THE RABBIT 305 



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

 internal ear, or membranous labyrinth, which is the essential 

 auditory organ and 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 waves of sound, and the external 

 auditory meatus, the canal which leads to the tympanic cavity. 



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

 meatus until the skull is reached ; note that the inner end of the 

 auditory meatus is surrounded by bone. The middle ear is the 

 space within the tympanic bulla, which is the spherical projection 

 near the base of the jaw and at the medial end of the auditory 

 meatus. With a strong scalpel cut away the ventral wall of the 

 tympanic bulla and expose its cavity; note the bony ring in its 

 lateral wall ; this ring supports the tympanum, or eardrum. Note 

 the vertical position of the tympanum ; anterior to it is a depres- 

 sion in the bone in which are located the three minute ear ossicles. 

 These bones are so small that it is difficult to study them in a dis- 

 section ; but if they are successfully removed they will be found to 

 be the following: the hammer, or malleus, which lies across the 

 dorsal portion of the tympanum; the anvil, or incus, which has 

 two projections, one of which articulates with the hammer, and 

 the other with the stirrup ; and the stirrup, or stapes, which fits 

 into the fenestra ovalis, an opening into the inner ear. 



The Internal Ear. The membranous labyrinth is an irregular sac 

 entirely inclosed in the bony capsule in the skull called the bony 

 labyrinth. It communicates with the middle ear by two openings 

 in the bony labyrinth, the fenestra ovalis, just mentioned, and the 

 fenestra rotunda, and is composed of three main portions: the 

 vestibule, the semicircular canals, and the cochlea. The bone in- 

 closing the membranous labyrinth is very hard and thick, and 

 a dissection of it is impossible without a special treatment of 

 the bone. 



The Central Nervous System; the Brain. Before removing the 

 brain from the cranial cavity cut away the skin and the muscles 

 from the head and the adjacent portions of the neck ; remove also 



