160 



HUMAN PHYSIOLOGY. 



image formed at the focal point ; the iris to the diaphragm, which by cutting 

 off the marginal rays prevents spherical aberration and at the same time 

 regulates the amount of light entering the eye; the ciliary muscle to the 

 adjusting screw by which distinct images are thrown upon the retina in spite 

 of varying distances of the 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 

 by the following figure : 



DIAGRAM OF A VERTICAL SECTION OF THE EYE. 



i. Anterior chamber filled with aqueous humor. 2. Posterior Chamber. 3. Canal o* 



Petit. 

 a. Hyaloid membrane, b. Retina (dotted line), c. Choroid coat (black line). 



d. Sclerotic coat. e. Cornea, f. Iris. g. Ciliary processes, h. Canal of Schlemm 



or Fontana. /. Ciliary muscle. From Holden' s Anatomy. 



The Dioptric or Refracting apparatus by which the rays of light enter- 

 ing 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 refrac- 

 tion at 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 re- 

 duced to three, viz : I. Cornea and aqueous humor. 2. The lens. 3. 

 The vitreous humor. The refracting surfaces may also be reduced to three, 



