Layer of eyes which forms ciliary apparatus?????
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Answer:
The eye (L., oculus; Gk, ophthalmos) (fig. 46-1A and B) lies in the cavity of the orbit and measures approximately 24 mm in diameter. The midpoints of the two pupils lie about 60 mm apart.
If the eye is too short in relation to the lens, near objects are focused behind the retina (hypermetropia: farsightedness or longsightedness). In contrast, if the eye is too long in relation to the lens, distant objects are focused in front of the retina (myopia: nearsightedness or shortsightedness) (fig. 46-1C).
The development of the eye is summarized in figure 46-2. The retina, which may be regarded as an extension of the wall of the brain, develops from neural ectoderm, whereas the lens and the anterior epithelium of the cornea are derived from somatic ectoderm. Neural crest and mesoderm also participate in ocular development.
Tunics of eye
The eyeball (globe or bulb) has three concentric coverings (figs. 46-3 and 46-4): (1) an external, fibrous tunic comprising the cornea and sclera; (2) a middle, vascular tunic comprising the iris, ciliary body, and choroid; and (3) an internal, nervous tunic, or retina.
External fibrous tunic
The cornea is the anterior, transparent part of the eye, and it forms about one-sixth of the circumference of the fibrous coat. Most of the refraction by the eye takes place not in the lens but at the surface of the cornea. The cornea is continuous with the conjunctiva and the junctional region is known as the limbus. The cornea is supplied by the ophthalmic nerve (from the fifth cranial nerve) by means of its ciliary branches. The eyelids close on stimulation of the cornea (corneal reflex, fig. 46-8A). The cornea is avascular and consists of five layers histologically: a largely collagenous substantia propria enclosed by anterior and posterior epithelia and limiting laminae.
When the cornea does not conform to a sphere but is more curved in one axis than in another, the condition is termed astigmatism.
Irritation of the eye, e.g., from a foreign body, causes hyperemia of the conjunctiva, which may also result from infection or allergic conditions (conjunctivitis). The posterior conjunctival arteries (from the palpebral arcades, fig. 46-10) become dilated and give a brick-red color to the conjunctiva.
Inflammation of the cornea (keratitis) or of the iris and ciliary body (iridocyclitis) causes dilation of the anterior ciliary arteries (from muscular branches of the ophthalmic, fig. 46-10), resulting in a rose-pink band of "ciliary injection". These vessels, unlike those of the conjunctiva, do not move when the conjunctiva is moved.
The sclera is the posterior, opaque part of the external tunic. Its anterior part can be seen through the conjunctiva as "the white of the eye". The sclera consists of fibrous tissue, and it receives the tendons of the muscles of the eyeball. Posteriorly, the fibers of the optic nerve pierce the sclera through a weak plate termed the lamina cribrosa (fig. 46-3).
External to the sclera, the eyeball is enveloped by a thin fascial sheath (so-called Tenon's capsule) that extends from the optic nerve to the sclerocorneal junction (fig. 46-3). The sheath separates the globe from the orbital fat and acts as a socket in which the eye moves as in a ball-and-socket joint. It blends with the sheaths of the muscles of the globe.
Hormonal disturbances (especially hyperthyroidism) may result in swelling of the orbital fat and extra-ocular muscles, causing protrusion of the eyes (exophthalmos).
An important, circular canal termed the scleral venous sinus (known to ophthalmologists as the canal of Schlemm) is situated at the sclerocorneal junction, anterior to a projection termed the scleral spur (fig. 46-5). The aqueous humor, formed by the ciliary processes, filters through intercellular channels leading from the anterior chamber to the venous sinus and drains by means of aqueous veins into scleral plexuses. The iridocorneal angle (between the iris and the cornea), also known as the angle of the anterior chamber or as the filtration angle, is very important physiologically (for the circulation of aqueous humor) and pathologically (in glaucoma).
Middle vascular tunic
The middle tunic, frequently termed the uvea, comprises the choroid, the ciliary body, and the iris, from posterior to anterior.