322 ANATOMY OF THE RABBIT 



may be studied. They are chiefly those already described in 

 connection with the skeleton (p. 186), but the following soft parts 

 may be identified. 



(a) The tympanic membrane (membrana tympani) is stretched 

 almost vertically across the lower end of the external 

 acoustic meatus. 



(b) The tensor tympani is a slender muscle, the origin of which 

 forwards from the alisphenoid is concealed. It is inserted 

 on the manubrium mallei. 



(c) The stapedius is a minute muscle arising from the periotic 

 bone above the cochlear fenestra and inserted on the stapes. 



(d) The chorda tympani is a delicate nerve which crosses the 

 tympanic cavity, lying between the long crus of the incus 

 and the manubrium mallei. 



The nerve is a continuation of the intermediate nerve, a sensory 

 root of the facial, which arises independently of the chief or motor root 

 and joins the facial in the facial canal of the periotic bone. Its peri- 

 pheral connections are with the lingual nerve and the submaxillary 

 ganglion. 



(e) The internal carotid artery traverses the carotid canal of 

 the tympanic bone. By breaking away the posterior 

 portion of the bulla, the entrance of the vessel into the 

 external carotid foramen may be seen. 



The dissection of the parts of the ear as here outlined includes only the 

 external and middle portions together with the associated acoustic nerve and its 

 entrance to the periotic bone. The parts of the internal ear (Fig. 47, p. 91) 

 are not readily made out without the use of special methods, though their position 

 may be estimated by carefully breaking away the surface of the ventral portion 

 of the periotic. They include (1) the bony labyrinth, consisting of a series of 

 connected spaces lodged in the interior of the petrous bone, and comprising the 

 cochlea, vestibulum, and the bony semicircular canals; and (2) the mem- 

 branous labyrinth, consisting of a second series of spaces contained within the 

 first, and comprising the duct of the cochlea, the sacculus, the utriculus, 

 and the semicircular ducts, together with their connections and the endo- 

 lymphatic duct and sac. The membranous labyrinth contains the sensory 

 portion of the ear and its cavity is occupied by a fluid material, the endolymph. 

 The wall is separated from that of the bony labyrinth by an extensive peri- 

 lymphatic space also occupied by a fluid material termed the perilymph. 



