SENSE OF HEARING. 887 



in the perception of sounds. Sounds which coincide with the funda- 

 mental tone of the tympanic membrane would so predominate as to 

 drown or confuse all other sounds. The tympanic membrane, however, 

 is free from this disadvantage, while it possesses in the highest degree 

 the power of being set in motion by an immense range of vibrations. 

 Thus, sounds may be recognized that are dependent upon thirty-two 

 vibrations a second up to such an extremely high pitch as sounds with 

 thirty-eight thousand vibrations a second will produce. This property 

 of the tympanic membrane by which it appreciates such a wide range of 

 sounds, while being free from any fundamental tone itself, is due to two 

 factors — in the first place, the funnel-shaped form of the membrane, and, 

 in the second plaee, its being damped by the chain of ear-ossicles. If a 

 sheet of india-rubber be stretched over a wide tube and be pressed by a 

 rod in the centre perpendicularly inward it will form a funnel-shaped 

 surface curved from within outward. It is evident that in such a mem- 

 brane the tension will vary at different parts, increasing toward the cen- 

 tre. Such a membrane, like the tympanic membrane, will have no funda- 

 mental tone, since its tension is not equal, while the tympanic membrane 

 will also have in principle the same form, since it radiates from the cen- 

 tre, within outward, in a convex form. The tympanic membrane, there- 

 fore, is not very extensible, but its tension is just sufficient to draw it 

 slightly inward from the centre without it being able to produce any 

 audible fundamental tone. 



On the other hand, great resistance is offered to the vibrations of the 

 tympanic membrane by its union with the auditory ossicles, which not 

 only deprive the membrane of every trace of a fundamental tone, so that 

 it can accommodate itself equally well to vibrations of every degree of 

 rapidity, but by loading the membrane entirely prevent the occurrence 

 of after-vibrations ; so that, therefore, in this respect the ear-bones act 

 like the dampers of the pianoforte, which fall upon the wire after every 

 note has been struck. 



Another point is worthy of attention in this connection. Since the 

 tympanic membrane possesses the shape of a funnel, the point of greatest 

 vibration must be situated somewhere between the apex and the edge, but 

 the force of all vibrations passes from the sides toward the centre and at 

 this point vibrations of the greatest intensity are produced. Moreover, the 

 tension of the tympanic membrane may be altered by muscular action in 

 a way to be directly described. The tympanic membrane is in direct 

 contact with the chain of ear-bones, and this serves to transfer the vibra- 

 tions of the tympanic membrane to the perilymph of the labyrinth, and, 

 likewise, by their points of attachment to different muscles, serve to vary 

 the tension of the membrana tympani and at the same time the pressure 

 on the lymph of the labyrinth. 



