194 PHYSIOLOGY. 



of the optic nerve. The ordinary sensibility of the eye is 

 derived from the branches of the fifth pair of nerves ; and 

 we have already seen that several pairs of nerves are sent to 

 its muscles. 



7. The minuteness with which objects are painted on the 

 retina is indeed wonderful. Standing on an eminence, the 

 eye takes in a landscape of hundreds of miles in extent ; and 

 all this is painted on the retina, though, as an English writer 

 calculates, a portion of the castle of Edinburgh, 500 feet 

 long and 90 feet high, occupies on the retina only the twelve 

 hundred thousandth part of an inch, when seen at a certain 

 distance. " A whole printed sheet of newspaper," says 

 Arnott, " may be represented on the retina on less surface 

 than that of a finger nail, and yet not only shall every word 

 and letter be separately perceivable, but even any imperfec- 

 tion of a single letter ; or, more wonderful still, when at night 

 an eye is turned up to the blue vault of heaven, there is por- 

 trayed on the little concave of the retina, the boundless con- 

 cave of the sky, with every object in its just proportions. 

 There a moon in beautiful miniature may be sailing among 

 her white-edged clouds, and surrounded by a thousand twink- 

 ling stars, so that to an animalcule, supposed to be within 

 and near the pupil, the retina might appear another starry 

 firmament with all its glory. If the images in the human 

 eye be thus minute, what must they be in the little eye of a 

 canary bird, or of another animal smaller still ! How wonder- 

 ful are the works of nature !" 



8. The Cornea is the term applied to the anterior tran- 

 sparent portion of the ball. It resembles a watch crystal in 

 shape, and it is received into a groove in the sclerotic coat, 

 in the same manner as a watch-glass is received into its 

 case, as in this cut. It is somewhat thicker than the sclerotic 



