224 



ANATOMY OF VERTEBRATES. 



1C9 



Delphinus tursio, a hog's bristle : having passed through tlie skin 

 and blubber, it makes a sudden bend upon itself, at Z>, and is then 

 continued by a course of about an inch and a half to the ear-drum, 

 where it rather suddenly expands : in the subcutaneous part of 

 its course the walls are strengthened by a few longitudinal carti- 

 lages with elastic connections, allowing of slight changes in length 

 and disposition ; but the walls are in contact throughout most of 

 the narrow part of the tube. The ear-drum is concave exter- 

 nally in DelphinidcR and Physeteridce ; but in a Balcenoptera 

 Hunter found it projecting with an unusual degree of convexity 

 into the dilated inner termination of the meatus. 



The density of the osseous tissue of the tympanic bone, ib. <?, 

 recalls that of the large otolites of fishes, and the almost free 

 suspension of this singularly shaped subconvolute mass suggests 

 that it may be affected, like those otolites, by the sonorous vibra- 

 tions which are propagated 

 through the water and strike 

 upon the outer surface of the 

 head of the Cetacea. How- 

 soever the ear-drum may be 

 agitated, whether by a pos- 

 sible entry and propagation 

 of air- vibrations through the 

 meatus, , b, or by an affection 

 of the dense and massive bone, 

 fig. 169, , supporting it, the 

 vibrations are conducted by 

 a triangular plate of fibrous 

 tissue, ib. f, to the handle of 

 the malleus, g, one margin of 

 the transmitting plate being 

 attached to about three- 

 fourths of the long axis of 

 the inner surface of the ear-drum ; but this is extended at e 

 beyond the circumference of the inner termination of the bony 

 meatus. The malleus articulates in the usual mammalian way 

 with the incus, h, and the inner eras of this with the stapes, z, the 

 thick, marginally rounded, elliptical base of which is deeply sunk 

 into the ' fenestra ovalis : ' there it is arrested by and presses 

 against the continuation of the lining membrane of the vestibule, 

 which, like the drum-membrane, is affected by the movements of 

 the attached ossicle : these are due to a ' stapideus ' muscle, 

 fig. 169, 0, inserted into the neck of the stapes so as to pull it at 



Tympanum and labyrinth, Balsena. xciv. 



