STRUCTURE OF THE HUMAN EYE. 165 



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 

 mechanical support. The sclerotica does not cover the 

 front of the eye ball, that portion being covered by the 

 cornea c, which forms the most prominent part of the 

 organ. The sclerotic coat forms what is usually called 

 the white of the eye, while the cornea covers the trans- 

 parent front, through which vision is performed. The 

 sclerotica is lined on the inside by the choroid coat, x, 

 which is chiefly made up of a tissue of fine blood ves- 

 sels, giving nourishment to the different parts of the eye. 

 The retina, r, is an exceedingly thin and delicate expan- 

 sion of the optic nerve o, situated within the choroid 

 coat. This is the immediate organ of vision. 



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 situa- 

 ted between the aqueous and vitreous humors. The 

 aqueous humor a, is a transparent, watery fluid which 

 occupies the most prominent portion of the organ, 

 immediately within the cornea. In this humor is situa- 

 ted the iris, i, a thin, circular membrane, which is of vari- 

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

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

 is called iris 9 i, which means rainbow. Through the iris 

 there 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 interi- 

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

 called the uvea. 



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 1 What is the choroid coat 7 

 What is the situation of the cornea 1 Where is the vitreous humor situ- 

 ated 1 What is the shape and situation of the crystalline lens ? What 

 is the iris ? What forms the pupil of the eye ? 



