104 OF HEARING. 



to (he circular furrow above mentioned by its spiuous 

 process which is directed forwards, and il lodges its 

 round head in the body of the incus. 



The incus is united to the head of the stapes by the 

 extremity of its long process which extends into the 

 cavity of the tympanum. 



The stapes, resting its base upon the fenestra ovalis, 

 runs towards the vestibule of the labyrinth, into which, 

 sounds, struck against the membrana tympani, are 

 propagated by the intervention of these three little 

 bones. 



249. The Eustachian tube* runs from the interior of 

 the fauces into the cavity of the tympanum : and the 

 inferior scala of the cochlea has the same direction ; 

 the opening of the latter, termed fenestra rotunda,-^ is 

 closed by a peculiar membrane. The true and prin- 

 cipal use of each is not sufficiently known. 



250. In the deepest part of the petrous bone is placed 

 the internal ear, consisting of three parts. 



First, of the vestibule, placed between the other two, 

 into which open not only the fenestra ovalis, but the 

 five orifices of the semicircular canals which lie pos- 

 teriorly, and the superior scala of the cochlea which is 

 placed anteriorly. 



The vestibule and semicircular canals loosely con- 



* Saunders, Anatomy of the human ear. Lond. 1806. fol. vol. i. ii. 



f- Scarpa, De Stntctura Faiestres Rotunda, &fc. Mutin. I/ 72. 8vo. 



J Comparative anatomy renders it most probable that the Eustachian tube 

 is subservient to the action of the membrana tympani. It is found in all 

 red-blooded animals which possess a membrana tympani, but is wanting in 

 fishes which are destitute of this membrane. The different opinions of the 

 moderns respecting its use may be found in Kiel's drchiv. fiir die Physiol. 

 T. ii. p. 18. iii. p. 165. iv. p. 10f). viii. p. 67. ix. p. 320, 



