154 



ON THE SENSES 



latory motion the fluid contained in the so-called labyrinth. It is no very easy 

 matter to make the structure, position and connection? of the system of small 

 bony levers interposed between the tympanum and the labyrinth perfectly clear; 

 we shall endeavor first to exhibit the priuciple of the arrangement by means of 

 some illustrative delineations : Fig. 3 represents the simplified form of the appa- 

 ratus seen in perspective ; Fig. 4, an ideal vertical section thereof. A little rod 

 of some length a h, which bears at its upper extremity a knob h, is by its lower 



half a c attached to the stretched 

 ^^* ' membrane of the tympanum T r, 



in such manner that it reaches 

 from the upper edge directly to 

 the middle of that membrane, thas 

 dividing the upper half of it into 

 two equal lateral portions, while 

 the upper half of the rod with the 

 knob projects freely into the cav- 

 ity of the tympanum. Through 

 the middle c of the rod, and cross- 

 ing it at right angles, (in Fig. 4 

 supposed to be perpendicular to 

 the surface of the paper,) passes 

 a solid transverse rod d e, both 

 ends of which are so secured by 

 ligaments to the bony wall of the 

 cavity near the border of the tympanum, that this 

 transverse rod can turn upon its own longitudinal 

 axis, but not be otherwise displaced. We may 

 imagine both ends surrounded by annular liga- 

 ments which are tightly fastened to the wall of 

 the cavity. The lower end a of the longer rod 

 bears a short arm af, with which it forms a right 

 angle, and which is perpendicular to the surface 

 of the membrane of the tympanum. The free 

 endy of this arm bears finally a stirrup-shaped ap- 

 pendage g, firmly articulated with it, whose foot- 

 plate is fitted like the piston of a syringe into an oval window-like opening of 

 the reservoir L, (the labyrinth,) containing a fluid; with the edges of this reser- 

 voir the appendage g is united by a membranous and very elastic border. The 

 action of the vibrations of the tympanic membrane on this apparatus must neces- 

 sarily be as follows : If the membrane be driven inwards by a wave of conden- 

 sation so that its curvature, as shown in Fig. 4, is increased and its centre a be 

 thus brought nearer to the reservoir L, the rod a b must follow the movement 

 and turn around the transverse rod d e (which serves as its axis of revolution) 

 in such manner that the part c a below the fulcrum c moves inwards and up- 

 wards with the tympanum, while the upper part c b, projecting into the cavity, 

 moves outwards and downwards, as is indicated by the directions of the arrows 

 in Fig. 4. The rod af, attached rectangularly to the former, will thus be lifted 

 up in the direction of the arrow proceeding from a, whereby its stirrup-like ap- 

 pendage g will be thrust deeper into the reservoir, and the elastic border by 

 which it is united to the edge of the opening will undergo extension. The fluid 

 in L receives this external pressure and is propelled before it. Were the walls 

 of this reservoir rigid and unyielding throughout its whole circuit the fluid could 

 not give way, and, since a fluid is compressible only by strong force, while that 

 of the air- wave is relatively small, the resistance thus opposed would render the 

 entrance of the appendage impossible, and mediately also the whole movement 

 of the apparatus and the tympanum. In order, therefore, that the external 



