of which are of the most interesting character, and most instructive to the student of pathology.

I propose now to pass from the review of those parts of the eye which lie within the range of the operations usually performed on the globe, and to devote the rest of this lecture to a brief sketch of the structure of the retina; for I should be unwilling to conclude the present short course without some account, however imperfect, of that portion of the eye, which, in a physiological sense, is the most essential of all, and which will be found to be as wonderful and elaborate in structure as it is important in function.

OF THE RETINA.

The retina is that peripheral nervous sheet on which the images of external objects are received. It is continuous with the optic nerve, and is expanded within the globe between the choroid and vitreous humor, as far forwards as the ora serrata, the situation of which has been already pointed out. Its surfaces may be styled choroidal or outer, and hyaloid or inner; and by these surfaces it is organically connected, on the one hand with the choroidal epithelium, on the other with the hyaloid membrane. It has the pinkish-gray colour of the surface of the cerebral convolutions, is very soft and easily torn, and is arranged in certain layers, the inner of which contain the bloodvessels that impart the pink tint to the whole, while the outer are nonvascular. All the superposed layers of the retina are thicker at the bottom of the eye, around the entrance of the optic nerve, than in front, near the ora serrata; and the entire nervous sheet becomes gradually thinner forwards, until it ends abruptly at the line indicated, being there continuous with that granular tissue which lines the ciliary processes of the choroid, and gives origin to the fibrous part of the suspensory ligament of the lens.

Constituents of the retina.-Now, the retina contains in itself all the structural elements which are to be found in other parts of the system, except nerve-tubules, which are not present in the human retina, nor in the retina of the higher animals, but only in the optic nerve; and it moreover contains, besides these, other structural elements not elsewhere met with, but peculiar to this part, and which

we are therefore led to suspect may be in some way or other subservient to the proper action of the retina as a recipient of the vibratory impressions of light.

The elements common to the retina with other portions of the nervous system are placed internally, or towards the hyaloid surface. These are:-1. Gray fibres, radiating on all sides from the entrance of the optic nerve, towards the anterior border of the retina, and being a continuation of the nerve-tubules of the optic nerve. They are gradually less abundant forwards, terminating in succession among the next mentioned elements. 2. Gray nervous matter, similar to the cineritious part of the cerebral convolutions (being an amorphous, finely granular matrix, containing nucleated nerve-vesicles). Caudate nucleated globules, analogous to those found in the ganglia, spinal cord, and certain parts of the brain. 4. Agglomerated granules, usually highly refractive, with very little intervening material, and allied to the nuclei of cells, such as are met with in some portions of the encephalon. Capillary bloodvessels are distributed among all but the last of these.

3.

The elements peculiar to the retina are situated externally, and together form the coat commonly known as Jacob's membrane. They are of two kinds-5. Columnar particles, or rods, arranged vertically in a single series; and 6. Bulbous particles, interspersed at regular intervals among the former. Both of these are found among the lower animals in many most remarkable modifications, some few of which I shall presently mention, ou account of their singularity, and to shew that they probably play an important part in the physiology of vision, though into the nature of their function we have as yet no particular insight. These elements, like the agglomerated granules, have no bloodvessels proper to them.

Of the gray fibres of the retina.-We may now pass these several elements more distinctly in review: and first, of the gray fibres. If we make a section of the coats of the eyeball through the part at which the optic nerve traverses them to join the retina, we see that this nerve becomes reduced in bulk as it is passing through the sclerotica, so that a transverse section of it, where it approaches the sclerotica, has nearly double the area of its intra-sclerotic termination, and the sclerotic canal is a truncated cone. We also observe that whereas the nerve behind, and for a little way within, the sclerotic

GREY FIBRES OF THE RETINA.

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canal, is opaque white, the tubules having their proper investment of white substance, it becomes gray and semi-transparent ere it touches the retina, and the retina itself has never any white glistening aspect such as the nerves have. In different animals, indeed, even among the mammalia, you will find great variety as to the precise point at which the nerve loses its whiteness, this point being sometimes only at the very junction of the nerve with the nervous expansion within; and in certain cases (of which the rabbit is the best example) the nerve advances a certain way within the choroid, and spreads out on the surface of the retina before it loses its whiteness, so that the retina in these animals appears to present a white area of an oval shape, and an eighth of an inch long, at the sclerotic aperture; and in some animals yet lower in the scale, nerve-tubes, with a very delicate layer of white substance, can be traced even further, and in more uniform distribution over the retina. But still it remains true, as I believe, for all, and certainly for man, that nerve-tubules, such as form the optic nerve, do not exist as a part of the retina, and that where they enter within the sclerotica, they are to be regarded as still the optic nerve in its course to the retina.

Now, what is the nature of the change in the constituent tubules of the optic nerve by which they lose their whiteness as they penetrate the sclerotica? They certainly do not terminate in the sclerotic canal; they cease to be characterized by their dark outline, and by their tendency to fall into the varicose or beaded state, but remain fibrous; in a word, they lose their white substance, but retain their axis or central fibre, and these fibrous parts coming together, advance and form the gray fibres of the retina. fibres of the retina. I have made many very thin sections through the nerve and retina in connection; and you will find, if you do the same, that these fibres, on entering the globe and encountering the hyaloid, pour themselves as it were on all sides in bundles over the hyaloid surface of the retina, and become coated at once, on their opposite or choroidal surface, with the elementary structures which I have enumerated as forming the other strata of the retina.

It follows, of course, that in the space occupied by the evolution of these grey fibres from the optic nerve, i. e. for the area of the inner orifice of the optic foramen or sclerotic canal, these other strata of the retina do not exist that the retina, in fact, does not exist; therefore it is no wonder that this spot should be blind-insusceptible

G

FIG. 13.

Section of the coats of the human eye at the entrance of the optic nerve, to shew the mode of origin of the layers of the retina. s, sclerotica; c, choroid; , plexiform bundles of optic nerve; o, line at which these lose their white substance; 9, grey fibres advancing to the retina, and becoming clothed on their choroidal surface with other layers, constituting r, the retina.

of stimulation by light. The blindness of the spot (proved by a wellknown experiment), in connection with the anatomical fact which I now point out, shows how essential to the visual power of the retina are its non-fibrous parts, so that we might almost say that the visual impression is received by the non-fibrous parts, and merely propagated by the fibrous; that the true retina is not an expansion of the optic nerve, but a nervous organ of independent structure, brought into co-operation with the brain through the nerve. But to proceed with the anatomy of the grey fibrous stratum.

In some animals (I allude particularly to some fishes) their radiation over the whole retina may be very easily made evident to any one by a slight maceration, for the retina shaken in water becomes divested of all but these fibres, and they seem to form a brush directly continuous with the nerve.

But in the higher animals it is usually more difficult to demonstrate such a disposition; both because this layer is much less readily detached from the rest, and because its fibres are disposed in bundles, which, after anastomosing together for some way, become blended with each other into a uniform lamina, and are lost among the grey nervous matter (2). In the fresh human retina they may be seen by looking directly on the inner surface, near the optic nerve, with a power of 50 diam. (fig. 14.)

The bundles there are large, but of different sizes, and anastomose so as to form very elongated meshes, in which large nucleated vesicles soon begin to appear. The bundles of the plexus are not cylindrical,

GREY VESICULAR MATTER.

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FIG. 14.

Anastomosing grey fibres of human retina, seen on their hyaloid surface, near the optic nerve: magnified.

but much compressed on their contiguous sides, so that, on a vertical section, they appear oval; otherwise this plexiform arrangement is a mere continuation of that of the bundles of tubules in the optic nerve, now spread out as a sheet, instead of being gathered up into a cylindrical cord.

Of the central artery and vein.—While speaking of the manner of evolution of the optic nerve at its coalition with the retina, it may be mentioned that the blood-vessels of the retina enter and leave it along the centre of the optic nerve, by two fibrous canals there provided, among the fibrous meshes in which the plexiform nerve-bundles lie. Arrived within the sclerotica, they subdivide and ramify upon the retina, the large branches which they form being, for a short way, interposed between the hyaloid and the plexiform grey fibres, but very speedily sinking in among these, and breaking up, by successive divisions, into the capillaries which supply and occupy the substance of those layers to which I have already described them to belong. Just within the ora serrata the plexus terminates by a marginal vessel.

2. Of the grey vesicular matter of the retina.-This lies contiguous to the hyaloid surface, in close relation with the last mentioned layer. It is the most vascular coat of the retina, and, in fact, receives the greater portion of the blood brought by the retinal artery. The capillaries form a very beautiful plexus, with meshes about as close as those of the grey matter of the cerebral convolutions, though arranged nearly on one plane. The walls of the capillaries are a simple membrane, with nuclei at intervals. It is easy, at a suitable period after death, to wash out the nervous matter from their intervals, and to obtain a separate view of the whole vascular system of the retina; in a perfectly recent specimen, also, the capillaries can be discerned among the matter of the layer now under consideration, often with the red corpuscles still within them. The finely granular matter of this

and