This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The eyeball has three main coats, viz.: a fibrous outer coat, called the sclerotic; a vascular middle coat, the choroid; and a nervous inner coat, the retina.
The sclerotic coat forms a firm protective covering or case for the delicate retina within. It is continuous posteriorly with the fibrous coat or dura of the optic nerve, which is a continuation of the dura mater of the brain. At the optic foramen, the dura mater splits into two layers; the outer layer forms the periosteum, while the inner forms the dural coat of the optic nerve. This nerve also, like the brain, has an arachnoid and a pial membrane. The sclerotic coat is continued forward over the front of the eye as the cornea. As it is essentially a membrane intended to be protective in its function, its diseases are those of weakness: thus, if the cornea is affected, it bulges forward and is called an anterior staphyloma; if the posterior part is affected, the sclera is stretched, and it forms a posterior staphyloma.
Anterior staphyloma may occur either rapidly as a small local protrusion, resulting from ulceration of the cornea or a wound, or it may be slow in forming, and involve nearly or quite the whole of the cornea, pushing it forward in the shape of a cone; this is called conical cornea. Posterior staphyloma occurs in near-sighted people, the anteroposterior diameter of the eye being longer than normal. If this posterior staphyloma or stretching of the eye becomes marked, the choroid atrophies and the functions of the retina are lost. The white sclera is seen with the ophthalmoscope, surrounding or to one side of the optic nerve.
Although the cornea has no blood-vessels, it still, from its exposed position, becomes inflamed (keratitis) and ulcerated, and eventually blood-vessels may develop into it from its periphery, constituting the disease known as pannus.
The weakest portion of the globe is at the junction of the sclerotic coat with the cornea. It is here that the sclera is thinnest. On this account, blows on the eye cause it to rupture usually at this point, the tear encircling the edge of the cornea for a variable distance (usually at its upper and inner quadrant) according to the force and direction of the injury. On healing, a staphyloma may form at this point.
The choroid or vascular coat of the eye contains the pigment or color of the eye. It is continued forward as the ciliary body (or processes) and iris. Being a vascular tissue, its diseases are inflammatory. If the choroid is affected we have choroiditis; if the ciliary region is inflamed, it is called cyclitis; and if the iris is inflamed we have iritis.
The retina or nervous coat of the eye is concerned in the function of sight and it, like other nerves, may be affected with inflammation, called retinitis. Sometimes it becomes loosened from the choroid beneath by a hemorrhage or rapid stretching of the sclera, constituting a detachment of the retina. Outside the disk is the macula lutea and fovea centralis or region of distinct vision.
Filling the interior of the eye is the jelly-like transparent vitreous humor, enclosed in the hyaloid membrane. In front of the vitreous humor is the lens; and the clear, limpid liquid between the anterior surface of the lens and the posterior surface of the cornea is the aqueous humor.
The lens, immediately behind the iris, is suspended in its capsule from the ciliary processes by its suspensory ligament or zone of Zinn. Between the ciliary processes and the sclera lies the ciliary muscle, which regulates the accommodation or focussing power of the eye. The ciliary processes are formed of convoluted blood-vessels supported by connective tissue and covered by the pigmented extension of the retina. This ciliary region is an exceedingly sensitive one and a serious wound of it usually means a loss of the eye.