218 



HUMAN PHYSIOLOGY. 



object from which the light rays emanate. The structures just enumerated 

 are those essential for normal vision. 



The relationship of the various structures composing the eyeball is shown 

 in Figure 31. 



The dioptric or refracting apparatus, by which the rays of light entering 

 the eye are so manipulated as to produce an image on the retina, consists of 

 the cornea, aqueous humor, crystalline lens, and vitreous humor. A ray 

 of light in passing through each of these media will undergo refraction at 



FIG. 33. DIAGRAM OP A VERTICAL SECTION OF THE EYE. 



(From Holden's "Anatomy") 



i. Anterior chamber filled with aqueous humor. 2. Posterior chamber. 3. 

 Canal of Petit, a. Hyaloid membrane. 6. Retina (dotted line), c. Choroid coat 

 (black line), d. Sclerotic coat. e. Cornea. /. Iris. g. Ciliary processes, h. 

 Canal of Schlemm or Fontana. *. Ciliary muscle. 



their surfaces and ultimately be brought to a focus at the retina. Inasmuch as 

 the two surfaces of the cornea are parallel and its refractive power practically 

 the same as the aqueous humor, the media may be reduced to three viz.: 



1. Cornea and aqueous humor. 



2. The lens. 



3. The vitreous humor. 



The refracting surfaces may also be reduced to three viz.: 



1. Anterior surface of the cornea. 



2. Anterior surface of lens. 



3. Posterior surface of lens. 



