The capillary net-work close to the pupillary margin of the iris. Anastomosis with vessels of outer coats. Anastomosis with branches of short posterior ciliary arteries. Anastomosis with chorioideal vessels. Course of vasa ciliar. Episcleral artery. Episcleral vein. Capillaries of lamina choriocapillaris. Circulus iridis major cut across. Branches to ciliary body. Branches to iris. Vena ciliar. Junction with the circulus iridis major.
Junction with lamina choriocapill. Arterial, and d 1. Venous episcleral branches. Arterial, and e 1. Arterial, and f 1. Venous branches to corneal border. Vena vorticosa. The Iris. It is a thin, circular, contractile disk, suspended in the aqueous humor between the cornea and lens, and perforated a little to the nasal side of its center by a circular aperture, the pupil.
By its periphery it is continuous with the ciliary body, and is also connected with the posterior elastic lamina of the cornea by means of the pectinate ligament; its surfaces are flattened, and look forward and backward, the anterior toward the cornea, the posterior toward the ciliary processes and lens.
The iris divides the space between the lens and the cornea into an anterior and a posterior chamber. The anterior chamber of the eye is bounded in front by the posterior surface of the cornea; behind by the front of the iris and the central part of the lens. The posterior chamber is a narrow chink behind the peripheral part of the iris, and in front of the suspensory ligament of the lens and the ciliary processes.
In the adult the two chambers communicate through the pupil, but in the fetus up to the seventh month they are separated by the membrana pupillaris. In front is a layer of flattened endothelial cells placed on a delicate hyaline basement membrane.
This layer is continuous with the endothelium covering the posterior elastic lamina of the cornea, and in individuals with dark-colored irides the cells contain pigment granules.
The stroma stroma iridis of the iris consists of fibers and cells. The former are made up of delicate bundles of fibrous tissue; a few fibers at the circumference of the iris have a circular direction; but the majority radiate toward the pupil, forming by their interlacement, delicate meshes, in which the vessels and nerves are contained. Interspersed between the bundles of connective tissue are numerous branched cells with fine processes.
In dark eyes many of them contain pigment granules, but in blue eyes and the eyes of albinos they are unpigmented.
The muscular fibers are involuntary, and consist of circular and radiating fibers. The color of the iris is produced by the reflection of light from dark pigment cells underlying a translucent tissue, and is therefore determined by the amount of the pigment and its distribution throughout the texture of the iris. The number and the situation of the pigment cells differ in different irides.
In the albino pigment is absent; in the various shades of blue eyes the pigment cells are confined to the posterior surface of the iris, whereas in gray, brown, and black eyes pigment is found also in the cells of the stroma and in those of the endothelium on the front of the iris.
The iris may be absent, either in part or altogether as a congenital condition, and in some instances the pupillary membrane may remain persistent, though it is rarely complete.
Again, the iris may be the seat of a malformation, termed coloboma, which consists in a deficiency or cleft, clearly due in a great number of cases to an arrest in development. In these cases the cleft is found at the lower aspect, extending directly downward from the pupil, and the gap frequently extends through the choroid to the porus opticus.
In some rarer cases the gap is found in other parts of the iris, and is not then associated with any deficiency of the choroid. Each of the two long ciliary arteries, having reached the attached margin of the iris, divides into an upper and lower branch; these anastomose with corresponding branches from the opposite side and thus encircle the iris; into this vascular circle circulus arteriosus major the anterior ciliary arteries pour their blood, and from it vessels converge to the free margin of the iris, and there communicate and form a second circle circulus arteriosus minor Figs.
The nerves of the choroid and iris are the long and short ciliary; the former being branches of the nasociliary nerve, the latter of the ciliary ganglion.
They pierce the sclera around the entrance of the optic nerve, run forward in the perichoroidal space, and supply the bloodvessels of the choroid. Other fibers from the plexus end in a net-work on the anterior surface of the iris. The fibers derived through the motor root of the ciliary ganglion from the oculomotor nerve, supply the Sphincter, while those derived from the sympathetic supply the Dilatator.
Membrana Pupillaris. The vessels of this membrane are partly derived from those of the margin of the iris and partly from those of the capsule of the lens; they have a looped arrangement, and converge toward each other without anastomosing.
About the sixth month the membrane begins to disappear by absorption from the center toward the circumference, and at birth only a few fragments are present; in exceptional cases it persists. The veins are darker in appearance than the arteries. The Retina tunica interna. Its outer surface is in contact with the choroid; its inner with the hyaloid membrane of the vitreous body.
Behind, it is continuous with the optic nerve; it gradually diminishes in thickness from behind forward, and extends nearly as far as the ciliary body, where it appears to end in a jagged margin, the ora serrata.
This forward prolongation consists of the pigmentary layer of the retina together with a stratum of columnar epithelium. The retina is soft, semitransparent, and of a purple tint in the fresh state, owing to the presence of a coloring material named rhodopsin or visual purple; but it soon becomes clouded, opaque, and bleached when exposed to sunlight.
Exactly in the center of the posterior part of the retina, corresponding to the axis of the eye, and at a point in which the sense of vision is most perfect, is an oval yellowish area, the macula lutea; in the macula is a central depression, the fovea centralis Fig.
At the fovea centralis the retina is exceedingly thin, and the dark color of the choroid is distinctly seen through it. About 3 mm. This is the only part of the surface of the retina which is insensitive to light, and it is termed the blind spot. Structure Figs. The pigmented layer consists of a single stratum of cells.
When viewed from the outer surface these cells are smooth and hexagonal in shape; when seen in section each cell consists of an outer non-pigmented part containing a large oval nucleus and an inner pigmented portion which extends as a series of straight thread-like processes between the rods, this being especially the case when the eye is exposed to light.
In the eyes of albinos the cells of this layer are destitute of pigment. Retina Proper. Stratum opticum. The stratum opticum or layer of nerve fibers is formed by the expansion of the fibers of the optic nerve; it is thickest near the porus opticus, gradually diminishing toward the ora serrata. When they reach the internal surface of the retina they radiate from their point of entrance over this surface grouped in bundles, and in many places arranged in plexuses.
Most of the fibers are centripetal, and are the direct continuations of the axis-cylinder processes of the cells of the ganglionic layer, but a few of them are centrifugal and ramify in the inner plexiform and inner nuclear layers, where they end in enlarged extremities. The ganglionic layer consists of a single layer of large ganglion cells, except in the macula lutea, where there are several strata. The cells are somewhat flask-shaped; the rounded internal surface of each resting on the stratum opticum, and sending off an axon which is prolonged into it.
From the opposite end numerous dendrites extend into the inner plexiform layer, where they branch and form flattened arborizations at different levels. Its central part fovea centralis is densely packed with cone cells for colour perception. At this point, the sense of vision is the most accurate and detailed. The inner part of the eyeball consists of the lens, the vitreous body and the two eye chambers. The lens is a transparent olive-shaped structure in the eye that has no blood vessels.
Lens and cornea see above work together to focus the light rays passing through the eyeball to the back of the eye, that is, to the retina, by bending or refracting them, thereby creating clear images of the environment perceived from different distances.
By adjusting its shape and size, the lens can change the focus. This process is called accomodation. The vitreous is a clear gelatinous mass held by collagen fibers. It is situated between lens and retina and comprises about two thirds of the entire eyeball.
By pushing the retina towards the choroid, the vitreous promotes keeping the retina in place. The anterior chamber of the eye is located between the iris and the cornea see above. The posterior chamber is the space between parts of the iris and the lens. Both chambers are filled with aquaeous fluid to nourish cornea and lens.
The human eye is a complex optical system that basically works like a camera: the iris serves as the aperture that controls the amount of light rays reaching cornea and lens photographic objective , and the retina works as the film.
Bending of light rays by cornea and lens serves to create sharp images on the retina. Its function is to provide nourishment to the outer layers of the retina through blood vessels. It is part of the uveal tract. Where is the fibrous tunic? Fibrous tunic of eyeball. Horizontal section of the eyeball. Cornea labeled at top, sclera labeled at center right. The sclera and cornea form the fibrous tunic of the bulb of the eye; the sclera is opaque, and constitutes the posterior five-sixths of the tunic; the cornea is transparent, and forms the anterior sixth.
Which eye structure supplies all others with oxygen and nutrients? What eye structure supplies all others with oxygen and nutrients? The choroid. What is the function of the fibrous tunic? What is the function of the lens? The lens is located in the eye. By changing its shape, the lens changes the focal distance of the eye. In other words, it focuses the light rays that pass through it and onto the retina in order to create clear images of objects that are positioned at various distances.
What is the purpose of the fat surrounding the eye? The space between the eye muscles and the bony wall of the orbit is filled with loose fat tissue. This soft tissue functions like a cushion if the eye is hit by an object. Fat tissue decreases in the elderly persons, the eyes sink into the orbit and the eyes look smaller but they are not smaller. What is the function of the muscles surrounding the eye? Specifically, the medial rectus muscle works to keep the pupil closer to the midline of the body.
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