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, 

 tympanic, 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 body of sphenoid bones. 



Rodentia. 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.) 



