THE SPECIAL SENSES 293 



(from scleros, hard) is composed of dense, white fibrous tissue, 

 and gives to the eyeball its firmness of figure and its white 

 color ; in front, it constitutes the part commonly called " the 

 white of the eye." It is one of the strongest tissues in the 

 body. It possesses very few vessels, and is not very sensitive. 

 It affords protection to the extremely delicate interior parts 

 of the eye, and the little muscles which effect its movements 

 are inserted into the sclerotic a short distance behind the 

 cornea (see Fig. 64, Y, z). It is perforated posteriorly to 

 admit the optic nerve. 



62. The choroid is the second or middle coat of the eyeball, 

 and lies closely attached to the inner surface of the sclerotic. 

 Unlike the latter, its structure is soft and tender; it is dark 

 in color, and possesses a great abundance of blood-vessels. Its 

 dark color is due to a layer of dark brown or chocolate-colored 

 cells spread out over its inner surface. This dark layer serves 

 to absorb the rays of light after they have traversed the trans- 

 parent structures in front of it. If the rays were reflected 

 from side to side within the eye, instead of being thus absorbed, 

 confused vision would result from the multitude of images 

 which would be impressed upon the optic nerve. 



60. This mechanism has been unconsciously imitated by the 

 opticians, who, when they make a microscope or telescope, take 

 care that the interior of its tube shall be coated with a thick 

 layer of black paint or lamp-black ; for without it, a clear 

 delineation of the object to be viewed is impossible. The 

 albinos, in whom these dark cells of the choroid are wanting, 

 have imperfect vision, especially in the daytime and in strong 

 lights. The dark cells are also wanting in white rabbits, 

 and other animals that have red or pink eyes; their vision 

 appears to be imperfect in the presence of a bright light. 



64. The Iris. Continuous with the choroid, in the front 

 part of the globe of the eye, is a thin, circular curtain, which 



62. The second or middle coat of the eyeball ? Its dark color ? 



68. Similar mechanism in microscopes ? The albinos ? White rabbits ? 



64. What is the iris ? Its construction ? How is the size of the pupil regulated ? 



