VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 569 



the number and distribution of blood-vessels, between the membrana tympani and 

 the iris, is a strong circumstance in confirmation of that membrane being endowed 

 with muscular action. 



In the horse, the membrana tympani is smaller than in man; its long diameter 

 is -2V of an inch; the short one &\ and it is almost quite flat, while in man it is 

 concave, which makes the difference of extent considerably exceed the difference in 

 the diameters. In the horse, the fibrous structure is not visible to the naked eye; 

 it is even indistinctly seen when viewed through a common magnifying glass; but 

 in a microscope it is very visible, and in every other respect agrees in structure 

 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 cir- 

 cular in its shape. In the goose, it is ^%- of an inch in its longest diameter, and 

 -gV in its shortest diameter. In the turkey -^ by ^. It is thinner in its coats in 

 birds than in the horse, and to the naked eye has no appearance of fibres; but 

 when viewed in a microscope, there is a visible radiated structure, not very unlike 

 the wire marks on common writing paper. 



In a former Lecture on the Structure of Muscles # , in which a comprehensive 

 view was taken of the subject, it was stated, that the organization necessary for 

 muscular contraction could exist in an apparent membrane, and that a fasciculated 

 structure was only necessary when muscular action was to be enabled to overcome 

 resistance. The coats of the taenia hydatigena were mentioned as an instance of 

 the first; and the human heart as the most complex of the 2d. In comparing the 

 membranae tympani of different animals, they afford a beautiful illustration of the 

 truth of this position. In birds, where from the smallness of its size the resistance 

 is very trifling, the membrane is very similar to the coat of an hydatid, only still 

 thinner. In the elephant, fibres forming fasciculi are very distinct. The mem- 

 brane of the horse, and that of the human ear, form the intermediate gradations. 

 The knowledge of a muscular structure in the membrana tympani, enables us to 

 explain many phenomena in hearing, which have not hitherto been accounted for 

 in a satisfactory manner. It is principally by means of this muscle that accurate 

 perceptions of sound are communicated to the internal organ, and that the mem- 

 brana tympani is enabled to vary the state of its tension, so as to receive them in 

 the quick succession in which they are conveyed to it. In the human ear, and in 

 that of birds, the radiated fibres of the membrana tympani have their principal at- 

 tachment to the extremity of the handle of the malleus, which is nearly in the 

 centre of the membrane. In the membrane of the elephant, which is oval, the 

 attachment to the handle of the malleus is at some distance from the centre. In 

 the horse, deer, and cat, which have the membrane still more oval than the ele- 

 phant, the handle of the malleus is situated in the long axis of the membrane, with 

 its extremity extending beyond the centre, reaching nearer to the circumference; 



* Phil. Trans, for 1795.— Orig. 



VOL. XVIII. 4 D 



