Chap. 40.] Why deaf Perfons open their Mouths. 381 



with an aqueous exudation, which is fuppofed to receive 

 and propagate to the nerves the vibratory motions 

 imparted by tfye air. When this fluid is collected in 

 too great quantity, or is compreffed by the ftapes, it 

 is fuppofed to efcape through two minute canals or 

 aqueducts, lately defcribed by Dr. Cotunni, a phyfi- 

 cian of Naples. One of the aqueducts opens into the 

 bottom of the veftibulum, and the other into the 

 cochlea, near the feneftra rotunda. They both pafs 

 through the os petrofum, and communicate with the 

 cranium ; they are lined with a membrane, which is 

 fuppofed to be a production of the dura mater. 



The manner in which found is propagated by pulfes 

 or undulations of the air has been fully, and, I truft, 

 clearly explained in a preceding part of this work *; 

 and from what has been now ftated it will appear, 

 that the ear is an organ admirably adapted for the re- 

 ception of thefe impreffions. Sound is, however, not 

 merely conveyed by the external cavity of the ear ; 

 but by means of the Euflachian tube, the air finds 

 admittance to the cavity of the tympanum, and the 

 effect of the vibrating air, entering the mouth, may- 

 be conveyed to the ear. Hence we perceive the rea- 

 fon why perfons who liften very attentively, and 

 perfons affected with partial deafnefs, open their 

 mouths. When we breathe, the air received by this 

 paffage preffes the membrana tympani outwards, and 

 when we make a very full infpiration, as in yawning, 

 this happens to fuch a degree as to prevent the impref- 

 fion of founds from without, and occafions a tempo- 

 rary deafnefs. 



Notwithftanding the labour of anatomifts in tracing 

 the intricate, fingular, and very curious ftructure of 



Sec bok v. chap. 10. 



the 



