164 SENSORIAL FUNCTIONS. 



it is difficult to make these principles understood with- 

 out the use of many diagrams, and lengthy explana- 

 tions ; and after all, we are obliged to refer to the eye 

 itself, the most perfect of optical instruments, in order 

 to illustrate these preparatory steps. We shall there- 

 fore begin with the structure of the eye, as the basis of 

 visual physiology, and after which, such explanations 

 will follow as will, it is hoped, make the subject in ques- 

 tion, both plain and interesting. 



The spherical form of the eye is preserved by firm 

 membranes of various thickness, called the coats of the 

 eye. The transparent media which these coats enclose, 

 and which refract the light so as to form a picture on 

 the retina are called the Humors of the Eye. There are 

 three principal coats or membranes, called the Scleroti- 

 ca, the Cornea, and the Choroid, besides which, there is 

 the Retina, which covers the back part of the eye. The 

 three humors are called the Aqueous, the Vitreous, and 

 Crystalline, the latter being a firm body, is usually called 

 the Lens. 



Horizontal section of the Human Eye. Fig. 99 rep- 

 resents a horizontal section of the right Human Eye. 



Fig. 99. 



How many coats has the eye, and what are their naiiesl How 

 many humors has the eye and what are their names 1 



