ORGANS OF THE SENSES. 



envelopes these delicate parts and separates them from the 

 solid parietes of the osseous labyrinth. The soft lapilli of 

 the labyrinth of vertebrated animals appear to consist of 

 minute rhomboidal crystals of carbonate of lime like those often 

 found lining the intervertebral foramina in other parts of the 

 vertebral column. 



The dense bones of the skull of birds closely envelope 

 every portion of their internal ear with a vibratile covering 

 of great hardness, and corresponding with the rare elastic 

 medium they inhabit. The semi-circular canals, so large and 

 free in the cranium of osseous fishes, are here even more 

 reduced in their dimensions than in reptiles, especially in 

 the water birds, but with enlarged ampuke at their ends for the 

 auditory nerves. The vestibule is narrow but more lengthened, 

 and the cochlea has assumed a more curved and spiral form 

 than even in the crocodiles, its spiral partition, formed by 

 two triangular cartilaginous folds extending nearly to its 

 apex, partially divides it into a vestibular and a tympanic 

 portion, communicating with the two foramina of the vestibule 

 and communicating with each other at the apex in the dilated 

 infundibulum or lagena. The cretaceous lapilli so large in the 

 membranous labyrinth of fishes and amphibia are now greatly 

 reduced and appear only in the form of strata of minute calcar- 

 eous crystals. The semi-circular canals are long and narrow 

 in the rapaceous birds, large and wide in the singing birds, 

 and comparatively small, short, and wide in the grallatores, the 

 palmipeds, and the gallinaceous birds. The tympanic cavity, 

 lengthened like the vestibule, has here a short external 

 meatus beyond the projecting convex membrana tympani to 

 which the cartilaginous malleal end of the anchylosed 

 tympanic ossicula is attached. Not only the tympanic 

 cavity is here surrounded by solid bone, but also the short 

 wide Eustachian tubes which sometimes unite together be- 

 fore they open into the posterior nares, and the drum of the 

 ear has its internal dimensions greatly encreased by the nu- 

 merous air-cells of the diploe, over the greater part of the 

 cranium, which now communicate freely with the interior of 

 both tympana. The short external auditory meatus removes 

 the membrana tympani from the level of the general surface 

 of the head, and the exterior concha, the last part of the 

 auditory organ to be developed, presents itself as a simple 



