Organ of Hearing, 1^3 



cartilage commonly found, and with the pendulous flaps of 

 the Elephant. The Meatus externuB is generally osseous. 



The Tympanic cavity is by no means always concealed in 

 the petrous bone, as it is in Man ; it is sometimes expanded 

 into a ^ bulla'; and may extend into the Squamosal and 

 Pterygoid bones*. The tympanic bone frequently retains its 

 freedom, and is more or less subservient to the support of the 

 drum membrane. The Basisphenoid sometimes f enters into 

 the formation of the tympanic cavity. 



The Three Auditory Ossicles (Otosteals) can in general be 

 distinguished ; viz., the Malleus, the Incus with the Os orbicu- 

 lare, and the Stapes, although their forms undergo consider- 

 able changes. Thus, the Malleus occasionally bears a close 

 resemblance to the * cartilage columellse' of Birds ; or it may 

 assume an expanded lamelli-form type ; or be bent, complet- 

 ing the circle for the drum-membrane, and expanding for its 

 attachment thereto ; or, lastly, it may be perforated. Again, 

 the Stapes may have a wide vacuity and slender crura, and 

 be traversed by a bony canal surrounding the trunk of a 

 nerve and vessel (arteria ophthalmica and maxillaris, or 

 arteria meningea media :|:) which pass through the stapes and 

 the tympanic cavity : or it may be columelliform, its oval 

 base supported on an imperforate stem, which may or may 

 not develope processes marking out the Incus : or it may 

 retain the normal Mammalian shape, and have very thick 

 crura, by which the perforation is obliterated or reduced to 

 a minimum. 



The bony Semicircular Canals occasionally project from the 

 petrosal capsule within the cranium : in many, though not 

 in all of the Mammalia, these canals open by five orifices 

 into the vestibule : there are nearly always three ampuUaj 

 present. 



* Sloth. t In Hedgehog (Insectivora) and Marsupialia. 



X In Cheiroptera. 



