfth of an inch, the entire fold being something more than an eighth in length. At first there is little or no appearance of a hole, but after the eye has remained for some time in the water, the fold begins to give way, and a small slit makes its appearance, which gradually widens, and assumes the appearance of a round hole. This hole is large in pro portion to the degree to which the fold has yielded ; and when the fold totally disappears, as it sometimes does, the transparent point gives the appearance which Sommerring re presents, of a hole with a yellow margin. If, instead of making the examination in this way from the outside, we view this part through the vitreous humour, the appearance of the hole is more remarkable ; but still that part of the retina is evidently projected forward be yond the level of the rest of that membrane. In the eye of a young man, which I had an opportunity of examining under peculiarly favourable circumstances, within five hours after death, I noticed the following appear ances. The cornea and iris having been cut away, and the lens removed from its situation, I placed the part in water, beneath one of the globular glasses, and held it so as to allow the strong light of a mid-day sun to fall directly upon it ; when the retina to the outside of the optic nerve presented unequivocally the ap pearance of being drawn or folded into the form of a cross or star, with a dark speak in the centre, surrounded by a pale yellow areola. I further satisfied myself of the prominence of the fold by holding a needle opposite to it, while the light shone full upon it, a shadow being thus cast upon the retina which deviated from the straight line when passed over the situation of the fold. To ascertain whether there is actually a hole in the retina, or merely a deficiency of nervous matter at this point, I allowed the eye to remain for some days in water, until the connexions of the parts began to give way. I then introduced a small probe between the retina and vitreous humour, the part still remaining in water, and bringing the blunt point of the instrument opposite the transparent spot, attempted to pass it through, but found I could not do so without force sufficient to tear the membrane. I also re moved the nervous matter by maceration and agitation in water, and on floating the vascular layer, found that I could no longer ascertain where the spot had originally existed, there being no hole in the situation previously occu pied by the transparent speck." It is remarkable that theforamcn of Sommer ring has not been found in the eyes of any of the manimalia except those of the guadrumana, in some of whom it has been detected by Home, Cuvicr, and others, but the extent to which it may be traced in this tribe has not been satis factorily ascertained. Dr. Knox, in a paper in the Memoirs of the Wernerian Natural History Society, announces the discovery of its existence in certain lizards. In the lacerta superciliosa he says, " the retina is very thick, and somewhat firm and opaque. Where the optic nerve enters the interior of the eye-hall, there is a distinct marsupiuni or black circular body, proceeding forwards apparently through the centre of the vitreous humour. Anteriorly, somewhat supe riorly and towards the mesial line or plane, we perceive, on looking over the surface of the retina Which regards the vitreous humour, a comparatively large transparent, nearly circular spot, through which may be distinguished the dark-coloured choroid. Close to this is gene rally placed a fold or reduplication of the retina, which is in general remarkably distinct. This fold or folds, (for there are more than one) either proceed from the transparent point towards the insertion of the optic nerve, or close to it. Sometimes the fold seems, as it were, to lie over the transparent point, and partly to conceal it from view ; or the point is formed in the edge of the fold itself, as in apes, but in general the fold runs directly from the insertion of the optic nerve upwards and inwards, pressing very close to the edge of the foramen centrale." The foramen was also seen in the lacerta striata, lacerta calotes, and others, while it was not to be detected in the gecko, crocodile, and some others. It was also subsequently discovered in the chameleon. The annexed figures represent the foramen of Sommerring in the human eye. A, shews the retina expanded over the vitreous humour : on the right is the place from which the optic nerve was cut away, and from which the ves sels branch out : on the left is the foramen of Sommerring, represented by a black dot sur rounded by a dark shade. B, shews the retina with a portion of the optic nerve. The exter nal membrane is turned down as in the pre ceding representation of the same structure in the sheep's eye, and the Iiiramen of Sommer ring, instead of a distinct hole, presents the appearance of a fold or depression with elevated sides. The wood-engraving does not admit of the delicacy of finish necessary to express per fectly this condition of the part.
Fig. 114.
A. B.
There is no part of the anatomy of the eye respecting which there has been so much diver sity of opinion as the anterior termination of the retina. It has already been stated that it extends to the posterior extremities of the ciliary processes, where it is discontinued, pre senting an undulating edge corresponding to the indented margin of this part of the carpus ciliare. Some assert that it extends to the mar gin of the lens, others that it is the vascular layer only which extends so fan, and others that the vascular layer extends over the lens. No one however at present, wbo describes from observation, denies the termination of the ner vous layer at the posterior margin of the ciliary body, although many insist upon the extension of the vascular layer to the circumference of the lens. The subject has received more attention than it deserves, as it involves no consideration of importance, either physiological or anato mical; but I am convinced from a very care ful scrutiny that no such layer extends between the ciliary processes of the choroid and those of the hyaloid membrane ; these two parts being mutually inserted into each other, as will pre sently be explained. In the paper above quoted in the Medico-Chirurgical Transactions I have explained what appears to me to be the arrangement of this part in the following words : " On removing the choroid, ciliary processes, and iris, we see the retina terminating with a defined (Imitated margin, about a quarter of an inch from the circumference of the lens : be tween this line of termination and the lens, the vitreous humour retains upon its surface part of the black pigment which covered the ciliary processes. If the eye be examined shortly after death, removing the black pigment from this part of the vitreous humour with a camel hair pencil, there is an appearance of, at least, the vascular layer being continued to the lens; this part not being so transparent as the rest of the hyaloid membrane, or so opaqu e as the retina.
From such an examination I was led to con clude that the vascular layer was continued to the margin of the lens, this part not being so transparent as the rest of the hyaloid membrane, or so opaque as the retina. From such an examination I was led to conclude that the vascular layer was continued to the margin of the lens, but I adopted a con trary opinion after I had witnessed the change which took place when the part had remained twenty-four hours in water : the retina then separating with a slight force, and frequently detached by the disturbance given in making the examination. If, after removing the choroid without disturbing the retina, the part be al lowed to remain in water for some days, the medullary part of the retina begins to give way, and may be altogether detached by agita tion in water, leaving the vascular layer firmly attached at the line of termination just de scribed. With all the care I could bestow, I have, however, never succeeded in separating this layer from the vitreous humour further. I f the maceration be continued for a few days longer, the vascular layer of the retina gives way, the larger vessels alone remaining attached at the original line of termination of the retina, and appearing to enter the hyaloid membrane at this part ; the appearance which at first so much resembled the vascular layer proceeding towards the lens remaining unchanged, being in fact part of the vitreous humour itself. The circumstance which has most strengthened the notion of the retina being continued forward to the lens is, that often on raising the choroid and ciliary processes from the vitreous humour, we find 'bow processes covered in several places by a fine semitransparent membrane insinuated between the folds ; this is supposed to be the vascular layer of the retina, but is really the corresponding part of the hyaloid membrane which is torn up, being firmly united to this part of the choroid." After this article had been prepared for press, I received an admirable monograph upon the retina by B. C. It. Langenbeck, son of the celebrated professor of that name in the Uni versity of Gottingen, in which the nature, structure, and relations of this most important and interesting part of the organ are subjected to a critical and elaborate inquiry. Ile advo cates the membranous nature of the black pig ment on the inner surface of the choroid, and gives an engraving of its organization as ascer tained by the microscope, resembling that given from the essay of Mr. Jones in the preceding pages. He devotes several pages to the de scription of the membrane which I found covering the medullary layer of the retina, and adds the testimony of a skilful anatomist in support of my description, sufficient to coun terbalance the convenient scepticism of certain writers better skilled in making plausible books than difficult dissections. The fibrous struc ture of the medullary layer of the retina is established, and a plate given of the peculiar nodulated condition of these fibres. The work concludes with an account of the morbid changes of structure observed in the retina, a subject which, notwithstanding its manfest importance, has not hitherto attracted the atten tion which it deserves. I am indebted to Dr. Graves for the following abstract of some recent investigations of Treviranus on the same subject. " From microscopical examinations Treviranus demonstrates that the cerebral mass, both medullary and cortical, consists of hollow cylinders containing a soft matter. These cylinders, extremely minute in the cortical substance, are somewhat larger in the medul lary, and still larger in the nerves. In the retina he finds, that after the optic nerve has penetrated the sclerotic and choroid, its cylin ders or nervous tubes spread themselves out on every side either singly or collected into bundles, each cylinder or collection of tubes bending inwards through the vascular layer, and terminating in the form of a papilla on the vitreous humour." Of the vitreous humour.—It has already been stated that the globe of the eye is divided into two chambers by the iris, the posterior of which is distended by a spherical transparent mass called the vitreous humour, which does not completely fill this chamber between the back of the iris and the hollow sphere of the retina, but is discontinued or compressed at a short distance from the back of the iris, having a narrow space between it and that membrane, called the posterior chamber of the aqueous humour. This trans parent mass is composed of water containing certain saline and animal ingredients, deposited in exquisitely delicate and perfectly transparent cellular membrane; hence it is capable of sus mining its own weight and preserving its form when placed in water, and in air presents the appearance of a gelatinous mass, scarcely de serving the name of solid. The cellular struc ture, in which the watery fluid is lodged, has been called the hyaloid membrane, and the whole mass denominated the vitreous humour. The fluid of the vitreous humour, according to Berzelius, is composed of water, containing about one and a half per cent. of animal and saline ingredients ; it has a saline taste, and acquires a slight opaline tint by being boiled. It consists of water 98.40, chlorurct of soda with a little extractive matter 1.42, a substance solu ble in water 0.02, and albumen 0.16. Its specific gravity is 1.059. When the hyaloid membrane is examined in its natural state, its cellular organization can scarcely be ascertained on account of its transparency ; but if it be suspended on the point of a pin until the fluid is allowed to drop out, it may be inflated with a fine blowpipe and dried, or if the whole be placed in strong spirit or weak acid, the mem brane becomes opaque, and its organization obvious. It has been supposed that the cells in which the fluid is lodged present a determi nate form, and attempts have been made to prove this by freezing the eye and examining the frozen fragments ; but any one who has seen the hyaloid membrane rendered opaque by acid must allow that the cells are too minute to admit of such investigation, and that the frozen masses, supposed to be the contents of cells, are merely fragments of the hyaloid membrane with their contained fluid. Although the hyaloid membrane is perfectly transparent, and the red particles of the blood do not circu late in its vessels, there can be little doubt that its growth and nutrition are effected by the circulation of a transparent fluid in vessels continuous with those conveying red blood. It is an established fact that transparent tex tures which in a natural state do not exhibit a trace of coloured fluid, when excited or inflamed, become filled with red vessels, as may be seen in the conjunctiva. It is there fore reasonable to admit that the hyaloid mem brane does not present a deviation from this general law. The fluid of the vitreous humour, it is to be presumed from analogy, is secreted by the vessels of the hyaloid membrane, and if no red vessels can be detected, the secretion must be accomplished by transparent ones. It has already been stated that the vascular layer of the retina adheres to the surface of the vitreous humour, and that the points of adhe sion are stronger along the course of the vessels than in the intermediate spaces; it is therefore most probable that the more superficial part of the sphere is supplied with transparent blood from the arteries of the retina, while a branch directly from the central artery, as it penetrates the pores opticus, enters behind, and extends to the back of the lens : such a branch can be injected in the fcetus, and is found to ramify on the back of the capsule of the lens; and in the eyes of large quadrupeds a transparent production, probably vascular, has been ob served proceeding from the entrance of the optic nerve into the mass of the vitreous humour. It is also probable that the ciliary processes of the choroid, which arc buried in the hyaloid membrane anteriorly, supply blood to that part of the sphere. That the vitreous humour undergoes changes analogous to those which take place in textures supplied with red blood, is proved by its hyaloid membrane being found opaque and thickened in eyes have been destroyed by internal inflam mation. A total disorganization of the vitreous humour is a frequent occurrence, the hyaloid membrane losing its cohesion to such a degree that the fluid escapes from the eye as freely as the aqueous humour when the cornea is divided in the operation of extraction ; and after the lens and its capsule have been removed by operations with the needle, opacity of the hyaloid membrane is occasionally, although rarely, observed. Allusion has frequently been made in books to an appearance in the eye denominated glaucoma, attributed, rather vaguely, to opacity of the vitreous humour; it appears, however, to be nothing more than the usual opacity of the lens which occurs in advanced life, seen through a dilated pupil. As an additional proof of the vascularity of the vitreous humour may be adduced the fact, that in the eyes of sheep, injured by blows in driving to the shambles, the vitreous humour is deeply tinged with red blood.