266 THE ANATOMY OF THE HORSE. 



the line of junction of the sclerotic and cornea, the iris passes across the 

 interior of the eye. This, which may be viewed as a dependency of the 

 choroid, is a muscular curtain perforated by an aperture termed the 

 pupil. The retina will be recognised as a delicate glassy layer lining 

 the greater part of the choroid. 



The refracting media of the eye are three in number, viz., (1) the 

 aqueous humour — a watery fluid enclosed in a chamber behind the 

 cornea ; (2) the crystalline lens (and its capsule) — a transparent soft 

 solid of a biconvex form, and placed behind the iris ; (3) the vitreous 

 humour — a transparent material with a consistence like thin jelly, and 

 occupying as much of the interior of the eye as is subjacent to the 

 choroid. 



Directions. — Another eye should be cleaned like the first, and used 

 for the more particular examination of the sclerotic and cornea. 



The Sclerotic is a strong, opaque fibrous membrane which in great 

 measure maintains the form of the eyeball, and protects the more 

 delicate structures within it. Its anterior portion, which is covered by 

 the ocular conjunctiva, is visible in the undissected eye, and is commonly 

 known as the " white of the eye." In form it is bell-shaped, and the 

 optic nerve pierces it behind like a handle. The point of perforation, 

 however, is not exactly at the centre of the summit of the bell, but a 

 little to its outer side. When the nerve is cut off close to the sclerotic, 

 the nerve-bundles appear as if passing through the apertures of a sieve, 

 and to this appearance the term lamina cribrosa is applied. The sheath 

 of the nerve passes on to the sclerotic around the point of perforation. 

 In front the rim of the bell becomes continuous with the cornea. The 

 outer surface of the membrane receives the insertion of the muscles of 

 the eyeball. The inner surface (which will afterwards be exposed) is of 

 a light brown colour, and is connected to the choroid by fine processes 

 of connective-tissue — the lamina fusca. The coat is thickest over the 

 posterior part of the eyeball, and is thinnest a little behind its junction 

 with the cornea. 



Structure. — The sclerotic is composed of connective-tissue, there being 

 a great preponderance of white fibres, but intermixed with these are 

 some fine elastic fibres. The bundles of fibres, which are disposed both 

 meridionally and equatorially, have a felted arrangement, but the surface 

 fibres are mostly longitudinal. The texture of the sclerotic is only 

 slightly vascular, the capillaries forming a wide-meshed network. It is 

 most vascular just behind the cornea. 



The Cornea is the anterior transparent portion of the outer coat of 

 the eyeball. It may be viewed as a part of the sclerotic specially 

 modified to permit the passage of light into the interior of the eye. Its 

 outline is elliptical approaching the circular, and its greatest diameter 

 is transverse. At its periphery it joins the sclerotic by continuity of 



