84 Orbit 



In the treatment of corneal ulcer atropine should be used, so that, 

 the pupil being dilated, the border of the iris may be unlikely to 

 prolapse. Then, to diminish the risk of the aqueous bursting through 

 the elastic layer, eserine must be used ; or paracentesis of the anterior 

 chamber may even be expedient. This operation is performed by intro- 

 ducing a slender blade through the sclero-corneal margin, taking 

 care that it does not pass between the layers of the cornea, and that, 

 in withdrawing it, the iris do not escape with the aqueous. 



In the case of a perforation of the central part of the cornea, 

 atropine should be used, so that the pupillary border of the iris may 

 be secured against collapse. But if the wound be near the periphery 

 the pupil should be made to contract by eserene. If, however, a piece 

 of the iris be already prolapsed, and it be found impossible to return 

 it, it must be cut off flush with the cornea, the stump being tempted 

 to return by the use of atropine, or eserene, as the case may be. 



With interstitial keratitis, which is often the result of inherited 

 syphilis, effusion takes place between the layers of the cornea, spoiling 

 its transparency, and giving it the appearance of ground glass. 



If inflammation run on to the formation of pus between the layers, 

 and the corneal abscess be not promptly evacuated, it may cause 

 perforation. Sometimes the inflammation is attended with separation 

 of the lamellae, so that the pus sinks to their lowest part, where it forms 

 a collection, the form, size, and tint of which are like the little white 

 crescent at the root of the nail (ow), and is therefore called onyx. 



Similarly pus in the anterior chamber forms a small,, crescentic 

 abscess, but this may be distinguished from onyx by its altering its 

 position as the head is inclined to one side. The pus must be let out 

 by paracentesis. 



The cornea is the most important of the refracting media, refraction 

 being the effect produced on rays of light passing from a rarer to a 

 denser medium. Should it be too convex a somewhat frequent con- 

 genital defect it overacts its part, and rays are brought to a focus before 

 they reach the retina. They have, therefore, to be somewhat scattered 

 before they reach the cornea by biconcave lenses. (Myopia, p. 86.) 



On the other hand, should the cornea be abnormally flat, the antero- 

 posterior axis of the eye-ball is too short, and the rays have to be 

 collected, somewhat before reaching the cornea, by the aid of convex 

 glasses. (Hypermetropia, p. 86.) 



If the curvature of the cornea be irregular, either in the vertical, 

 horizontal, or oblique diameter, the rays come unevenly to a focus, 

 some of them missing their mark, the error being called astigmatism 

 (a, priv., ort'y/Lia, mark}. Permanent unevenness with a resulting astig- 

 matism is more apt to follow the extraction of a cataract when the 

 incision is made through the cornea than when made through the 

 corneo-scleral tissue. 



Sometimes the cornea grows thin and prominent, conical it is not 



