STRUCTURE OF THE EYE. 



Tliis organ is, for these reasons, a subject of inquiry and expo- 

 sition, which must be regarded with, the most lively interest by 

 every one, whatever be his station, who is endowed with the least 

 understanding or reflection. But besides the general considera- 

 tions here developed, it is also to be remembered that, without a 

 previous knowledge of the structure and functions of the eye, it is 

 impossible to comprehend the use and application of the innumer- 

 able optical instruments which have been invented to aid its 

 defects, whether natural or accidental ; to repair the ravages of 

 time, and to supply to age a renovated and re-invigorated organ of 

 vision ; to replace the diseased optical membrane removed by the 

 knife of the surgeon, and thus restore sight where absolute blind- 

 ness had ensued ; to bring within the range of accurate vision 

 objects rendered indistinctly visible, or altogether invisible, either 

 by reason of their remoteness or minuteness. These admirable 

 instruments can be easily rendered intelligible, provided a general 

 knowledge of the structure and functions of the eye be first 

 obtained, but not otherwise. 



"We purpose, therefore, to devote the present tract to a popular 

 and simple exposition of the eye, and more particularly, the 

 human eye. 



3. The eyes, as they exist in the human species, have the form, 

 as is well known, of two spheres, each about an inch in diameter, 

 which are surrounded and protected by strong bony sockets 

 placed on each side of the upper part of the nose. The external 

 coating of these spheres is lubricated by a fluid secreted in adjacent 

 glands, and spread upon them from time to time by the action of 

 the eye-lids in winking. 



The eye-balls are moved by muscles connected with them within 

 the socket upon the principle known in mechanics as the ball and 

 socket joint. 



A front view of the eyes and surrounding parts is shown in 

 fig. 1, a section of them made by a horizontal plane through the 

 line A B, which passes through the centre of the front of the eye- 

 balls, being shown in fig. 2 (see p. 49). 



4. The external coating c D F E consists of a strong and tough 

 membrane, called the sclerotica, or sclerotic coat. A part of this 

 membrane is visible when the eye-lids are open at w, and is called 

 the white of the eye. In this part of the eye-ball there is a 

 circular opening, covered by a thin and perfectly transparent shell 

 D G F, called the cornea. This cornea is more convex than the 

 general surface of the eye-ball, and may be compared to a watch- 

 glass. It is connected round its edge with the sclerotica, which 

 differs from it, however, both in colour and opacity, the sclerotica 

 being white and opaque, while the cornea is perfectly colourless 



E 2 51 



