THE DENTITION. 285 



enclosed by the tympanic bone which often projects like a vesicle 

 (tympanic bulla), but is in communication with cavities in the neigh- 

 bouring cranial bones. The tympanic cavity is largest in the Cetacea, 

 in which the sound is not transmitted, as in the terrestrial animals, 

 by the tympanic membrane ami "he auditory ossicles to the fenestra 

 ovalis of the vestibule, but is conducted mainly by means of the 

 bones of the head through the air of the tympanic cavity to the 

 fenestra (/. rotunda) of the unusually large cochlea, and thence to 

 the perilymph of the scala tympani. The three semicircular canals, 

 with the vestibule and cochlea, are very firmly embedded in the 

 petrous bone, which in the Cetacea is only connected by ligaments 

 with the neighbouring bones. The Eustachian tubes open in the 

 Cetacea alone into the nasal passages, in all other Mammals into the 

 pharynx. An external ear (pinna} is wanting in the Monotremes, 

 many Pinnipedes, and in the Cetacea, in which the external meatus 

 outside the convex tympanic membrane is represented by a solid 

 cord; it is rudimentary in the aquatic animals which are able to 

 close the external opening of the ear by a valvular apparatus, and in 

 the burrowing Mammalia. In all other cases it consists of a Tory 

 variously- shaped external appendage, supported by cartilaginous 

 pieces and usually moved by special muscles. 



The sense of touch is mainly located in the skin of the ends of the 

 extremities (tactile corpuscles on the tips of the fingers and on the 

 surface of the hand of Man and tjie Apes); also on the tongue, 

 proboscis, and lips, in which long bristle-like tactile hairs (vibrissce), 

 embedded in follicles, with peculiar nervous ramifications, are very 

 generally present. 



The sense of taste has its seat principally at the root of the tongue 

 (papilla circumvallatce, compare fig. 89, vol. i), but also on the soft 

 palate, and is far more highly developed than in any other class of 

 animals. 



Dentition. At the entrance to the digestive organs the jaws are 



almost always armed with teeth. Only individual genera as 



Echidna, Manis, and Myrmecophaga are entirely without teeth, 

 while the whalebone Whales, which bear on the inner surface of 

 the palate vertical horny plates (whale-bone), arranged in transverse 

 rows (fig. 673), possess teeth, at least in the foetal condition. Horny 

 teeth, produced by hardening of papillae of the buccal mucous mem- 

 brane, are present in Ornithorhynchus and Rhytina. 



The dentition of the Mammalia is never so much developed as that 

 of Fishes and Eep tiles ; and the teeth, which are wedged into alveoli, 



