COMPARATIVE ANATOMY OF THE EAR. 377 



forms the boundary of the auditory cavity is unconsolidated, and a mem- 

 brane which stretches over the otherwise vacant space presents the first 

 rudiment of the fenestra ovalis. With the exception of the amphioxus, 

 all vertebrated animals have a special organ of hearing, which, in suc- 

 cessive tribes, presents an interesting increase of complexity, beginning in 

 the cyclostomes with a sac in the cranial cartilages filled with water, 

 nerve fibrils distributed on its walls, and an otolith included, but no ex- 

 ternal communicating aperture. From this, in succession, the various 

 portions which are to be developed in perfection in the higher races grad- 

 ually appear: the myxine has one semicircular canal arising from the 

 vestibule, the lamprey has two, the higher forms have three. As the case 

 may be, a portion of the cartilage or bony parietes is deficient, and, again, 

 the fenestra ovalis is the result. Though in the osseous fishes there is 

 neither tympanum or cochlea, in some few the rudiments of the former 

 begin to exist. The naked amphibia have no cochlea, and only one fe- 

 nestra, answering to the ovalis : to this is fitted a stapes ; but in lizards 

 and scaly serpents there is a general advance, these having a conical 

 cochlea. As we pass through them the plan is earned out ; the tympan- 

 ic cavity and its chain of bones, the Eustachian tube, and cochlea ap- 

 pear; and with the rudiment of the cochlea there is presented in the tym- 

 panic cavity a second aperture, answering to the fenestra rotunda. In 

 birds the structure offers a continued improvement, commencing on a 

 plan analogous to that of the scaly amphibia, but exhibiting a speedy 

 development. . The membrana tympani is composed of several layers ; 

 the cavity of the drum communicates with cells in the cranial bones, the 

 analogues of the mastoid cells ; a bony Eustachian tube crosses to meet 

 its fellow of the opposite side, and open in a common aperture. The os- 

 sicles consist of a malleus, a staff-shaped intermediate bone, and a flat 

 stapes, resting on the fenestra ovalis. As if to show a tendency to the 

 form it is eventually to assume, this bone sometimes presents a forked 

 appearance, the preparation for a stirrup shape. As regards this bone, 

 birds and mammals may be said to overlap, for in its more developed 

 condition in birds it bifurcates, but in the lower mammals, as the kanga- 

 roo, it is still cylindric. In birds of prey the semicircular canals are 

 large, the cochlea fairly developed, though as a straight or slightly-curved 

 tube, containing its scake and vibrating lamina : the vestibule has ear- 

 stones. Through the monotremata this condition of construction is con- 

 tinued into the perfect mammals : all the aerial tribes have external ears, 

 and full development is reached in the auditory mechanism of man. 



Now if we collate the facts here presented with the requirement of the 

 condition of life which each of these successive races seems to demand, we 

 shall find that the remark heretofore made, that the semicircular canals 

 are for the recognition of the qualities of sound, is strikingly borne out, 



