558 MANUAL OF PHYSIOLOGY. 



way as would cause irregularity of its distribution, and thus dazzle 

 and interfere with the distinctness of the image. The choroid 

 also is elastic, and can move over the neighboring sclerotic ; it 

 can be drawn forwards by the contraction of the radiating ciliary 

 muscle, which acts as a tensor of the choroid membrane. 



The iris has a special power of motion, by means of which the 

 opening in it can be made smaller, so as to regulate the amount 

 of light admitted to the eye, and cut off more or less of the rays 

 which would pass through the margin of the dioptric media. 

 The importance of this will be better understood further on. 



Within the choroid coat, and in immediate contact with it, is 

 the nervous coat, or retina, formed by the expansion of the optic 

 nerve, which pierces the sclerotic a little obliquely, entering it 

 somewhat to the nasal side of the axis of the eye. The retina 

 lines all the back part of the eyeball, and stretching forwards, 

 becomes fused with the ciliary processes, where, however, the 

 nervous elements of the coat are wanting. The fibrils of the 

 optic nerve reach the inner surface of the coats of the eye, and 

 lie in immediate relation to the transparent medium, which occu- 

 pies the greater part of the bulb. The fibres then lie internally 

 to their terminals, which turn outwards and are set against the 

 choroid coat. The ultimate nerve-endings are situated in pig- 

 mented protoplasmic cells, which form the outermost layer of 

 the retina. 



THE DIOPTRIC MEDIA OF THE EYEBALL. 



The transparent substances which fill the eyeball are, together 

 with the cornea, commonly called the dioptric media. The aque- 

 ous humor lies in contact with the posterior surface of the cornea, 

 and just fills the prominence which is formed by this part of the 

 eye. It is in this fluid that the movable iris is stretched and 

 separates the aqueous department of the eye into an anterior and 

 posterior chamber. The vitreous humor occupies much the larger 

 share of the eyeball. It lies in apposition to the retina, being 

 separated from it only by a thin transparent structure, called the 

 hyaloid membrane, which incloses the clear gelatinous vitreous 

 humor, and is fused with the ciliary part of the retina and cho- 



