CHAPTER XI. 



Sense of sight. Organ of vision. Globe of the eye; sclerotic; cornea; 

 choroid ; ciliary ring ; ciliary body; ciliary process; iris; pupil; nvea; 

 pigment; retina; vitreous body; hyaloid membrane; crystalline; anterior 

 and posterior chambers; aqueous humour. Muscles of the eye. Con- 

 junctiva. Eyelids, eyelashes. Lachrymal apparatus. Vision ; func- 

 tions of the retina, reversed images; functions of the iris; optic centre, 

 visual angle, visual impressions, single or mixed, adaptation of the eyt to 

 distances, myopia, presbyopia; achromatism; single and double vision 

 with two eyes, stereoscope; alternation in the action of the eyes; persist- 

 ence of impressions on the retina; accidental images; irradiation; acci- 

 dental aureola; Daltonism; apparent motion of objects. Optic nerve. 

 Movements of the eye. Extent of vision. 



Organ of vision. The visual apparatus consists of the 

 globe of the eye and its appendages, which are the eyelids 

 and eyebrows, the motor muscles of the eye, and the lach- 

 rymal apparatus. 



The globe of the eye. The globe of the eye is generally 

 described as a spheroid, to which the segment of a smaller 

 sphere is applied in front, and this definition is exact to the 

 senses if it is not so mathematically. The walls of the globe 

 of the eye are formed principally of two fibrous membranes; 

 one white and opaque the sclerotic (sderos, hard)- which 

 envelops the two posterior thirds of the globe; and the other 

 transparent, and resembling a horny plate, from whence its 

 name, cornea. The sclerotic is one of the strongest fibrous 

 membranes in the body; it is white on its external surface, 

 and of a brownish-red colour internally; it is thicker at the 

 posterior portion of the eye, where it opens to allow the pas- 

 sage of the optic nerve, than in front, where it terminates in 

 a circular hollow or slope, into the border of which the cornea 

 is set like a watch-glass. The two membranes are united by 



