THE EYE *507 



about I inch. The plane of this circle would therefore divide 

 the eyeball into two halves— an anterior half, consisting of the 

 corneal segment and the front part of the sclerotic segment, and a 

 posterior half, consisting of the back part of the sclerotic segment. 

 The meridian is represented by a line encircling the eyeball at right 

 angles to the equator, and passing through the anterior and pos- 

 terior poles. 



Posteriorly the eyeball receives the optic nerve, which pierces 

 the sclerotic coat at a point about |^ inch to the inner side of, and 

 about ^^ inch below, the posterior pole. 



The eyeball is composed of three coats, concentrically arranged, 

 as follows : (i) an external coat, consisting of an opaque part, called 

 the sclerotic, and a transparent part, called the cornea ; (2) a middle 

 coat, which is vascular, pigmented, and muscular, and consists of 

 (a) a posterior part, called the choroid coat, {h) an anterior part, 

 called the iris, and (c) an intermediate part, representing the ciliary 

 body ; and (3) an internal coat, called the retina. 



These three coats enclose a cavity which contains the refracting 

 media, these being three in number, as follows : (i) a fluid, called 

 the aqueous humour, which lies between the cornea and the crystal- 

 line lens, where it occupies the anterior and posterior chambers, 

 into which this region is divided by the iris ; (2) a solid body, called 

 the crystalline lens, which lies behind the aqueous humour ; and 

 (3) a soft gelatinous body, called the vitreous body, which occupies 

 the large space behind the crystalline lens. 



Coats of the Eyeball. Refracting Media. 



External = sclerotic and cornea. Aqueous humoior. 



Middle = choroid, ciliary body, Jind iris. Crj^alline lens. 



IntCTnal = retina. Vitreous body. 



External Coat. 



Sclerotic Coat. — The sclerotic (white of the eye) is a strong white 

 fibrous coat of great density, which surrounds the posterior five- 

 sixths of the eyeball, and maintains the shape of the organ. An- 

 teriorly it unites, and becomes continuous, with the cornea, which 

 it slightly overlaps. The junction of the two is indicated by a 

 slight groove, called the sulcus sclerce, and the union is known as 

 the corneoscleral junction. Posteriorly the sclerotic is pierced by 

 the optic nerve, this part being known as the optic entrance. It 

 is situated, as stated, at a point about ^ inch to the inner 

 side of, and about ^ inch below, the posterior pole of the 

 eyeball. The part of the sclerotic corresponding to the optic 

 entrance is pierced by a number of openings for the passage of the 

 fasciculi of the optic nerve, and is hence called the lamina cribrosa. 



Around the optic entrance there are numerous minute openings 

 for the ciliary vessels and nerves, and in this situation the dura- 

 matral sheath of the optic ner\'e blends with the sclerotic. About 

 midway between the optic entrance and the comeo-scleral junction 



