394 ELEMENTARY. ANATOMY. [LESS. 



almost universal, the only exceptions being the Lampreys, 

 which have but two canals, and the Myxine, which has but 

 one the whole organ in it consisting of but one circular 

 canal. 



A cochlea is present in all Mammals and Sauropsida, but 

 only in the former is it spirally coiled, and not in all of 

 them, it bein ,* but slightly twisted in Monotremes as in Sauro- 

 psidans. In man's own division, the Monodelphous Mammals, 

 the cochlea may be less coiled than in him, namely only one 

 and a half times (e.g. the Hedgehog) ; or more so than in 

 him, as in the Paca, which has it coiled five times. 



The MEMBRANOUS LABYRINTH may, as in Fishes, be en- 

 closed by hard structures (bone or cartilage) in such a way 

 .that no fenestrae are left unsolidified, but at the same time 

 the bony chamber (or otocrane) may be widely open inter- 

 nally, as e.g. in the Carp, Cod, &c. 



The opening of the primitive involution of the integument 

 to form the ear may persist throughout life, as in the Rays, 

 where it opens on the top of the head by a valvular aperture. 

 The external auditory meatus may be quite absent, as in 

 Fishes and Tailed-Batrachians, and the tympanic membrane 

 may appear on the surface of the cranium (though covered 

 with a layer of skin), as in many Frogs and Lizards. 



The internal part of the primitive cleft (i.e. the Eustachian 

 tube) may be absent, as in Fishes ; or the two tubes may 

 unite into a single and median aperture, as in Birds and 

 the Toad Pipa; or they may open (as in the Crocodiles) by 

 three apertures one median and two lateral ones each 

 lateral Eustachian tube forming a singularly complex and 

 contorted canal. They may have narrow openings into the 

 pharynx, as in the higher Vertebrates, or wider ones, as in 

 Frogs. 



The auditory ossicles may be very different from those of 

 man in form and relative size. Thus their relations have 

 been asserted to be as follows : 



In the malleus of man the processus gracilis creeps down, 

 as it were, to the Glasserian fissure, but it remains much 

 smaller than the manubrium. As we descend in the animal 

 scale we find these proportions reversed, till in the Echidna 

 the very large malleus has an enormous processus gracilis, 

 which descends in close contiguity to the lower jaw. In the 

 Sauropsida the enormous malleus becomes segmented, one 

 portion constituting the os quadrature, and the other forming 

 the os articulare of the mandible. 



