cular and pigmentary membrane; it is of a dark brown color externally, and of a deep black internally.

The outer portion is made up of a rich network of vessels, the branches of the ciliary arteries and veins; the inner portion, the pigmentary layer, is a delicate membrane formed of hexagonal cells, containing black pig

ment.

The Function of the choroid is mainly to absorb the rays of light which pass through the retina, and thus prevent them from interfering with the distinctness of vision by being again reflected upon the retina.

The Iris is a circular, muscular diaphragm, placed in the anterior por

SCLEROTIC COAT REMOVED TO SHOW THE CHOROID, CILIARY MUSCLE AND NERVES. a. Sclerotic coat. b. Veins of the choroid. c. Ciliary nerves. d. Veins of the choroid. e. Ciliary body. f. Iris.-From Holden's Anatomy.

tion of the eye, and perforated a little to the nasal side of the centre by a circular opening, the pupil; it is attached by its periphery to the point of junction of the sclerotic and cornea. It is composed of a connective tissue stroma, blood vessels and non-striated muscular fibres, circular and radiating. The circular fibres surround the margin of the pupil like a sphincter, and are controlled by the 3d pair of nerves; the radiating fibres (dilators of the pupil) radiate from the centre toward its circumference, and are controlled by the sympathetic system of nerves.

The Ciliary muscle is a grayish circular band, consisting of unstriped muscular fibres, about one-eighth of an inch long, running from before

backward; beneath the radiating fibres are small bands of circular fibres running around the eye. It arises from the line of junction of the sclerotic, cornea and iris; passing backward it is attached to the outer surface of the choroid; it is the principal agent in accommodation, and innervated by the 3d pair of nerves.

The Retina forms the internal coat of the eye; in the fresh state it is a delicate, transparent membrane, but soon becomes opaque and of a pinkish tint; it extends forward almost to the ciliary processes, where it terminates in the ora serrata. In the posterior portion of the retina, at a point corresponding to the axis of vision, is a rounded, elevated yellow spot, the limbus luteus, having a central depression, the fovea centralis; about 1 of an inch to the inner side is the point of entrance of the optic nerve, where it spreads out to assist in the formation of the retina. The arteria centralis retina pierces the optic nerve near the sclerotic, runs forward in its substance and is distributed in the retina as far forward as the ciliary processes.

The Retina consists of nine distinct layers, from within outward, supported by connective tissue. 1. Membrana limitans interna. 2. Fibres of optic nerve. 3. Layers of ganglionic corpuscles. 4. Molecular layer. 5. Internal granular layer. 6. Molecular layer. 7. External granular layer. 8. Membrana limitans externa. 9. Layer of rods and cones.

In the Fovea centralis, at the point of most distinct vision, all of the layers disappear except the layer of rods and cones, which becomes somewhat longer and more slender.

The Aqueous humor is a clear fluid, alkaline in reaction, occupying the anterior chamber of the eye; this chamber is bounded in front by the cornea, posteriorly by the iris.

The Vitreous humor forms about four-fifths of the entire ball. It supports the retina, and is excavated anteriorly for the reception of the lens; it is transparent, of a jelly-like consistence, and surrounded by a structureless, transparent membrane, the hyaloid membrane.

The Crystalline lens is situated immediately behind the pupil, in the concavity of the vitreous humor. It is inclosed in a highly elastic, transparent membrane, the capsule. The lens is a transparent, doubly-convex body, of an inch transversely, 4 of an inch antero-posteriorly; it is held in position by the suspensory ligament, formed by splitting of the hyaloid tunic, the external layer of which passes in front of the lens, the internal layer behind it. Its function is to refract the rays of light and bring them to a focus upon the retina.

Vision. The eye may be regarded as a camera obscura, in which images of external objects are thrown upon a screen, the retina, by means of a double convex lens.

The Essential Conditions for proper vision are: 1. Certain refracting media, e. g., cornea, aqueous humor, and crystalline lens, by which the rays of light are so disposed as to form an image. 2. A diaphragm, the iris, which, by alternately contracting and dilating, increases or diminishes the amount of light entering the eye. 3. A sensitive surface, to

DIAGRAM OF A VERTICAL SECTION OF THE EYE.

1. Anterior chamber filled with aqueous humor. 2. Posterior chamber. 3. Canal of Petit.

a. Hyaloid membrane. b. Retina (dotted line). c. Choroid coat (black line). d. Sclerotic coat. e. Cornea. f. Iris. g. Ciliary processes. h. Canal of Schlemm or Fontana. i. Ciliary muscle. (From Holden's Anatomy.)

receive the image and transmit the luminous impressions through the optic nerve to the brain. 4. A contractile structure, the ciliary muscle, which can so manipulate the lens as to enable external objects to be seen at near or far distances.

The Refracting Apparatus, by which parallel rays of light are brought to a focus on the retina, consists mainly of the crystalline lens, though aided by the cornea and aqueous humor. A ray of light passing through the pupil is refracted and concentrated by the lens at a given point pos

terior to it. For the correct perception of images of external objects, the rays of light must be accurately focused on the retina; in order that this may be accomplished, the lens must have a certain density and a proper curvature of its surfaces. When the lens is too convex, its refracting power is greatly increased, the rays of light are brought to a focus in front of the retina, and the visual perception becomes dim and confused. When it is too flat, the rays are not focused at all, and the resulting perception is the same. The Crystalline lens, therefore, produces a distinct perception of the outline and form of external objects.

Action of the Iris. The iris, consisting of contracting and dilating fibres, transmits and regulates the quantity of light passing through its central aperture, the pupil, which is necessary for distinct vision.

If the light be too intense or excessive, the circular fibres contract under the stimulus of the 3d pair of nerves, and the aperture is diminished in size; if the quantity of light be insufficient, the dilating fibres contract under the stimulus of the sympathetic, and the pupillary aperture is

increased in size.

The Retina, which is formed partly by the expansion of the optic nerve, and partly by new nervous structures, is the membrane which receives the impressions of light. Its posterior surface, which is in contact with the choroid, and especially the layer of rods and cones, is the sensitive portion, in which the rays of light produce their effects.

The point of most distinct vision is in the macula lutea, and especially in its central depression, the fovea, which corresponds to the central axis of the eye; it is situated about of an inch to the outside of the entrance of the optic nerve. It is at this point that images of external objects are seen most distinctly, while all around it the perceptions are more or less obscure; at the macula all the layers disappear except the layer of rods and cones.

Blind Spot. At the point of entrance of the optic nerve is a region in which the rays of light make no impression, owing to the absence of the proper retinal elements; the fibres of the optic nerve being insensible to the action of light.

The course which a ray of light takes is as follows: After passing through the cornea, lens, and vitreous humor and the layers of the retina, it is finally arrested by the pigmentary layer of the choroid; here it excites in the layer of rods and cones some physical or chemical change, which is then transmitted to the fibres of the optic nerve, and thence to the brain, where it is perceived as a sensation of light.

The Accommodation of the eye to vision for different distances is accomplished by a change in the convexities of the lens, caused by the action of the ciliary muscle. When the eye is accommodated for vision at far distances, the structures are in a passive condition and the lens is flattened; when it is adjusted for vision at short distances, the convexities of the lens are increased.

When the Ciliary muscle contracts and draws the choroid coat forward, the suspensory ligament is relaxed and the lens becomes more convex, in virtue of its own elasticity.

Optical Defects. Astigmatism is a condition of the eye which prevents vertical and horizontal lines from being focused at the same time, and is due to a greater curvature of the eye in one direction than another.

Spherical aberration is a condition in which there is an indistinctness of an image from the unequal refraction of the rays of light passing through the circumference and the centre of the lens; it is corrected mainly by the iris, which cuts off the marginal rays, and only transmits those passing through the centre.

Chromatic aberration, in which the image is surrounded by a colored margin, from the decomposition of the rays of light into their elementary parts, is corrected by the different refractive powers of the transparent media in front of the retina.

Myopia, or short-sightedness, is caused by an abnormal increase in the antero-posterior diameter of the eyeball; the lens being too far removed from the retina, forms the image in front of it, and the perception becomes dim and blurred. Concave glasses correct this defect, by preventing the rays from converging too soon.

Hypermetropia, or long-sightedness, is caused by a shortening of the antero-posterior diameter; the lens consequently focuses the rays of light behind the retina. Convex glasses correct this defect, by converging the rays of light more anteriorly.

Presbyopia is a loss of the power of accommodation of the eye to near objects, and usually occurs between the ages of 40 and 60; it is remedied by the use of a convex eye-glass.

Accessory Structures. The muscles which move the eyeball are six in number; the superior and inferior recti, the external and internal recti, the superior and inferior oblique muscles. The four recti muscles, arising from the apex of the orbit, pass forward and are inserted into the sides of the sclerotic coat; the superior and inferior muscles rotate the