ORGANS OF PERCEPTION. 201 
ceous body. There are no semicircular canals. In the Crus- 
tacea, the sac consists of a scaly cylinder ; the one extremity 
closed by an elastic membrane, exposed to the action of the 
element in which the animal resides; and the other extre- 
mity open for the entrance of the nerve. Neither semi- 
circular canals nor cretaceous bodies are obvious. 
In Quadrupeds, the labyrinth occupies the inner part of 
the ear ; and on its external side there is a cavity, termed 
the Tympanum, or barrel of the ear. This cavity is irre- 
gular in its form, according to the species. It is closed on 
all sides; behind by the labyrinth, laterally by the osse- 
ous parietes, and externally by an elastic membrane, term- 
ed membra tympani, or drum of the ear. The inner wall of 
this cavity, which is formed by the labyrinth, contains two 
openings ; the one termed the fenestra ovalis, which com- 
municates with the vestibule and semicircular canals; and 
the other termed the fenestra rotunda, which opens into 
the cochlea, but whose orifice is closed by a membrane. 
From the floor of the barrel, a tube, osseous at its commence- 
ment, and cartilaginous in the remaining part, takes its rise, 
which terminates in the back of the mouth in the palate, 
called the Kustachian tube. Other holes may be observed 
in different species, leading imto adjacent cells. The ex- 
ternal membrane, by which the tympanum is closed, is at- 
tached to the osseous circle which forms the walls. It may 
be regarded as a continuation of the skin through the ex- 
ternal ear, and of the integuments of the mouth through 
the Eustachian tube; it is dry, transparent and elastic. 
Within the barrel there are three bones, which form an 
immediate connection between the drum and the fenestra 
ovalis. The first, which is termed malleus or hammer, ad- 
heres to the drum; and is articulated with the central bone, 
meus or anvil, which is again united to a branch of the 
stapes or stirrup, whose base rests upon the fenestra ovalis. 
