134 



ANATOMY OF VERTEBRATES. 



The labyrinth or internal ear consists of the * vestibule,' three 

 semicircular canals, and beginning of the cochlea. The vestibule 

 is smaller in proportion to the other parts, but is longer than in 

 Reptilia. The superior semicircular canal, fig. 55, h' , is usually 

 the largest, as in the Owl ; but it is relatively smaller in most 

 Cantores : the external canal, z, inosculates at m with the horizon- 

 tal one k, but the chief communications of the canals are through 

 the medium of the vestibule. The ends of the canals where the 

 acoustic nerves enter are expanded into ' ampullge,' ib. /, and the 

 nerves are supported in exquisitely delicate vascular membranes 

 lining the canals, and slightly projecting into the ampulla. 



The cochlea is represented by an obtuse osseous conical cavity, 

 fig. 55, ?z, longer than in the Crocodile, very slightly bent, mth the 



concavity directed backward. Its 

 interior is occupied by two small 

 cylinders of fine cartilage, each a 

 little tmsted, and united by a 

 thin membrane at their origin and 

 termination. They proceed from 

 the osseous bar, which separates 

 the two foramina, communicating 

 respectively, the one, ' foramen 

 rotundum,' mth the vestibule, the 

 other, ' foramen ovale,' A\dth the 

 tympanum. The sulcus, which is 

 left between the cartilages, is dilated near the point, and accom- 

 modates the same branch of the auditory nerve, which is sent 

 to the cochlea in Mammals. This nerve spreads in fine fila- 

 ments upon the united extremity of the cartilaginous cylinders. 

 The cavity is divided by the presence of the cartilages into two 

 'scalae,' the anterior of which communicates with the vestibule 

 and is not closed; the posterior scala is shorter, and w^ould 

 communicate with the tympanum by the foramen ovale, Avere it 

 not closed by a membrane. Besides these parts the cochlea 

 still contains a trace of the cretaceous substance which forms so 

 conspicuous a part of the organisation of the internal ear in 

 Fishes. The Struthious Birds manifest their closer relation to 

 the Reptilia by having the cochlea smaller in proportion to the 

 other parts than in the ears of birds of flight. 



The cavity of the tympanum has been already described, p. 62 : 

 besides the communications Avith the air-cells of the surrounding 

 bone, it is continued by the ' eustachian' tube, fig. 55, e, to the 

 palate : to the membrane closing the ' foramen ovale' is applied the 



Organ of hearing, Owl. 



