302 LECTURE XXXII. 



is, however, no doubt that the muscles, with which the difi^ent parts of 

 the ear are furnished, are concerned in accommodating the tensionN/f soihe ' 

 of them to the better transmission of sound ; 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 sounds 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 oftew 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 the 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 termi- 

 nations of the nerves, which form a loose membranous tissue, almost float- 

 ing in it, while another portion of them is distributed on the surface of 

 three semicircular tubes or canals, opening at both ends into the cavity, 

 and a third portion supplies the cochlea, a detached channel, which appears 

 to be arranged 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 affecting the fluid 

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

 the flexibility 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 series 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 respecting the functions of every part of the ear, since its 

 structure varies so much in different animals, that we cannot pronounce 

 with certainty respecting the indispensable necessity of any one arrange- 

 ment for the perfection of the sense. And even in the case of the human 

 ear, many of these parts may be spared without great inconvenience ; thus, 

 we hear very perfectly, by means of impressions communicated to the 

 teeth, and through them to the large bones of tjie head ; and even when the 

 membrane of the tympanum, and all the small bones of the ear have been 

 destroyed by disease, the undulations of the air still continue to affect tho 

 organ in the usual manner.* (Plate XXV. Fig. 349... 351.) 

 * Douglas, De Aure Humana, 4to, Bonon. 1704. 



