THE FROG 197 



is the sensitive part of the eye. At the back of the chamber 

 is an opening through which the optic nerve enters, on. After 

 entering the eye the nerve spreads out on the retina, where it 

 is affected by the light entering the eye. The chamber of the 

 eye is divided into two parts by a large spherical, transparent, 

 crystalline lens, held in position by several bands of fibers, 

 shown at I. Anteriorly the lens is partly covered by an opaque 

 membrane, really a continuation of the choroid, which grows out 

 from the wall of the chamber on all sides. This is the iris, and 

 it covers the outer part of the lens, except in the middle, where 

 the lens is not covered. This opening is the pupil, and serves 

 to allow light to enter. The iris contains pigment cells, which 

 give the eye its color. Each of the two chambers of the eye 

 is filled with a transparent fluid. That lying between the 

 cornea and the lens is the aqueous humor, and that back of the 

 lens, which is rather more solid, is the vitreous humor. The 

 retina, which lines the eye chamber, is an extremely complicated 

 organ, made of hundreds of thousands of end organs of sensi- 

 tive nerves. It is a complex of neuron bodies, dendrites, and 

 axons (Fig. 96 B) t and is highly sensitive to the light, which 

 is focused upon it by the lens. Attached to the ball of the eye 

 are six muscles, by means of which it can be rotated in any 

 direction: 



The ears. The frog has no external ears. Just back of the 

 eyes are two rounded, flat depressions, each formed by a mem- 

 brane which covers the real ear. If the thin skin which covers 

 this area be removed, a rather tough, flat membrane will be 

 found beneath, which is the tympanic membrane proper. This 

 membrane extends over a shallow conical cavity, called the 

 tympanum or ear-drum. This cavity connects below with 

 the mouth through the eustachian tube. Extending across 

 this is a slender bar of bone and cartilage, called the columella. 

 This is attached to the membrane at one end and connected 

 with the inner ear at the other, and transmits vibrations of 

 the membrane to the inner ear, the real organ of hearing. 



