ADDITION. 185 



the sclerotic^. 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. 



Next to the eye, the organs of Hearing are more com- 

 plex and refined than those of any other sense. Indeed, 

 certain parts of the mechanism of Audition, are not less 

 exquisitely formed, or less striking and wonderful in 

 their functions than the most admirable parts of the 

 organs of vision. 



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 

 physiology of the Ear. 



Sound is the result of vibratory motions in the parti- 

 cles of a sonorous, or sounding body, which motions 

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

 Ear. 



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. 



The sounding body gives an impulse to the air in 

 every direction, and which is propagated from one par- 

 ticle 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 7 What is sound 1 

 How is sound communicated to the Ear 7 Does sound pass through a 

 vacuum, or not? 



16* 



