STRUCTURE OF THE HUMAN EYE. 161 



The sclerotic coat, s, is that which surrounds all the 

 others, being the exterior coat of the eye. This is a firm, 

 dense membrane, and gives the eye-ball its chief mechani- 

 cal support. The sclerotica does not cover the front of 

 the eye-ball, that portion being covered with the cornea, 

 c, which forms the most prominent part of the organ. 

 The sclerotica coat forms what is usually called the white 

 of the eye, while the cornea covers the transparent 

 iront, through which vision is performed. The sclerot- 

 ica is lined on the inside by the choroid coat, x, which 

 is chiefly made up of a tissue of fine blood-vessels, giving 

 nourishment to the different parts of the eye. The retina, 

 r, is an exceedingly thin and delicate expansion of the 

 optic nerve, o, situated within the choroid coat. This is 

 the immediate organ of vision. 



485. Of the three humors, the vitreous, v, occupies more 

 than three parts of the whole globe of the eye. It con- 

 sists of a transparent jelly, which has somewhat of a 

 glassy appearance, and hence its name vitreous, or glassy. 

 The crystalline humor, has the shape of a double-convex 

 lens, and occupies the front part of the eye, being situ- 

 ated between the aqueous and vitreous humors. The 

 aqueous humor, a, is a transparent, watery fluid, which 

 occupies the most prominent portion of the organ, imme- 

 diately within the cornea. In this humor is situated the 

 iris, i, a thin, circular membrane, which is of various 

 colors in different persons, being black, in the black, and 

 blue, in the blue eyed. On this account, this part is 

 called iris, which means rainbow. Through the iris, 

 tl.ere is a central perforation, called the pupil, p, through 

 which the light or the images of objects pass to the 

 retina. The iris is fixed to the choroid coat by a white 

 elastic ring, called the ciliary ligament, g. The interior 

 surface of the iris is lined with a dark brown pigment, 

 called the uvea. 



486. Structure of the Iris. The structure of the iris 

 is very peculiar, being composed of two layers of con- 



What is the situation of the sclerotic coat ? What is the choroid coat ? 

 What is the situation of the cornea ? Where is the vitreous humor situ- 

 ated? What is the shape and situation of the crvstalline lens ? What 

 is the iris * What forms the pupil of the eve ? 



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