Chap. 40.] Ears of different Animals. 377 



wards, or their ears are flexible, becaufe they move 

 their head for the moft part with greater difficulty 

 than man. Man again, who mud equally hear founds 

 coming from all quarters, but efpecially fuch as are 

 fent from about his own height, has his external ear 

 placed in a vertical manner, fomewhat turned for- 

 ward. In fhort, wherever we fee a peculiarity in the 

 make of this organ in any creature, we mall, with 

 very little reflection, difcbver this form to be 'more 

 convenient for that creature than another. The ani- 

 mal alfo has the power of directing the cone of the 

 ear to the fonorous body without moving the head \ 



The membrana tympani is a membrane confifring 

 of feveraj. laminse. Externally there is the epidermis, 

 under this the vafcular cutis, and, laftly, a dry, elaflic, 

 fhining, and pellucid fubftance. Thefe laminse are 

 connected by their cellular fubftance. The membrana 

 tympani is never naturally perforated, and the paflage 

 of fmoke from the mouth through the external ear, 

 mentioned by fome authors j-, is fabulous, except, 

 perhaps, in fome cafes, where a perforation had been 

 accidentally made by violence or difeafe. By the 

 action of particular mufcles, the membrana tympani 

 is preferved in a degree of tenfion fit for receiving 

 the imprefiions of the air. Under it runs a branch of 

 the fifth pair of nerves, called the chorda tympani. 



The membrana tympani is ftretched before a round- 

 iili cavity of the os petrofum, hence called the tym- 

 panum or drum, and which is about feven or eight 

 lines wide, and half as many in depth. This cavity 

 is increafed in the adult by a communication with the 

 cells of the maftoid procefs, which do notexift in the 

 foetus. Within, the tympanum is lined by a moift and 



* Monro on Comparative Anatomy. 

 f By Dr. Goldfmith in particular. 



vafcular 



