764 PHYSIOLOGY. 



the handle of the malleus, also passes inward, from the fact that the 

 head of the malleus pulls the articulating surface of the incus out- 

 ward. The long process of the incus and the handle of the malleus 

 vibrate in the same direction. When the long process of the incus 

 moves inward it gives an impression to the stapes, with which it 

 articulates almost at right angles. The stapes cannot be torn out 

 of the oval window by the Sigle pneumatic speculum when the tym- 

 panic membrane is drawn outward, as the incus only follows the 

 malleus for a certain distance, the malleus sliding in the joint to 

 complete its motion. The malleus and incus are fixed by ligaments 

 in such a way that motion is only possible in to-and-fro vibrations 

 around the so-called axis of rotation, one end of which is found at 

 the origin of the anterior part of the anterior ligament of the 

 malleus, and the other end in the short process of the incus. The 

 ossicles of the ear act like a compound lever; the short process of 

 the incus is the fulcrum ; rhe power is applied to the umbo, in which 

 the handle of the malleus ends; and the resistance is the base of 

 the stapes. The length of the handle below the axis of the malleus 

 is one and one-half times that of the head above the axis. But the 

 range of excursion is only two-thirds that of the handle and drum- 

 head, whilst the power of movement of the head of the malleus will 

 be one and one-half times more than that of the handle. By this 

 means, according to Helmholtz, vibrations are diminished in extent 

 but increased in force. The chain of ossicles vibrates as a whole, 

 and not by molecular vibration. The tympanic membrane is twenty 

 times the size of the oval window ; hence the movement of the mem- 

 brane of the oval window is smaller in extent, but about thirty times 

 greater in power. When sound impinges against the tympanum, the 

 tympanic membrane moves inward with the attached handle of the 

 malleus, and the head of the malleus moves outward. The incus fol- 

 lows these movements ; the body of the incus swings outward and the 

 long process moves inward, which pushes the stapes into the oval 

 window. 



Thus the ossicles and the fluid in the labyrinth do not form a 

 mass vibrating independently, but as one body. 



Tensor Tympani. The tensor tympani reflex has its sensory 

 nerves from the trigeminus and its motor nerve from the same source. 

 When one tensor tympani contracts, the tensor of the opposite side 

 also contracts. 



In rare cases the tensor tympani is under the control of the will. 



