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4. A thick granular layer of the function, of which we are ignorant and 5. The membrana Jacobi, which its discoverer, Dr. Jacob, Professor of Physiology in the Royal College of Surgeons, first described in the "Philosophical Transactions," 1819. His mode of preparing it is as follows:

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Bacillar, or Jacob's.

Outer granular.

Middle granular.

Inner granular.

Grey granular.

Nerve cells.

"Having procured a human eye, within fortyeight hours after death, a thread should be passed through the layers of the cornea, by which the eye may be secured under water, by attaching it to a piece of wax previously fastened to the bottom of the vessel-the posterior half of the sclerotic having been first removed. With a pair of dissecting forceps in each hand, the choroid coat should be gently torn open and turned down. If the exposed surface be now carefully examined, an experienced eye may perceive that this is not the appearance usually presented by the retina. Instead of the blue-white reticulated surface of that membrane, a uniform villous structure, more or less tinged by the black pigment, presents itself. If the extremity of the ivory handle of a dissecting knife be pushed against this surface, a breach is made in it, and a membrane of great delicacy may be separated and turned down in folds over

Optic fibres.

Section of the Layers of the Retina, after Kölliker.

the choroid coat, presenting the most beautiful specimen of a delicate tissue which the human body affords. If a small opening be made in the membrane, and the blunt end of a probe introduced beneath, it may be separated throughout without being turned down, remaining loose

Jacob's Membrane detached.

over the retina; in which state, if a small particle of paper or globule of air be introduced under it, it is raised so as to be seen against the light, and is thus displayed to great advantage; or it is sometimes so strong as to support small globules of quicksilver dropped between it and the retina, which renders its

[graphic]

membraneous nature still more evident. If a few drops of acid be added to the water after the membrane has been separated, it becomes opaque and much firmer, and may thus be preserved for several days, even without being immersed in spirit."

Dalrymple next described it as a serous sac, lining the choroid and retina. It is composed of rods which readily separate from each other, and the outer end of each is curved into a crook; from this appearance it has been called the bacillar layer. Some of the rods take a radiating direction.

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In the centre of the retina is the "punctum aureum, or yellow spot, which Sömmering discovered, in 1791, in a young man drowned in the Rhine a few hours before. The vessels and nerve-tubes of the retina do not pass over it, but curve about it. The spot has decidedly a small aperture in its centre, for through it the

interior of the eye can be seen. A yellow pigment, which water will dissolve out, gives a golden halo round it, so that its diameter exceeds half a line. Jacob's layer is continued across it and the colour is much brighter when it is removed. It has been only found in man, quadrumana, and a few reptiles; so that Sir E. Home's theory-that through it the absorbents entered- -seems improbable. In an erudite paper in the "Atlantis," July, 1858, Prof. Hayden, of the Catholic University, thus expresses his opinion upon its function :

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"I have been led to the conclusion that unity of visual perception with two eyes is inseparably associated with the yellow spot of Sömmering, and may invariably be inferred from its presence, and that this property is limited to a circle, having as its centre the foramen centrale, and a radius of about 04 inch. This circle I propose to designate as the region of visual unity,' within which the rays of light proceeding from an object must fall, in order to produce a single perception from a binocular impression. Outside this circle, and concentric with it, is another, having a radius of 0.533 inch the latter I would name the region of visual duality,' because I believe that the image of an object depicted within it, and outside the region of unity in both eyes simultaneously, produces a double perception in the sensorium. That portion of the retina extending from the region of duality to the ora serrata, is probably insensible to luminous impressions, and serves some other purpose in the economy of the eye, with which we are still unacquainted.'

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The Optic Nerve arises by the optic tracts from the quadrigeminal bodies and thalami. The tracts wind round the crus cerebri, being flattened against it by the middle lobe, and, on the olivary process, form with its fellow the optic commissure. In this body, besides a few vesicles which give it a glanglionic character, there are 4 kind of tubules: 1. Those which pass from one

tract, along the outer side of the corresponding nerve, to the eye of the same side. 2. Those which cross from one tract through the commissure to the opposite nerve, and are by far the most numerous. 3. Those which run from one tract back to the other, being thus intercerebral, and present in such animals as the mole, in which the optic nerves are wanting. 4. Those which are inter-retinal, by passing from one nerve to the other, and which were first described by that able physiologist and great physician whose loss we have recently had to deplore, Dr. Mayne, in Todd's "Cyclopædia." In the cod, all the fibres of one tract pass to the opposite nerve. Wollaston, in order to account for single vision with 2 eyes, stated that all the fibres of one optic nerve-say, the right-go to the right side of both the right and left eyes. As the optic nerve enters the optic foramen, the dura mater sends a strong sheath about it, which joins the sclerotic and dips down through the fibres, producing the appearance, at the entrance into the eye, of a "lamina cribrosa." The nerve suddenly diminishes before it passes through, owing to the white substance of Schwann ceasing, and ends by a small papilla, which is marked by the porous opticus, a spot produced by the central artery. The optic nerve enters about 1 lines to the inner side of the central axis, as it follows the direction of the orbit, whereas the eyes are parallel in man. Dr. Power, when lecturer on Anatomy in the Richmond Hospital School, discovered in the loligo and other animals, that the fibres of the optic nerve decussate behind the sclerotic, and he thinks this anatomical arrangement may explain erect vision, and "that the correct impressions made upon the sensorium is the result of the rays of light impinging on the terminations of the nervous fibres in the retina, which, crossing each other in their course backward, run to be attached to opposite points of the sensorium.' The arrangement, however, cannot be discovered in many

animals, and to render the explanation satisfactory, the fibres should be shown to decussate in every direction. His valuable memoir may be found in the "Dublin . Journal of Medical Science," 1843,

and one of the figures from it is here reproduced.

Light. The properties of light will be briefly discussed before considering the mechanism of that most admirable of optical apparatuses, the human eye.

Sir I. Newton believed

light to be an emission of particles from a luminous body; whereas, strangely enough, he applied the

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second theory, which is now more Decussation of the Fibres of the Optic Nerve. generally accepted, to another imponderable-sound. Boyle also suggested a molecular theory. Huygens first suggested that light consisted in a succession of waves from the luminous object, and this theory has obtained powerful support from the great work of the Rev. Dr. Lloyd, Vice-Provost of Trinity College, Dublin. Light travels with such rapidity that it is supposed to pass from the sun to the earth, a distance of 95,000,000 miles, in 8 min. 13 sec., or at the rate of 191,515 miles per second. Light is not homogeneous, but if passed through a prism is decomposed, owing to their unequal refrangibility, into 7 primitive coloursnamely, violet, 109; indigo, 47; blue, 48; green, 46; yellow, 27; orange, 27; red, 56. The numbers after each indicate their proportion, if the spectrum be supposed to measure 360. According to Sir David Brewster, it is composed of 3 primary colours-blue, yellow, and red. If these colours of the spectrum be passed through another prism, they are re-composed into white light.

A ray of light falling on a polished surface is reflected or thrown back at an angle, which is equal to that of

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