218 MAMMALIAN ANATOMY 



In the macerated skull it opens widely on the side of the head by 

 the large external auditory meatus, and communicates behind with the 

 inner chamber of the bulla through an opening above the common 

 septum. 



It is pierced in front, close to the base of the skull, by three canals : 

 the inner and larger is the Eustachian opening ; the middle canal, 

 more or less confluent with the inner, is the Glaserian fissure, in a 

 part of which is formed the small canal of Huguier. 



In the temporal bone, before the removal of the soft parts, the 

 external auditory meatus is closed by the drum of the ear or tympanic 

 membrane, and the Eustachian canal lodges the partly cartilaginous 

 Eustachiau tube, which, by opening in front on the back wall of the 

 pharynx, puts the middle ear in communication with the throat. The 

 Glaserian fissure admits to the tympanum the tympanic branch of the 

 internal maxillary artery, and the canal of Huguier gives exit to the 

 chorda tympani nerve. 



The middle ear contains the ear-ossicles, three minute bones which 

 form a chain across its posterior and upper part ; two small muscles 

 which move the ossicles ; and nerves and blood-vessels. The greater 

 part of the cavity, however, is empty. 



The roof of the tympanum is wide in front and narrow behind. 

 It is formed principally by the inferior surface of the petrous of the 

 temporal, but a strip of the inferior surface of the squamous runs along 

 the outer edge, and at the anterior end are seen on the outside the 

 ventral surface of the introverted upper edge of the ectotympanic, and 

 on the inside a triangular area of the inferior surface of the alisphenoid 

 continued forward, dorsal to the inner part of the edge of the ecto- 

 tympanic, as the roof of the Eustachian opening (Fig. 137). 



The middle of the roof is occupied by the large oval fossa for the 

 tensor tympani muscle. Behind and at the outer side of this fossa is 

 a second fossa, which lodges the heads of the two larger of the ear- 

 ossicles, the malleus and the incus. On the median side of this latter 

 fossa is the anterior end of a bridge of bone coming from the squamous 

 and joining the petrous. On the inside of this 1.) ridge is the slit for 

 the stapedius muscle. In front of the large fossa for the tensor 

 tympani is the prominent, flattened, hooked process of the anterior edge 

 of the ectotympanic. At the base of the hook on the outside are a 

 groove and pit in which the long process of the malleus is attached by 



