LESSON IX 

 THE ORGAN OF SIGHT 



1. The General Structure of the Eye.— Every sense- 

 organ consists of two parts ; the essential part, consisting 

 of the structures in wliich the sensory nerve supplied to 

 the organ terminate, and in which the impulses which 

 pass up that nerve are generated, and the accessory part, 

 arranged so as to bring the agent, which affects the organ, 

 to bear upon the essential part. In the case of the 

 eye, the accessory structures are so ■ complicated and 

 their action so striking that they seem, at first sight, 

 to form the greater part of the whole sense-organ. 

 Hence we may, perhaps with advantage, consider the 

 accessory parts first, and then pass on to the essential 

 structures. 



The accessory organs, by means of which the physical 

 agent of vision, light, is enabled to act upon the expan- 

 sion of the optic nerve, comprise three kinds of appar- 

 atus : (a) a " water- camera," the eyeball; (h) muscles 

 for moving the eyeball ; (c) organs for protecting the 

 eyeball, viz. the eyelids, with their lashes, glands, and 

 muscles ; the conjunctiva ; and the lachrymal gland and 

 its ducts. 



The ball, or globe, of the eye is a globular body, moving 

 freely in a chamber, the orbit, which is furnished to it 

 by the skull. The optic nerve, the root of which is in 



