326 THE BIOLOGY OF THE FROG chap. 



gain the impression that the eye is pressed into the head by 

 the closure of the lids. The opening of the eye, on the 

 other hand, while generally accompanied by the protrusion 

 of the eyeball through the contraction of the levator bulbi 

 muscle, is partly effected by means of an independent 

 muscle, the depressor membrance nictitantis. In the closure 

 of the eye the lower lid and nictitating membrane are pulled 

 over all but a small part of the exposed surface. 



Through the transparent cornea may be seen the colored 

 iris, in the center of which is an oval aperture, the pupil. 

 Just behind the pupil lies the transparent crystalline lens, 

 which, unlike the lens of higher animals, is of a nearly 

 spherical form, although somewhat more flattened on the 

 anterior face. It is made up of concentric layers like the 

 coats of an onion. On its outer surface is a single layer of 

 epithelial cells, the remaining portion consisting of trans- 

 parent fibers whose general direction is parallel with the 

 optical axis of the eye. The whole lens is surrounded by 

 a very delicate transparent capsule which is attached along 

 the outer margin to the ciliary body by means of radiating 

 fibers. 



The cavity of the eye is divided by the lens and its sup- 

 porting fibers into an outer chamber which contains a trans- 

 parent watery fluid, the aqueous humor, and a larger inner 

 chamber filled by a more solid transparent substance known 

 as the vitreous humor. The vitreous humor is permeated 

 by numerous fibers which form a sort of supporting network 

 which contains free cellular elements and numerous blood 

 vessels. The circulation in the latter as seen by the oph- 

 thalmoscope has been described by several observers and 

 with especial fullness by Hirschberg and Schleich. 



The wall of the posterior chamber of the eye is formed 

 of three tunics, the scle?-otie coat, which has been described 



