LESSON 94.] ORGANS OF VISION IN THE HIGHER ANIMALS. 301 



watery medium around them, they have little necessity for an aqueous 

 humor, and their cornea is flat. To preserve this flatness in front, 

 the sclerotic coat is thickened and consolidated ; and it is also to 

 prevent its assuming the spherical form in Birds, by the equal pres- 

 sure of the contained fluids, that the sclerotic is there strengthened 

 with bony plates, which preserve the tubular form of the eye, and 

 the great convexity of the cornea in that class. 



1364. The crystalline lens in Fishes is composed of minute trans- 

 parent fibres, disposed in concentric layers, and united FIG. 427. 

 by their serrated edges, as seen in the Codfish (Fig. 



427), the layers increasing in density from the sur- 

 face to the centre of the lens. 



1365. The organs of vision are smallest in such 

 Fishes as burrow in the mud and sand ; they are 

 larger in predaceous Fishes, which frequent the dark 



_ & . . r . Fibres of the crys- 



depths of the ocean, than in those which reside on taiiine lens, Codfish. 

 the shallow coasts, or in fresh waters. Ciliary processes are rarely 

 developed in this class. 



LESSON XCIV. 



ORGANS OF VISION IN THE HIGHER ANIMALS 



1366. As the eyes, we must henceforth consider, possess a more 

 complicated structure than the organs hitherto examined, it appears 

 to be desirable to give a brief enumeration of the parts which col- 

 lectively form a visual organ in the higher animals. 



1367. The globe of the eye is composed of tunics and humors. 

 The tunics are three in number, the 



1. Sclerotic and cornea. 



2. Choroid, iris, and ciliary processes. 



3. Retina, and zonula ciliaris. 



1368. The sclerotic (skleros, hard) and cornea form the external 

 tunic of the eye-ball, and give to it its peculiar form. The sclerotic 

 is much thicker behind than before ; it is pierced at its posterior 

 surface by the optic nerve, ciliary nerves, and arteries. The cornea 

 is attached to its anterior part, by means of a bevelled edge ; its an- 

 terior surface is also covered by a thin tendinous layer, the tunica 

 albuginea (white tunic), which is covered for a part of its extent by 

 the mucous membrane of the front of the eye, the conjunctive mem- 



