520 THE NERVOUS SYSTEM. 



lined posteriorly with a nervous expansion, the retina ( 9 ), which is 

 sensitive to light, and which receives the luminous rays admitted 

 through the cornea. The cavity of the eyeball is therefore like that 

 of a room with but one window, where all the light enters in front, 

 and strikes the back wall of the apartment. There are, in addition to 

 the above-mentioned parts, a transparent refracting body with convex 

 surfaces, the crystalline lens ( 8 ), by which the light is concentrated at 

 the retina ; a perforated muscular 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 ( 5 ), lining the 

 inner surface of the sclerotic and the posterior surface of the iris, thus 

 preventing reflections within the eye, and absorbing the light which 

 has once passed through 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, while the optical glasses 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 

 resistance, is the external protective membrane of the eyeball. It is 

 composed of condensed connective tissue, similar to that of the fasciae 

 and fibrous membranes in general; and toward its anterior 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. 130, j), which penetrates it from 

 behind at its 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 derives its name from its horny con- 

 sistency and appearance, forms the anterior part of the wall of the eye- 

 ball. It occupies the nearly circular space left at this situation by the 

 deficiency of the sclerotic, with which it is continuous at its edges ; 

 the difference in the physical appearance of the two being that the 

 sclerotic is white and opaque, while the cornea is colorless and trans- 

 parent, 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, and projects from the front of the eyeball, like 

 a small dome set upon a larger one. Its outline, where it joins the 

 edge of the sclerotic, is a little oval in form, its transverse diameter 

 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 lachrymal 

 secretion, distributed over it by the movements of the eyeball and lids. 



At the outer border of the cornea there is a small circular canal, the 

 canal of Schlemm (Fig. 130, 4 ), enclosed in this part of the wall of 

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



