THE EYE. 817 



of this globe, or orbital cavity, the muscles that move it, the protective mem- 

 branes or eyelids, the membrana nictitans or accessory eyelid, and, lastly, the 

 lachrymal apparatus, which concurs in the protection of the ocular globe by 

 the fluid it incessantly throws out upon its surface. 



ARTICLE I. THE ESSENTIAL ORGAN OF VISION, OR OCULAR GLOBE. 



(Fig. 383.) 



(Preparation. The eye should be as fresh as possible. All the fat and muscles 

 should be carefully removed with scissors, the optic nerve being allowed to remain.) 



The globe, or ball of the eye, is a spherical shell, whose interior is filled 

 with fluid or semifluid parts, named the humours or media of the eye. The 

 wall of this shell is formed by a continuous, very resisting, colourless 

 envelope, limpid and translucid in its anterior portion, which constitutes the 

 transparent cornea, and white and opaque for the remainder of its extent, 

 known as the sclerotica. 



On the inner face of the sclerotica is a second tunic the choroid : a black 

 membrane that lines the posterior face of the retina, and which, near where 

 the two constituent portions of the external envelope unite, projects into the 

 interior of the eye an elliptical diaphragm with a large opening in its centre 

 the iris. Immediately behind this disc is suspended or set, like a rose- 

 brilliant, in the centre of a circular zone depending from the choroid, 

 a biconvex body the crystalline lens, one of the media of the eye, and 

 which divides the interior of its cavity into two compartments : a posterior, 

 very large, occupied by the vitreous humour ; and an anterior, itself divided 

 by the iris into two chambers of unequal dimensions, which contains the 

 aqueous humour. 



Viewed externally, and as a whole, the organ resulting from the union 

 of all these parts represents a globular body, the anterior region of which 

 corresponds to the cornea, and is more convex than the other points : a 

 circumstance that tends to increase the antero-posterior diameter of the eye. 

 But as this ocular sphere, to which is added, in front, this segment of a 

 smaller sphere, is sensibly depressed from before to behind, it results that 

 the other two principal diameters the vertical and transversal offer 

 about the same dimensions as the first ; Girard has even stated that the 

 latter is the least. With an eye hardened by chromic acid, we have 

 found that the transverse diameter measured O m ,036, and the vertical O m ,040 

 (1-417 x 1'575 inches). 



Two paragraphs will be devoted to the description of the constituent 

 parts of the globe : one for the membranes, the other for the media. 



THE MEMBRANES OF THE EYE. 



1. The Sclerotic. (Fig. 383, 6.) 



The sclerotic is a white, very solid membrane, forming in itself about 

 four-fifths of the external shell of the eye. 



Its external face, in relation with the recti muscles and adipose tissue, 

 receives posteriorly, though lower than the middle, the insertion of the 

 optic nerve, which passes through it and the choroid to form the retina. Its 

 internal face is loosely united to the choroid by vessels, nerves, and 

 cellular tissue. 



In front, the sclerotic shows an elliptical opening whose greatest diameter 



