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 Eustachian tube. Other holes may be observed 

 in different species, leading into 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, 

 incus or anvil, which is again united to a branch of the 

 stapes or stirrup, whose base rests upon the fenestra ovalis. 



