.SIGHT. 597 



part of the chorioid has not the black pigment of other parts, but a pigmentum 

 lucidum or a tapetum, shining with a blue or green metallic lustre. In mammalia, 

 to compensate for the smaller extent of motion in the head, the eye has more 

 means of motion than in birds. In aquatic mammalia, the eye has many affinities 

 with the structure of the organ in fishes. In cetaceous animals, for example, 

 the cornea is flat, the aqueous humour inconsiderable ; the lens large, dense, 

 and spherical. Intermediate forms of the organ are observed in the semi- 

 aquatic, the walrus, seal, beaver, otter. " Thus," says my able colleague, 

 "we observe these complicated optical instruments, the most universal and 

 the noblest organs of sense, gradually advancing to perfection from the monad 

 to the man, where all their internal essential parts, and all their external 

 accessory apparatus, are the most exquisitely finished and adjusted ; and it is 

 chiefly through these means that he is enabled to provide for his wants, to acquire 

 the materials of thought, and to enjoy the sublime spectacle of nature." Dr. 

 Grant, 11. cc. See also Dr. Roget, 1. c. 



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