ON THE SOURCES AND EFFECTS OF SOUND. 3S7 



and it is equally certain that their operation is not absolutely necessary in 

 the process. 



The external ear serves in some measure for collecting the undulations of 

 soiinds transmitted through the air, and reflecting them into the auditory 

 passage, at the bottom of which they strike against the membrane of the 

 tympanum or drum, which, being larger and more moveable than some of 

 the subsequent parts, is capable of transmitting a stronger impulse than 

 they would immediately receive. In the same manner we may often feel 

 the tremors produced in a sheet of thick paper, held in the hand, by the 

 agitation of the air, derived from a loud sound, which would not otherwise 

 have affected the organ of touch. The impulse received by tlie membrane 

 of the tympanum is conveyed by the hammer and anvil, two small bones, 

 which together constitute a kind of bent lever, through a third minute flat- 

 tened bone, to a fourth called the stirrup, which serves merely as a handle 

 to its basis, a plate covering the orifice of a cavity called the vestibule, and 

 communicating the impulse to the mucous fluid which fills this cavity. The 

 fluid of the vestibule, thus agitated, acts immediately on the terminations ' 

 of the nerves, which form a loose membranous tissue, almost floating in it, 

 Avhile another portion of them is distributed on the surface of three semi- 

 circular tubes or canals, opening at both ends into the cavity, and a third 

 portion supplies the cochlea, a detached channel, which appears to be ar- 

 ranged with singular art as a micrometer of sound. It resembles the spiral 

 convolutions of a snail shell, and if uncoiled, would constitute two long 

 conical tubes connected at their summits, the base of one opening into 

 the vestibule, that of the other being covered by a membrane only, which 

 separates the fluid from the air contained in the general cavity of the ear, 

 or the tympanum. It is evident from the properties of fluids moving in 

 conical pipes, that the velocity of any impulse atfecting the fluid at the 

 base of the cone must be extremely increased at its vertex, while the flexi- 

 bility of the membrane at the base of the second channel allows this motion 

 to be effected without difficulty. It has also been supposed that a scries 

 of fibres are arranged along the cochlea, which are susceptible of sympathetic 

 vibrations of different frequency according to the nature of the sound which 

 acts on them; and, with some limitations, the opinion does not appear to be 

 wholly improbable. We must, however, reason with great caution respect- 



