SENSE OF SIGHT. 609 



concentrated at the level of the retina ; a perforated muscular curtain 

 or diaphragm, the iris (7), placed in front of the lens,' which regulates 

 the quantity of light admitted through its central orifice, the pupil ; and 

 finally a vascular membrane with an opaque layer of blackish-brown 

 pigment, the choroid (s), which lines the whole inner surface of the 

 sclerotic and the posterior surface of the iris, thus preventing reflec- 

 tions within the eye, and absorbing all the light which has once passed 

 through the substance of the retina. The construction of the eyeball, 

 in its general arrangement as an organ of vision, is not unlike that of 

 a photographic camera; where the sensitized plate at the back part 

 represents the retina, the blackened inner surface of the box the choroid, 

 w r hile the lenses of the tube in front perform the office of the crystalline 

 lens and cornea of the eyeball. 



Sclerotic Coat. The sclerotic, so named from its toughness and re- 

 sistance, is the external coat and protective membrane of the eyeball. It 

 is composed of condensed layers of connective tissue, similar to those 

 of the fasciae and membranous tendons in general ; and toward its an- 

 terior third it receives the tendons of the external muscles of the eyeball, 

 which become fused with its substance. Posteriorly it is continuous 

 with the neurilemma of the optic nerve (Fig, 189, i), which penetrates it 

 from behind at its point of entrance into the eyeball. A portion of the 

 sclerotic is visible anteriorly through the conjunctiva, forming the so- 

 called "white" of the eye. 



Cornea. The cornea, which derivesMts name from its firm consistencjr 

 and homogeneous appearance, resembling that of horn, forms the anterior 

 part of the wall of the eyeball. It is inserted into the nearly circular 

 space left at this situation by the deficiency of the sclerotic, with the 

 texture of which it is continuous at its edges ; the difference in the phy- 

 sical appearance of the two being that the sclerotic is white and opaque, 

 while the cornea is colorless and transparent, so that the colored iris 

 and dark pupil are visible through its substance. The surface of the 

 cornea has a sharper curvature than that of the sclerotic, so that it pro- 

 jects from the front of the eyeball, like a smaller dome set upon a larger 

 one. Its outline, where it joins the edge of the sclerotic, is a little oval 

 in form, the transverse diameter of the cornea, in man, being slightly 

 longer than the vertical. At its centre, it is about 0.8 millimetre in 

 thickness, becoming a little thicker at its edges. Its anterior surface is 

 kept polished and brilliant by the watery secretion of the lachrymal 

 glands, distributed over it by the frequent movements of the eyeball 

 and the lids. 



At the outer border of the cornea, where it joins the sclerotic, and 

 where the tissues of the two membranes pass into each other, there is a 

 small cavit}', running, in the form of a circular canal, the canal of 

 Schlemm (Fig. 189, 4), through the thickness of this part of the wall 

 of the eyeball. The inner wall of the canal of Schlemm is composed of 

 elastic and tendinous tissue, and gives attachment to the fibres of the 

 ciliary muscle on the one hand, and on the other to the outer border of 



