Of the retina. — This is the third spheri cally disposed membrane entering into the structure of the eye, and may be considered the most essential of all, being that which is endowed with the peculiar description of sensibility which renders the individual con scious of the presence of light. It is as exactly fitted to the inside of the choroid as that membrane is to the sclerotic, but does not extend to the anterior margin of the choroid as that structure extends to the anterior margin of the sclerotic. The retina is destined to be penetrated by the rays of light, which, reflected from surrounding objects, are collected to form images on the bottom of the eye, consequently its extension as far forward as the choroid or sclerotic is unnecessary, and nature makes no thing superfluous. It is discontinued at the posterior extremities of the ciliary processes of the choroid, at the distance of about an eighth of an inch from the anterior margin of that membrane.
The retina is evidently the optic nerve ex panded in the bottom of the eye in the form of a segment of a sphere. That nerve differs, in some respects, in construction from the other nerves of the body. In its course from the hole in the bone through which it enters the orbit until it enters the eye, it is of a cylindrical form, and proceeds in a waving line to its desti nation. The medullary fibres are involved in a tough strong material, not separable into cords or bundles as in other nerves, but constituting a cylinder of collected tubes, from the divided extremity of which the medullary matter may be squeezed in as soft and pulpy a foini as it exists in the brain. It is not easy to determine by anatomical investigation, whether the medullary material is disposed in tubes or in a cellular structure, but as that material is universally disposed in a fibrous form, both in brain and nerve, it is more than probable that it is so ar ranged here. These cerebral fibres involved thus in a cylindrical bundle of tubes, techni cally called ncuri/cma by modern anatomists, is covered externally by a fine transparent membrane, adhering to it so closely that it re quires some care to separate it; and this is again covered by a tube of strong fibrous mem brane, the sheath of the optic nerve continued from the dura muter to the sclerotic, to which membrane it adheres so firmly, that it cannot be separated except by the knife. Formerly the sclerotic was considered to be a continuation of the dura mater, and much importance, in a pathological point of view, was attached to the circumstance, but although both structures are of the fibrous class, the sclerotic is very different in texture, and the adhesion between them is not more remarkable than any other of the numerous adhesions which occur between fi brous membranes.
Where the optic nerve enters the eye, it is contracted in diameter, as if a string had been tied round it, and then passes through a hole in the sclerotic, to which it adheres. When
seen from the inside, after removing the retina and choroid, it appears in the form of a circu lar spot, perforated with small holes, from which the medullary material may be expressed. This is the lamina cribrosa of Albinos, consi dered to be a part of the sclerotic, but which is .really nothing more than the terminating ex tremity of the nerve.
The optic nerve does not enter the eye in the centre of the globe, but about an eighth of an inch to the side of it, assuming the centre to correspond to the extremity of a line passing from the middle of the cornet, through the centre of the eyeball to its back. The nerve is generally described and represented as pro jecting in the form of a round prominence, as it enters the eye ; but this is not, I believe, the state of the part dining life, but is produced by the contraction of the neurilema pressing out the medullary matter in this form. As the nerve enters the eye, it immediately expands into and constitutes the retina, the medullary fibres separating and spreading out on the sphe rical vitreous humour. The expansion of the nerve in separate fibres cannot be distinctly seen in the human eye, but may be recognized with some care in the eye of the ox, and with out difficulty in that of the hare and rabbit, where it divides into two bundles, as has been well described by Zinn in the GUttingen Com mentaries.
The retina does not consist of medullary or cerebral fibrous matter alone. As the brain has its pia muter and arachnoid membrane, and the nerve its neurikma, this nervous struc ture has its appropriate provision for its sup port and the distribution of its vessels. This is the vascular layer, first accurately described by Albinos. It is a delicate transparent mem spews a few shreds of the membrane remaining a week or more after birth.
brane, of such strength, that when detached, it may be moved about in water, and freely ex amined without breaking. It adheres so firmly to the hyaloid membrane of the vitreous hu mour in the fresh eye, that it cannot be sepa rated entire, and the medullary fibres adhere so closely to its external surface, that they can not be detached at all in the form of a distinct membrane. To demonstrate the vascular layer, the sclerotic should be carefully removed, leav ing a portion of the optic nerve freed from its sheath ; the choroid should then also be re moved under water, by tearing it asunder with a pair of forceps in each hand. The vitreous humour, covered by the retina only, should then be allowed to remain about two days in the water, at the end of which time the me dullary layer softens and separates into flakes, which may be scraped from the vascular layer beneath by passing the edge of a knife gently over it, after which the vascular layer may be detached by careful management, and sus pended in a bottle from the optic nerve.