SENSE OF SIGEIT. 8'j 



25. The iris is contractile, and its principal use is to regulate 

 the quantity of light which should penetrate to the bottom of the 

 eye ; when the light is too vivid, it contracts, and consequently 

 diminishes the pupil, through which the rays must pass to reao h 

 the retina; in the dark on the contrary the pupil is enlarged. 



26 Thechoroid membrane, which lines the internal face of the 

 globe of the eye, is covered with a sort of black varnish, which 

 bsorbs all the luminous rays, not necessary for vision. 



27. Images painted, if we may use this term, upon the retina, 

 are transmitted to the brain through the medium of the optic 

 nerve. 



28. The accessary parts of the apparatus of vision are of two 

 kinds; the one is designed to protect the globe or ball of the eye, 

 the oiher to move it and give the required direction to fulfil its 

 functions in the best manner. 



29 The protecting organs of the eye are: 1st, the orbit, 2nd, 

 the eye-lids, 3rd. the lachrymal apparatus 4th, the eye-brows. 



30. The n>'lrit is a great bony cavity, hollowed out in the face 

 on each side of the nose. It has the form of a cone, the base of 

 which is open and 'directed forward; its parietes are formed, 

 above by the frontal bone ; below, by the superior maxillary bone ; 

 externally, or outwardly, by the malar or cheek bone, and inter- 

 nally by the bones which belong partly to the nose. The bottom 

 of the orbit is pierced by a large hole, which communicates with 

 the cranium, and gives passage to the optic nerve. 



31. The ball of the eye is set into this cavity and rests upon a 

 sort of cushion formed of fat. It is protected in the same way 

 on all sides except in front, and there we find the eye-lids. 



32. The eye-li.ls are moveable curtains, stretched in front of 

 the ball of the eye. On the outside they are formed of the skin ; 

 internally they are lined by a smooth membrane which is reflected 

 over the front of the eye upon the sclerotica, and this membrane 

 is called the membronn conjunctiva ; between these two mem- 

 branesthe conjunctiva, and the skin there is placed a thin plate 

 of fibrous and resisting substance, called tarsus or palpebral car- 



ilage, as well as muscles which serve to move these organs. In 



25. What is the action of the iris? 



26. What is the use of the black covering 1 of the choroid coat? 



27. How are images upon the retina transmitted to the brain? 



28. What are the uses of the accessary parts of the apparatus of vision' 

 2'D. What are the protecting organs of the ^ye ? 



30. What is the orbit ? 



31. Upon what does the ball of the eye rest? 



32. What are the eye lids? Of what do they consist? What is then 

 number? 



3 



