200 PHILOSOPHY OF ZOOLOGY. 



containing a fluid, through which the nervous filaments are 

 dispersed. These cavities are of two kinds. In the first, 

 there are tubular semicircular holes, which are three in 

 number, and denominated semicircular canals, which open 

 into a cavity termed the vestibule ; the second is termed 

 the cochlea, which likewise communicates with the vestibule, 

 and consists of a tube revolving round a conical axis, hol- 

 low like a turbinated shell, describing, according to the 

 species, from one turn and a half to three turns and a half. 

 These cavities are filled with a fluid which has been term- 

 ed the Water of Cotunnius. 



The auditory nerve takes its rise from the inferior sur- 

 face of the fourth ventricle of the brain, and penetrates the 

 cavities of the labyrinth by traversing an infinite multitude 

 of perforations, which it fills by its minute and anasto- 

 mosing filaments. 



In Birds, the labyrinth consists of nearly the same parts 

 as in quadrupeds. In these, however, the cochlea is less 

 perfectly developed, and merely consists of a short hollow 

 bony process, divided into two scalae. The semicircular 

 canals are not hollowed out of an os petrosum, but consist 

 of tubes, united by cellular bone. In Reptiles the cochlea 

 is still more imperfect ; and in many species it can scarcely 

 be said to exist. The semicircular canals are apparent ; 

 and there is a bag containing gelatinous pulp, in which the 

 filaments of the nerves are distributed. In this sac there 

 is a substance of the consistence of starch, which may be 

 regarded as occupying the place of the cochlea in the high- 

 er classes. In Fishes, the semicircular canals still exist ; but 

 there is no trace of a cochlea. The sac of gelatinous pulp, 

 contains three cretaceous bodies, varying in hardness in the 

 different races. The nervous filaments embrace these bo- 

 dies. Among the cephalopodous Mollusca, the labyrinth 

 consists of a simple sac, inclosing the pulp and a single creta- 



