9<y On the StruSure and Vfes efth Membrana Tyntpattl, 



circumference to the handle of the malleus ; from near this handle, a fmall trunk fent off 

 branches, in a radiated manner, which anaftomofed with thofe which had an oppofitc 

 courfe. 



This correfpondence, in the number and diftribution of blood veflels, between the 

 membrana tympani and the iris, is a ftrong circumftance in confirmation of that membrane 

 being endowed with mufcular aiStion. 



In the horfe, the membrana tympani is fmaller than in man ; its long diameter is ^^ths 

 of an inch ; the fhort one ^o^ths ; and it is almoft quite flat, while in man it is concave, 

 which makes the difference of extent confiderably exceed the difference in the diameters, 

 In the horfe, the fibrous ftrufture is not vifible to the naked eye ; it is even indiftinftly 

 feen when viewed through a common magnifying glafs ; but in a microfcope it is very 

 vifible, and in every other refpedl agrees in ftru£ture with the membrane in the human 

 ear, and in that of the elephant. 



In birds, the membrana tympani is larger in proportion than in the quadruped, and 

 more circular in its fliape. 



In the goofe, it Is -j^ths of an inch in its longeft diameter, and" tV^^is in its fhorleft 

 diameter. In the turkey, -j^ths by ^st^s. It is thinner in its coats in birds than In the 

 horfe, and to the naked eye has no appearance of fibres ; but, when viewed in a microfcope, 

 there is a vifible radiated ftru£lure, not very unlike the wire marks upon common writing 

 paper. 



In a former Le£lure upon the Stru£lure of Mufcles,* in which a comprehenfive view 

 was taken of the fubjeft, it was ftated, that the organization neceffary for mufcular con- 

 tradion could exift in an apparent membrane, and that a fafciculated flrudure was only 

 neceffary when mufcular aftion was to be enabled to overcome refiftance. The coats of 

 the Taenia hydatigena were mentioned as an inftance of the fir.1 ; and the human heart as 

 the mod complex of the fecond. In comparing the membranse tympani of different 

 animals, they afford a beautiful Illuftration of the truth of this pofition. 



In birds, where from the fmallnefs of its fize the refiftance is very trifling, the membrane 

 is very fimilar to the coat of an hydatid, only ftill thinner. In the elephant, fibres forming 

 fafciculi are very diftin£l:. The membrane of the horfe, and that of the human ear, form 

 the intermediate gradations. 



The knowledge of a mufcular ftruflure in the membrana tympani, enables us to explain 

 many phaenomena in hearing, which have not hitherto been accounted for in a fatlsfadtory 

 manner. It is principally by means of this mufcle that accurate perceptions of found are 

 communicated to the internal organ, and that the membrana tympani is enabled to vary 

 the ftate of its tcnfion, fo as to receive them in the quick fucceffion in which they are 

 conveyed to it. 



• Philofophical Tranfaftions for the year 1795. 



In 



