204 THE SPECIAL SENSES. 



vessels. The Sclerotic (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 consti- 

 tutes 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 pro- 

 tection to the extremely delicate interior parts of the eye; 

 and the little muscles which effect its movements are in- 

 serted into the sclerotic a short distance behind the cornea 

 (see Fig. 48, Y, z). It is perforated posteriorly to admit the 

 optic nerve. 



62. The Choroid is the second or middle coat of the eye- 

 ball, and lies closely attached to the inner surface of the 

 sclerotic. Unlike the latter tunic, 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 transparent 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. 



63. This mechanism has been unconsciously imitated by 

 the opticians, who, when they make a microscope or tele- 

 scope, take care that the interior of its tube shall be coated 

 with a thick layer of black paint or lamp-black; for with- 

 out it, a clear delineation of the object to be viewed is im- 

 possible. 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. 



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



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



