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 scale. 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- 
erclasses. 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- 
