378 



hitchcock's anatomy 



FUNCTIONS OF THE EYE. 



692. The primary and obvious use of the eye is to receive 

 the form and color of different objects, and to transmit them 

 to the brain. 



693. Use of the Coats — Sclerotica — Cornea — Choroid— 

 Black Paint — Iris — Ciliary Processes — The Retina— The 

 use of the firm and hard sclerotic coat is to give a firm attach- 

 ment to the muscles which move the eye, and also secure pro- 

 tection to the delicate parts within. The cornea is tough but 

 transparent, so that the anterior portion of the eye may be 

 firm and at the same time admit the rays of light. The cho- 

 roid coat furnishes the blood to the eye, it being for the most 

 part entirely made up of blood-vessels. It also secretes the 

 black paint (Fig. 288), which is of great service in absorb- 

 ing the superfluous or wandering rays of light which otherwise 

 would obscure the image on the retina. A similar provision 

 is made by painting black the tube of the telescope and mi- 

 croscope. The iris regulates the amount of light entering the 

 ball, by its contraction and expansion. This function is per- 

 formed by the radiated and circular muscular fibers, the 

 latter of which contract by the stimulus of strong light; 

 but if the lio;ht be feeble, the radiated contract, and thus 

 enlarge the pupil. The retina, with the vitreous humor 

 in front and the black paint directly behind it, receives 

 the image of the object before the eye. It bears the same 

 relation to the other parts of the eye as the silvered or glass 

 plate does in the camera of the photographist. This impres- 

 sion is carried to the brain by means of the optic nerve. 



694. Use of the Humors — Aqueous— The Lens — Vitreous c 

 — Of the three humors or media of the eye, the aqueous is 

 the least important, since if by accident or operation it is re- 



G92. State the use of the eye. 693. Give the use of the sclerotic coat. Of the cornea. 

 What does the choroid furnish to the eye ? Of what service is the black paint ? * What 

 does the iris do? What is the function of the ciliary processes? What ia the use of the 

 retina ? 694. Which is the least Important of the humors ? 



