TADPOLES AND FROGS. 
191 
Sight. The optic lobes just mentioned are con¬ 
nected with a pair of eyes very much like our own. 
Each eye is really a camera obscura with its lens and 
darkened walls. The image of an external object is 
thrown on the retina , which corresponds to the ground 
glass of a camera and is composed of nervous tissue 
forming the end of the optic nerve. The frog sees well, 
B 
Fig. 149.—Striped Muscle-fibres. A, a fibre much magnified; B, a 
fibre breaking up into fibrillae. 
as is evidenced by the precision with which he strikes 
a fly with his tongue. 
Hearing. On either side of the head is a dark cir¬ 
cular spot which indicates the position of the ear. This 
ear is wholly internal. Under the skin is a membrane, 
or ear-drum, and under that a cavity, the middle ear. 
This cavity connects with the mouth by the eustachian 
tube as in man. Back of this cavity is another, the 
inner ear, containing the nerves which conduct sound 
vibrations to the brain. 
Smell. In the nasal cavities fibres of the olfactory 
nerve are spread out to receive the sensations of smell. 
