THE VITREOUS HUMOR AND IRIS. 507 



to the retina; the quantity of which is, cceteris paribus, directly 

 proportioned to the size of the pupillary aperture. The pos- 

 terior surface of the iris is coated with a layer of dark pig- 

 ment, so that no rays of light can pass to the retina, except 

 such as are admitted through the aperture of the pupil. 



The ciliary muscle is composed of organic muscular fibres, 

 which form a narrow zone around the interior of the eyeball, 

 near the line of junction of the cornea with the sclerotic, and 

 just behind the outer border of the iris (Fig. 178). The outer- 

 most fibres of this muscle are attached in front to the inner 

 part of the sclerotic and cornea at their line of junction, and, 

 diverging somewhat, are fixed to the ciliary processes, and a 

 small portion of the choroid immediately behind them. The 

 inner fibres, immediately within the preceding, form a circular 

 zone around the interior of the eyeball, outside the ciliary 

 processes. They compose the ring formerly called the ciliary 

 ligament. 



The function of this muscle is to adapt the eye for seeing 

 objects at various distances. The manner in which it effects 

 this object will be considered afterwards (p. 511). 



The contents of the ball of the eye are surrounded and kept 

 in position by the cornea, and the dense, fibrous membrane 

 before referred to as the sclerotic, which, besides thus incasing 

 the contents of the eye, serves to give attachment to the va- 

 rious muscles by which the movements of the eyeball are 

 effected. These muscles, and the nerves supplying them, have 

 been already considered (p. 425, et seq.}. 



Of the Phenomena of Vision. 



The essential constituents of the optical apparatus of the eye 

 may be thus enumerated : A nervous structure to receive and 

 transmit to the brain the impressions of light ; certain refract- 

 ing media for the purpose of so disposing of the rays of light 

 traversing them as to throw a correct image of an external 

 body on the retina ; a contractile diaphragm with a central 

 aperture for regulating the quantity of light admitted into the 

 eye ; and a contractile structure by which the chief refracting 

 medium shall be so controlled as to enable objects to be seen 

 at various distances. 



With the help of the diagram below (Fig. 184), represent- 

 ing a vertical section of the eye from before backwards, the 

 mode in which, by means of the refracting media of the eye, 

 an image of an object of sight is thrown on the retina, may be 

 rendered intelligible. The rays of the cones of light emitted 

 by the points A B, and every other point of an object placed 



