SENSITIVE SYSTEM. 399 



Eustachian Tube. — The cavity of the tympanum holds a com- 

 munication with the external air through a canal named, after 

 its discoverer (Eustachius), the Eustachian tube. This tube 

 communicates with the cavity of the tympanum by what (in the 

 dried bone) appears to be nothing more than a fissure, from 

 which, having passed through the jietrous portion of the tempo- 

 ral bone, it becomes cartilaginous in its composition, and pro- 

 ceeds for some distance gradually expanding in caliber, until ul- 

 timately it opens into i\\Q guttural sac, formed at the back of the 

 fauces. One side of the tube is clothed by the levator palati and 

 stylo-pharyngeus muscles*. 



Labyrinth. 



The labyrinth, in which are deposited the organs more imme- 

 diately concerned in the function of hearing, is an exceedingly 

 irregular cavity, comprising the vestibule, semicircular canals, 

 and cochlea. 



The cavity of the tympanum we found contained air, having 

 a communication with the atmosphere without; but within the 



tance is tti'ice as great ; and, consequently, vibrations communicated by 

 the membrana tympani to the extremity of the manulirium of the malleus, 

 will be transmitted by the head of that bone to the incus with twofold 

 intensity. 



Again, we perceive the same principle brought into operation in the mo- 

 tions of the incus. One of the processes of this bone is received into a de- 

 pression in the wall of the tympanum in such a manner that the centre of 

 motion proves to be in the direction of a line drawn through the middle of 

 the body of the bone; so that the extremity of the other — its long pro- 

 cess (to which the orbicular bone is attached) — performs a greater sphere 

 of motion than the part receiving the impression from the head of the mal- 

 leus : the consequence of which is, that but a trifling degree of motion 

 given to the body of the incus must become very much more perceptible 

 from augmentation before it reaches the orbicular bone. 



The OS obiculare appears to have been interposed in the manner it is, 

 in order that an accurate perpendicular impulse might be communicated to 

 the stapes : had this bone not been where it is, the vibration from the long 

 process of the incus must have been transmitted to the stapes in an obl'ujue 

 direction, the result of which would have been confusedness and indistinct- 

 ness in audition. 



The stapes, resting as it does flat against the membrane closing the 

 fenestra ovalis, and receiving these full and distinct impulses, imparts the 

 benefit of them to the membrane, and thus the sensation becomes most 

 impressive and perfect. 



* The design of the Eustachian tube appears to be to admit of a free 

 circulation of air in and out of the cavity of the tympanum. Air, from be- 

 ing retained within the cavity, must necessarily become heated and rare- 

 fied, a condition in which it is less suitable to transmit sounds with full 

 intensity ; and therefore a renewal of it takes place through the Eustachian 

 tube. 



