100 COMPARATIVE ANATOMY. 



where it resembles that of the bird, is a large heliciform 

 chamber abundantly supplied with filaments of the acoustic 

 nerve, and divided longitudinally by a solid lamina. Otolithes 

 granular. Middle ear traversed by chain of bones called, 

 from within outwards, stapes, incus, and maleus. The vidian 

 nerve escapes through tympanum, as in Aves. Communica- 

 tion with diploe is seen in the openings between the middle 

 of the ear and mastoid process. 



The temporal bone of mammals is divided into three por- 

 tions squamous, mastoid, and petrous and is usually de- 

 scribed as arising from six centres of ossification : squamous, 

 t} 7 mpanic, zygomatic, styloid, mastoid, and petrous. But the 

 petrous portion in reality arises from two distinct centres of 

 ossification (opisthotic and pro-otic), the mastoid (epi-otic) in 

 part only from one. 



In the human skull the petrous portion of the bone, as 

 thus formed, is a very complicated adjustment of bony ele- 

 ments, viz.: an inferior opisthotic bone containing the lower 

 part of cochlea, and a superior, pro-otic, sheltering the greater 

 part of the vestibule, the upper part of the cochlea, the an- 

 terior vertical semicircular canal, part of the posterior verti- 

 cal canal, and the external semicircular canal. (Huxley.) The 

 epi-otic is confined entirely to the mastoid process.* 



Monotremata. Cochlea smaller than in other mammals ; 

 semicircular canals relatively large. Tympanic ossicles an- 

 chylosed, resembling columnella of reptiles and birds. In 

 Echidna (porcupine anteater) external auditory canal long and 

 wavy. 



Marsupialia. The middle ear often extends into petrous 

 portion of temporal and bod} 7 of sphenoid bones. 



Eodentia. Bony portion of external auditory canal often 

 extended, and directed backwards, as in Lepus (hare). 



* Although in man the tympanic bone becomes early united with the squa- 

 mous, it remains separate from it in many of the lower animals ; in some of 

 these, as in Cetacea, it is joined to the petrous and mastoid, and forms, with 

 them and the styloid, a bone which continues distinct from the squamous. 

 The mastoid portion remains quite separate in some animals, and occasion- 

 ally, though very rarely, in the human skull. (Humphrey.) 



