78 THE EYE. 



LESSON 36. 



Structure of the Eye. 



Mem'branous, consisting of a web of several sorts of fibres inter- 

 woven together. 



Op'tic, producing vision, subservient to vision. 



Sclerot'ica, (pronounced skle-rot'-i-ca,) derived from a Greek word 

 signifying hard. 



The body of tlie eye is of a spherical form. It has two 

 membranous coverings ; the external one is called the scle- 

 rotica ; this has a projection in that part of the eye which 

 is exposed to view, called the cornea, because, when dried, 

 it has nearly the consistence of very tine horn, and is suffi- 

 ciently transparent for the light to obtain free passage through 

 it. The second membrane, which lines the cornea, and en- 

 velopes the eye, is called the choroid; this has an opening 

 in front just beneath the cornea, which forms the pupil, 

 through which the rays, of light pass into the eye. The 

 pupil is surrounded by a circular border, M'hich is a part of 

 the choroid and called the iris, composed of a sort of net- 

 work, which contracts or expands according to the force of 

 the light in which it is placed. If a person sits looking 

 towards a window, the pupils of his eyes appear very small, 

 and the iris large. When he turns from the window, and 

 covers his eyes with his hands, so as entirely to exclude tlie 

 light for a few moments, the pupils will be enlarged and the 

 iris diminished. Tliis is the reason vyhy the eyes suffer pain^ 

 when from darkness they suddenly come into a strong light ; 

 for the pupil being dilated, a quantity of rays must rush in 

 before it has time to contract. And when we go from a 

 strong light into obscurity, we at first imagine ourselves in total 

 darkness ; for a sufficient number of rays cannot gain ad- 

 mittance into the contracted pupil to enable us to distin- 

 guish objects : but in a few minutes it dilates, and we clearly 

 perceive objects which were before invisible. 



The choroid is imbued with a black liquor which serves 

 to absorb all the rays that are irregularly reflected, and to 

 convert the body of the eye into a more perfect camera ob- 

 scura. Y/itliin these coverings of the eye-ball are contained 

 three transparent substances, called humours. Tlie first 

 ©ecupies the space immediately behind the cornea, and is 



