[EYE AND EAR 347]
The cornea.—This is almost circular in shape. It is convex anteriorly and projects forward from the sclerotic in the same manner that a watch glass does from its case. This layer covers what we call the pupil.
The second tunic or coat (membrane) is formed from behind forward by the Choroid, the ciliary body and the Iris. The choroid is the vascular and dark coat covering the posterior five-sixths of the globe. The ciliary body connects the choroid to the circumference of the iris. The iris is the circular muscular septum (division) which hangs vertically behind the cornea, presenting in its center a large rounded opening, the pupil.
The choroid is a thin highly vascular membrane of a dark brown or chocolate color and is pierced behind by the optic nerve and in this situation is firmly adherent to the sclerotic.
The ciliary body comprises three muscles for its make-up and connects the choroid to the circumference of the iris.
The Iris (rainbow) has received its name from its various colors in different individuals. It is a thin, circular shaped, contractile curtain, suspended in the aqueous (watery) humor behind the cornea and in front of the lens, being perforated a little to the nasal (nose) side of its centre by a circular opening, the pupil, for the transmission of light. By its circumference it is continuous with the ciliary body, and its inner or free edge forms the margin of the pupil. The anterior surface of the iris is variously colored in different individuals and marked by lines which converge toward the pupil.
The Retina.—This is a delicate membrane, upon the surface of which the images of external objects are received. Its outer surface is in contact with the choroid; its inner, with the vitreous (glass) body. Behind it is continuous with the optic nerve; it gradually diminishes in thickness from behind forward. The retina is soft, semi-transparent and of a purple tint in the fresh state. Exactly in the centre 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 spot, called after its discoverer, the yellow spot or Macula lutea of Sommering.
Refracting Media.—The aqueous humor completely fills the anterior and posterior chambers of the eyeball. The anterior chamber is the space bounded in front by the cornea; behind by the front of the iris. The posterior chamber is a narrow chink between the peripheral part of the iris, the "suspensory ligament" of the lens and the "ciliary processes."
[348 MOTHERS' REMEDIES]
The vitreous body forms about four-fifths of the entire globe, It fills the concavity of the retina and is hollowed in front, forming a deep cavity, for the reception of the lens. It is perfectly transparent and of the consistency of thin jelly. The fluid from the vitreous body resembles nearly pure water. The crystalline lens enclosed in its capsule is situated immediately behind the pupil, in front of the vitreous body. The lens is a transparent, double-convex body. It is more convex on the posterior than on the anterior surface. The rays of light go through this body and converge to a point at the back of the retina.