246 VERTEBRATE ZOOLOGY 



The interior of the eye is divided into two principal chambers 

 by the lens and the iris. The posterior cavity is filled with a 

 jelly like mass called the vitreous humor ; the anterior cavity, with 

 a fluid mass called the aqueous humor. 



Exercise 36. Draw a diagram showing the structure of the eye. Care- 

 fully label all the parts. 



The Ear. The organ of hearing is made up of three parts: 

 the internal ear, or membranous labyrinth,— the essential auditory 

 organ, which is situated in a bony capsule at the side of the 

 cranium; the middle ear, or tympanic cavity, in which are the 

 ear ossicles and the tympanic membrane ; and the outer ear. 



The outer ear is composed of the external ear, which extends 

 above the head and receives the vibrations of sound, and the 

 external auditory meatus, the canal which leads to the tympanic 

 cavity. The external ear, whose movements are controlled by 

 small muscles, is a thin plate of cartilage, covered on both sides 

 by integument. 



The Middle Ear. Cut away the walls of the external auditory 

 meatus until the bony walls of the skull are reached. At the inner 

 end of the meatus is the tympanic membrane, or eardrum, sur- 

 rounded by a ridge of bone. Note its shape and the character 

 of the membrane. The middle ear lies in the tympanic bulla, 

 which is the spherical projection of the skull just back of the 

 external ear. Entirely free this from the muscles which may be 

 attached to it. With a strong scalpel cut away the ventral wall 

 of the bulla and expose its interior. This is divided into a medial 

 and a lateral chamber by a bony partition extending from its 

 lateroventral wall. Of these two chambers the lateral one is the 

 tympanic cavity proper and contains the ear ossicles; from the 

 forward end the Eustachian tube goes to the nasopharynx. Note 

 the round hole in the dorsal wall ; this is the fenestra rotunda. 



Carefully cut away the partition between the chambers until 

 the inner surface of the tympanic membrane appears, and study 

 this and the minute ear ossicles. There are three of these bones : 

 the hammer, or malleus ; the anvil, or incus ; and the stirrup, or 

 stapes. The hammer consists of two parts, — a slender handle and 



