VER TEBRA TES. 303 



In the sharks this ear-sac is placed behind and medial 

 to the spiracle (p. 308). In the higher vertebrates the 

 spiracle becomes closed on the outside, but the rest of the 

 structure remains, and is known as the Eustachian tube, 

 and as its outer end comes between the ear and the external 

 world, one or more bones usually extend across the tube 

 to convev the sound-waves to the sac. This forms the 



V 



middle ear. In the frogs the outer end of the Eustachian 

 tube is closed by the large tympanic membrane on the side 

 of the neck. 



In the higher vertebrates an external ear occurs. This 

 consists of a tube leading inward to the tympanic mem- 

 brane, and to this tube are frequently added structures to 

 catch and reflect the sound-waves into the tube. It should 

 be mentioned that the ear is more than an organ of hearing ; 

 it is also an organ for maintaining the balance, for if the 

 ear or the auditory nerve be injured the animal can no 

 longer maintain its equilibrium. 



The eye (fig. 1 13) is built on the plan of a photographic 

 camera. The essential parts are a lens which brings the 

 rays of light to a focus on the retina, and means for causing 

 the image on the retina to stimulate the optic nerve. To 

 these are added various accessory structures for protection, 

 for regulating the amount of the light, etc. In the lower 

 forms eyelids are absent,, but higher in the scale folds of flesh 

 are developed which can close over the organ. Many ani- 

 mals have three of these eyelids, two working vertically, 

 the third, the nictitating membrane, drawn from the inner 

 angle of the eye over the transparent cornea. This nictitat- 

 ing membrane occurs in the eye of man as a small fold 

 (semi-lunar fold}, which has entirely lost its primitive 

 protective function. 



Over the whole globe of the eye is a tough layer, the 



