888 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



The ear-ossicles form a jointed chain of bones connecting the mem- 

 brane with the oval window. As is well known, solids are capable of 

 conducting sound-waves by being thrown into molecular vibration, and 

 the rapidity of such conduction is greater on account of the greater 

 elasticity and density than in the atmosphere. From the very fact of 

 the greater rapidity of the conduction through solids the wave length 

 will be longer, but the conduction of sound through the ear-ossicles is 

 entirely distinct from such molecular vibrations. In the transmission 

 of sound-waves from the tympanic membrane to the labyrinth the chain 

 of bones vibrates as a whole. The average wave length of medium 

 tones in the ear varies from one-half to one meter, while in solid bodies 

 it is still greater. The ear-bones are by no means immovably fixed. 

 From their small mass they are extremely light, so that an impulse acting 

 on one end will set the whole chain of bones in motion. Consequently, 

 when waves of sound strike against the t} r mpanic membrane the vibra- 

 tions are transmitted directly to the ear-bones, and the}' vibrate in a 

 transverse direction and carry the vibrations to the oval window by 

 vibrating in mass and not through molecular vibration. 



The mode of movement of the ear-ossicles has been a subject of 

 considerable study. As is known, the handle of the malleus is attached 

 to the tympanic membrane through its entire length, while its head pro- 

 jects above the edge of the membrane into the tympanic cavity. Besides 

 this, the malleus is fixed by ligaments in such a way that motion is only 

 possible in a to-and-fro vibration around the so-called axis of rotation, 

 which lies in a plane almost parallel to the tympanic membrane and passes 

 through the neck of the malleus. When the handle of the malleus is 

 drawn inward its head will, of course, move in the opposite direction, and 

 as the handle of the hammer is set in vibration the anvil will also be set 

 in motion through its articulation with the head of the hammer. The 

 incus is only loosely connected by a ligament passing through its short 

 process to the posterior wall of the tympanic cavity in front of the open- 

 ings of the mastoid cells, while its mode of articulation with the malleus 

 has been compared by Helmholtz to the action of cog-wheels ; so that 

 when the handle of the malleus moves inward to the tympanic cavity the 

 incus and its long process, which is parallel with the handle of the malleus, 

 also passes inward, from the fact that the head of the malleus pulls the 

 articulating surface of the incus outward. Therefore, the handle of the 

 malleus and the long process of the incus vibrate in the same direction. 

 As the long process of the incus moves inward it gives an impression to 

 the stirrup-bone, with which it articulates almost at right angles. If, 

 however, as by a great condensation of air in the tympanum, the tym- 

 panic membrane is moved outward it, of course, draws the handle of the 

 malleus with it, and, as a consequence, the hammer-head is forced inward, 



