AUDITION. 181 



the sclerotica. The long tendon of this muscle, t, after 

 passing through the loop of the other, which acts as a 

 pulley, is conducted through a circular sheath to the under 

 part of the eye, where it is attached to the lower portion 

 of the nictitating membrane. By the joint action of these 

 two muscles, the membrane is instantly drawn over the 

 front of the eye-ball. Its return is effected by its own 

 elasticity, which is sufficient to carry it back to its place 

 in the inner cornea of the eye. 



AUDITION, OR HEARING. 



559. Next to the eye, the organs of hearing are more 

 complex and refined than those of any other sense. In- 

 deed, certain parts of the mechanism of audition are not 

 less exqu'sitely formed, or less striking and wonderful in 

 their functions, than the most admirable parts of the organs 

 of vision. 



560. Principles of Acoustics. -Acoustics is the science 

 which treats of the origin, propagation, and effects of 

 sound. For an epitome of this science, the author must 

 refer to his " System of Natural Philosophy," while at 

 present, only such a view of the subject will be stated as 

 is absolutely necessary, in order to understand the physiol- 

 ogy of the ear. 



561. Sound is the result of vibratory motions in the 

 particles of a sonorous, or sounding body, which motions 

 are first communicated to the air, and by the air to the 

 ear. , 



562. Sound does not, like light, pass through void space, 

 it being proved by experiment that the report of a bell, 

 struck in a vacuum, is not heard, though the blow is seen, 

 and near the ear. 



563. The sounding body gives an impulse to the air in 

 every direction, and which is propagated from one parti- 

 cle to another, in a circle, in the same manner that the 



What is said of the organs of hearing, when compared with those of 

 vision ? What is taught by the science of acoustics f What is sound ? 

 How is sound communicated to the ear ? Does sound pass through a 

 vacuum or not ? 



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