If we turn down the lower lid and turn up the upper, we see a red membrane called the conjunctiva (connecting).  This is the mucous membrane of the eye.  It lines the inner surface of both lids and is reflected over the fore part of the Sclerotic and Cornea—­two other coats of the eye, The palpebral or eyelid portion of the conjunctiva is thick, opaque, highly vascular (filled with blood vessels) and covered with numerous papillae.  It turns back (reflects) over the Cornea, but it consists only of a very thin structure (epithelium) forming the anterior layer of the cornea and is, in health, perfectly transparent.  Upon the sclerotic it is loosely attached to the globe.  When the conjunctiva becomes inflamed it is called (Conjunctiv(a)itis) conjunctivitis.  The sclerotic-cornea forms the external tunic (coat) of the eyeball, the sclerotic being opaque and forming the posterior five-sixths of the globe; the cornea, which forms the remaining sixth (the front white part that is plainly seen) being transparent.  The sclerotic (means dense and hard) serves to maintain the form of the globe, the eyeball.

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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.