SCIENCE OF COMMON THINGS. 263 



Eetina. Iris. Pupil. Cornea 



nerve, which conveys the sensation to the brain. If this nerve were cut, 

 notwithstanding the eye might be in other respects perfect, the sense of 

 sight would be destroyed. 



1704 Of what parts does the eye consist f 



The eye is of globular form, and is composed of three 

 coats or membranes, called the sclerotic, the choroid, 

 and the retina; and three humors, denominated the 

 aqueous ', the crystalline, and the vitreous. 



1705 What is meant by the " retina of the eye ?" 



The network which lines the back of the eye is called 

 the retina ; it is composed of an expansion of the optic 

 nerve. 



1706 What is that portion of the eye called ivhich in some persons is 

 blue, in others gray or hazel ? 



It is called the iris. 



17*O 7* In the centre of the iris is a circular "black opening : what is this 

 called 1 



It is called the pupil. But this spot is not a black 

 substance, but an aperture, which appears black only 

 because the chamber within it is dark. It is properly 

 speaking the window of the eye, through which light 

 is admitted, which strikes on the retina. 



17*O8 Does light admitted through the pupil to the retina produce vision ? 



Yes, provided the light enter in sufficient quantity. 



How by the arrangement of the several parts of the eye are we 

 enabled to see? 



The rays of light falling upon the cornea, enter the 

 interior of the eye through the pupil, and by the joint 

 action of the cornea and crystalline lens are brought to 

 a focus at the back part of the eve, upon the retina. 

 Here an image is formed, and the impression it makes 

 is conveyed along the optic nerve to the brain. 



17*1O What is meant "by Hie " cornea of the eye ?" 



All the outside of the visible part of the eyeball. 



Fig. 77 represents the interior construction of the eye. It is composed, 

 in the first place, of the cornea, a, a transparent membrane in front of the 

 globe of the eye. Next is the sclerotic coat, t, which joins on the cornea, 

 and upon which the external form of the eye depends. The cornea is 

 united to, or fixed in, the sclerotic coat, like the glass into the case of a 

 watch : d, c represents the iris, with an opening in it, forming the pupil 



