ORGAN OF HEARING IN MAMMALIA. 225 



nn angle of 45 with the plane of the base : and, by depressing 

 this into the { feiiestra,' to make tense the closing curtain and set 

 in motion the perilymph. The muscle, ib. it, which seems to 

 answer to the ( tensor tympani,' degenerates, in most Cetacea, into 

 tissue which becomes ossified and fixes the malleus to the wall of 

 the tympanum. In all Cetacea the otosteals are dense and massive. 

 In Delphinus leucas the stapes is imperforate, as in the Walrus, 

 fig. 170, c : in Phoccena communis, ib. B, there is a small hole. 

 From the inner or mesial and anterior end of the tympanum the 

 Eustachian canal is continued, which terminates, as shown by the 

 probe, d, in fig. 168, in the fauces above the posterior part of the 

 bony palate which has been cut away to expose it, the parts being 

 displayed from below. The tube is membranous throughout, not 

 traversing any bony canal. In the porpoise its inner surface is 

 provided with folds like valves, with the free margin directed to 

 the nasal outlet of the tube : this part communicates with sinuses, 

 some leading to a cellular structure, others compared by Hunter 

 i to the large bag on the basis of the skull of the horse.' * The 



o o 



cetacean labyrinth is excavated in a petrosal capsule, fig. 169, b, m, 

 of the same dense kind of bone as the tympanic, but of an 

 irregular shape, and attached by a short, thin, easily fractured 

 plate to the tympanic. The usual mammalian characters here 

 prevail : the cochlea, k, is indeed relatively larger, compared 

 with the semicircular canals, than in other Mammals, and differs, 

 in Delphinidce, in the greater extent and form of the ( scala 

 vestibuli,' which more resembles a complete tube than the half of 

 such divided in the direction of its axis : it also describes an 

 oblique sigmoid curve 011 leaving the vestibule before it com- 

 mences its spiral turns, which are two and a half in number, and 

 rather more depressed than usual. The aqueductus cochleae is 

 large in Delphinidce. The fenestra ovalis is bordered by a rim 

 for the stapes. The fenestra rotunda is relatively large, and is 

 divided, the lower canal passing along the wall of the scala ves- 

 tibuli and conveying a part of the cochlear nerve. The acoustic 

 nerve is large. 



In Man it has been found that the fall of a drop of water on 

 that with which the meatus has been filled affects the air in the 

 tympanum, so as to produce a sensible impression of sound. 2 The 

 membrane closing the fenestra cochlea transmits its vibrations to 

 the fluid filling the corresponding cone or ( scala,' which would be 

 propagated at the apical communication along the other cone to 

 the vestibule : the Cetacea, with their meatus and ear-drum in a 



J xciv. p. 3S2 (1787). 2 See Carlisle's experiment in cii", p. 207. 



VOL. III. Q 



