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serpents, however, they closoly resemble the caduci-branchiate am- 

 phibia, and by their vestibular saculus, containing solid cretaceous 

 bodies, they resemble the osseous fishes. The cavity of the tym- 

 panum is larger in the saurian than in the ophidian reptiles. As 

 we ascend to the higher orders, we meet with a cochlea, which is 

 the last part of the internal ear that attains perfection, slightly cnrved 

 and divided into a scala tympani and scala vestibuli. 



In birds the organ of hearing resembles that of the crocodile ; 

 there is no cartilaginous external ear, and but a rudimentary concha, 

 which, however, is compensated for, especially in rapacious birds, 

 by a peculiar arrangement of feathers around the external meatus, 

 which in general they can erect at will so as to catch distant sounds, 

 and by that means either flee from danger, or pursue their prey 

 through dark and gloomy places. The external ear of owls is fur- 

 nished with a crescentic fold of integument in the form of a valve. 

 The oval tympanum, surrounded by bone, communicates with the 

 air-cells of the cranium by three large openings, and with the 

 fauces by two short wide Eustachian tubes which open by a single 

 orifice in the swan at the back of the posterior nares. There is but 

 one bone in the tympanum analogous to the stapes, with processes 

 rudimentary of the malleus and incus, and moved by a single tensor 

 muscle, which comes from the occiput and increases the convexity 

 of the membrana tympani by drawing it outwards. The cochlea, 

 though more developed than in reptiles, has not yet reached perfec- 

 tion, and the other parts of the internal ear present no remarkable 

 peculiarities different from the inferior grades of the next class. 



In the mnmmalia, generally, the organ of hearing is distinguished 

 by the development of a true cochlea ; by an increased number of 

 auditory hones ; by the formation of external canal, and by the 

 addition of an external movable ear. We will meet, however, with 

 evident, though gradual, transitions from the simple state of organ 

 already seen in the inferior classes, up to man, where it has attained 

 its most complex and perfect condition. The concha is very small 

 in otarite, beavers, and otters, and wholly absent in the cetacea, 

 seals, walruses, the mole, the manis, and the ornithorhynchus. The 

 aquatic shrew and other mammalia which frequently go into the 

 water, form an approach to the crocodlile in having the external 

 auditory opening furnished with a valve. This external orifice in 

 the dolphin is merely large enough to admit a pin, and from it a 

 long narrow winding passage leads to the tympanum through the 

 fat which lies under the skin. As we ascend through the mam- 

 malia, residing more exclusively on the land, the concha acquires 

 greater size, and by the development of cartilage and powerful 

 muscles it becomes to enjoy very free and varied motions. It is 

 large, movable, and directed backwards in the ruminantia, pachy- 

 dermata, cheiroptera; and especially in the timid and feeble rodentia ; 

 and in the carnivora it is small and inclined forwards. 



The cavity of the tympanum communicates with the mastoid cells, 

 contains four movable ossicula and three distinct muscles ; it is 



