160 SENSOR1AL FUNCTIONS. 



the use of many diagrams, and lengthy explanations ; 

 and after all, we are all obliged to refer to the eye itself, 

 the most perfect of optical instruments, in order to illus- 

 trate these preparatory steps. We shall therefore 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 question both plain 

 and interesting. 



483. The spherical form of the eye is preserved by 

 firm membranes of various thicknesses, 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, beside 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. 



484. Horizontal Section of the Human Eye. Fig. 99 

 represents a horizontal section of the right human eye. 



Fig. 96. 



How many coats has the eye, and which are their names ? How many 

 humors has the eye and what are their names ? 



